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Episode 12: Vaccine Safety 6 - Expecting Protection: building vaccine confidence for moms-to-be
- Heather McSharry, PhD
- May 28
- 28 min read
Updated: 6 days ago
For informational purposes only. Please talk to your doctor about getting vaccinated.
Welcome to my series devoted to helping parents sort through the propaganda and fear-mongering regarding vaccines.
Episode Summary
Pregnant people are bombarded with advice—and when it comes to vaccines, fear often drowns out facts. In this episode, Heather cuts through the noise to explain how vaccines work during pregnancy, why they protect both mother and baby, and what the latest research says about safety. From flu and COVID-19 to Tdap and RSV, Heather unpacks the evidence, tackle misinformation, and offer clarity for anyone navigating prenatal care in an age of uncertainty. Expecting Protection is a science-grounded guide for building vaccine confidence—one honest conversation at a time.
Listen here or scroll down to read full episode.
What's up folks!
Today, we’re tackling an incredibly important topic: vaccine safety and recommendations during pregnancy.
Pregnancy is a time of profound change, and while it’s exciting, it also makes the body more susceptible to certain infections. These infections can pose serious risks not only to the pregnant person, but also to the developing baby. Fortunately, vaccines are one of the best tools we have to reduce these risks — but they’re also a source of concern and misinformation for many people.
Our discussion today is grounded in evidence from peer-reviewed research, public health agencies like the CDC and WHO, and professional societies like ACOG and the Royal College of Obstetricians and Gynecologists.
NOTE: The best defense is to make sure you are up to date on all your vaccines before you get pregnant. Being up to date will help protect you and your child from serious, preventable diseases. But the next best thing is getting the recommended vaccines while you're pregnant.
Immune system changes during pregnancy
In addition to all the other changes pregnancy brings, it also brings profound immunological changes. Contrary to the common idea of “immune suppression,” pregnancy actually involves a dynamic modulation of the immune system. To tolerate the genetically distinct fetus, the maternal immune system shifts towards an anti-inflammatory state in certain trimesters, while also mounting specific pro-inflammatory responses when needed for implantation and labor.
This fascinating balance ensures that the fetus is not rejected as “foreign,” but it also makes pregnant individuals more susceptible to certain infections and sometimes more likely to have severe disease outcomes. For example, pregnant people are at greater risk of serious complications from influenza, COVID-19, or pertussis, and more likely to require hospitalization or intensive care. Even foodborne infections like listeriosis are more serious for expecting moms. This immune modulation is why pregnant individuals need extra protection from vaccines, which aren't just safe but critical for the health of the mother.
Infections can harm not only the pregnant person but also the fetus, so maternal vaccination is just as critical for the health of the baby. For instance, pertussis (whooping cough) can be deadly for newborns in their first months of life, as they’re too young to be vaccinated themselves. This is where the dual protection concept comes in: maternal antibodies, transferred across the placenta, provide crucial passive immunity to the baby in those vulnerable first months.
This is why it’s essential for researchers studying how pregnant individuals respond to infections to also examine how the placenta itself reacts to these pathogens. Some infections might not directly harm the pregnancy but can make both the mother and fetus more susceptible to other infections. In such scenarios, preventive measures, such as vaccination, become crucial—and the earlier these are administered, the better. Bottom line is that careful consideration of the risks and benefits of vaccination throughout all stages of pregnancy is vital.
Vaccine Safety in Pregnancy
And that brings us to vaccine safety in pregnancy. And I want to be very clear here, and emphasize that the vaccines recommended during pregnancy — such as flu, Tdap, COVID-19, and RSV — have been thoroughly studied in the context of pregnancy. They are inactivated or subunit vaccines, designed to work with the maternal immune system’s unique adaptations to provide safe, effective protection. For more info about inactivated and subunit vaccines, check out my What's In Vaccines episode.
Vaccines Recommended During Pregnancy:
Flu vaccine— as soon as it's available during flu season.
Tdap (tetanus, diphtheria, and whooping cough) vaccine — as soon as possible during your third trimester, even if you got it before pregnancy.
An updated COVID-19 vaccine— if you haven't received it yet.
RSV vaccine — September- January, between 32 and 36 weeks of pregnancy.
Of those listed above, The American College of Obstetricians and Gynecologists (ACOG) says two should be given during each pregnancy: Flu and Tdap. So let's cover those first.
Influenza (Flu) Vaccine: Influenza, or the flu, is a highly contagious respiratory illness caused by influenza viruses. It usually comes on suddenly and causes symptoms like fever, cough, sore throat, body aches, and fatigue. While most people recover in about a week, the flu can cause serious complications in vulnerable groups — including young children, older adults, pregnant people, and those with chronic health conditions. Each year, seasonal flu causes millions of illnesses and hundreds of thousands of hospitalizations worldwide.
Numerous case reports and studies show that pregnancy itself increases the risk of complications from seasonal influenza and they are more likely to experience severe complications from flu than the general population. Severe infections have also been seen among postpartum women in the two weeks after giving birth, especially during the 2009 H1N1 swine flu pandemic.
One large study from Nova Scotia, following about 134,000 pregnant women between 1990 and 2002, found that third-trimester hospital admissions during flu season were five times higher than the year before pregnancy, and more than twice as high as in non-flu seasons. These hospitalizations were even more common among women with underlying health conditions.
So the real risk of maternal infection here seems to be for the moms-to-be rather than the infants. Although influenza during pregnancy can lead to delivery complications like preterm labor or cesarean delivery, studies show that babies born to mothers with laboratory-confirmed influenza are not at higher risk of low birth weight, birth defects, or low Apgar scores compared to babies born to uninfected mothers.
However, I want to be clear that that is for maternal infection during pregnancy. Infection of newborns and babies younger than 6 months old is very dangerous. These babies have higher rates of hospitalization, more prolonged intensive care unit (ICU) stays, and higher fatality rates than almost any other age group. And these babies cannot yet be vaccinated themselves for flu, they are too young. This is why it's so important for moms to get their flu vaccine, so they can pass on their maternal antibodies to the babies in utero and after birth in breast milk.
Annual flu vaccination is strongly recommended for all pregnant people during any trimester. Studies show that flu vaccination during pregnancy reduces the risk of flu-associated hospitalization by about 40–50% in pregnant people. It also cuts the risk of flu-related hospitalization in infants by about 72%. Those are huge benefits of the vaccine!
So, which flu vaccines are OK for moms-to-be? Inactivated influenza vaccine can be safely and effectively administered during any trimester of pregnancy and is the best way to protect yourself and others from this potentially serious disease. To clarify, the inactivated flu vaccine is the one that's injected. The nasal vaccine is a live attenuated vaccine and is not given to pregnant people because of a theoretical risk of transmission, though that risk is very low.
This is the standard for the US, UK, EU, Australia, and Canada. Globally, the World Health Organization also strongly recommends inactivated flu vaccines during pregnancy in all countries, especially where flu is a significant public health concern. This underscores how widely supported these safe and effective vaccines are around the world.
And now let's look at what exactly is in a flu shot. The formulations will differ slightly across the various licensed versions, but they will be similar. As an example I'll use one that's really common in the US and other countries: Fluzone.
Flu Vaccine Sidebar: A little info that will help us understand what's in this vaccine, is that last year, experts decided that for the 2024–2025 season, the Fluzone flu vaccine would switch from being quadrivalent to being trivalent. In other words, it would only contain three virus strains instead of four: two A strains and one B strain from the Victoria lineage. This change reflects the global disappearance of the B/Yamagata strain, showing how quickly viral circulation can evolve and shape vaccine recommendations.
Now this vaccine, and similar inactivated flu vaccines, are standardized for the hemagglutinin protein content. Hemagglutinin, or HA is the main part of the virus that our immune system recognizes that will elicit the immune response we're going for. This is also referred to as the antigen. Hemagglutinin is the H in H5N1 that you hear about with bird flu. The N part, or NA, is neuraminidase, another important viral protein that is present in small amounts in these vaccines but isn’t directly measured or standardized in them because it's not the main immunogen we're after. Now, an interesting thing about these flu vaccines is that they don't use whole inactivated virus (like they do for other vaccines like Yellow Fever, Japanese Encephalitis, and Tick-Borne Encephalitis). For flu they actually break down the whole influenza virus and use only the fragments containing the HA protein. This is what they refer to as a split virus in the package insert. Now that we know that, let's take a look at the ingredients in the vial. For more detailed explanations of vaccine components and why they are safe, please check out my What's in Vaccines episode!
Fluzone (Sanofi Pasteur), is a sterile solution for intramuscular injection. Here's the package insert. Just FYI, when you are looking at a package insert, the place where you find the ingredients is called the Description and is usually Section 11, though the page number varies between inserts.
Each dose contains:
Antigens:
Inactivated influenza virus hemagglutinin (HA) from four strains (hence the quadrivalent in the name):
2 influenza A strains (H1N1 and H3N2)
1 influenza B strains (Victoria lineage)
Stabilizers:
Sodium phosphate-buffered isotonic sodium chloride solution: maintains pH of the vaccine formulation within a safe and stable range (close to physiological pH).
The sodium chloride component adjusts the vaccine’s osmolarity to be similar to that of human cells (isotonic), minimizing discomfort or tissue irritation at the injection site.
Residuals: trace amounts of some things left over from manufacturing.
Formaldehyde: Used during the virus inactivation process to kill the virus, ensuring it won't cause disease while still allowing the immune system to recognize the viral antigens. I really go into why this is safe in What's in Vaccines episode.
Octylphenol ethoxylate (also known as Triton X-100): a detergent or surfactant used to disrupt the viral envelope in the virus “splitting” process.
Preservatives:
The multi-dose vial contains thimerosal as a preservative.
Single-dose, pre-filled syringe presentation is manufactured and formulated without thimerosal or any other preservative.
NOTE: Thimerosal does not contain the dangerous type of mercury. It contains ethylmercury, not the more dangerous form of mercury called methylmercury. Ethylmercury is processed by the body much more quickly and does not build up or cause harm at the low levels used in vaccines. Because of misinformation campaigns and public misperception, manufacturers decided to take it out of most vaccines even though it's safe. In flu vaccines, it is sometimes in multi-dose vials, which have a greater risk of contamination, but never in single dose vials. See my episode on What's in Vaccines for more debunking of vaccine component myths.
So now you know the ingredients and that they are safe. Let's move on to the next vaccine recommended in pregnancy, Tdap.
Tdap (Tetanus, Diphtheria, acellular Pertussis): Whooping cough, also known as pertussis, is a highly contagious respiratory infection caused by the bacteria Bordetella pertussis. It's characterized by uncontrollable coughing fits, sometimes followed by a "whoop" sound when breathing in. While it can affect anyone, it's particularly dangerous for babies and young children.
Maternal Tdap vaccination is not only safe in pregnancy it is critical for protecting newborn infants against pertussis. Babies do not begin to develop their own immunity to whooping cough until they receive their first vaccinations at around two months of age. This gap leaves them vulnerable during their earliest months, precisely when they face the greatest risk of contracting whooping cough and suffering severe, life-threatening complications.
So Tdap is recommended during every pregnancy, ideally between 27–36 weeks’ gestation. And it's important to understand that protective antibodies against whooping cough peak about 2 weeks after getting vaccinated but it takes time to pass them to the baby. The recommended timing of vaccination maximizes the transfer of maternal antibodies, significantly reducing the risk of pertussis in newborns. Now, in case you didn't catch that, I said get vaccinated early in each pregnancy. This is because The amount of whooping cough antibodies in the pregnant woman's body also decreases over time. This is true even if pregnancies are only a year or two apart. Doing so allows each baby to get the greatest number of protective antibodies and best protection possible against whooping cough.
about one-third of infants with pertussis require hospitalization, and 1% of those cases die. Pertussis is one of the significant causes of illness and death in infants. It can be fatal with severe complications such as pulmonary hypertension, heart failure, and encephalitis.
Vaccination after delivery leaves baby without protective antibodies
When the mother doesn't get Tdap during pregnancy and has never received it before, she can get it after the baby is born. Once the mother develops antibodies after vaccination—about 2 weeks—spreading whooping cough to her newborn is less likely. However, the baby will still be at risk for catching whooping cough from others.
Now, I would be remiss if I didn't mention that some early studies suggested a small increase in the risk of chorioamnionitis — an inflammation of the fetal membranes — following Tdap vaccination during pregnancy. However, even in those few studies, there was no increase in adverse outcomes for those pregnancies. In addition, this potential link has not been consistently replicated in subsequent studies, which also confirm no increased risk of adverse outcomes for infants of women vaccinated with Tdap during pregnancy. Furthermore, extensive research and ongoing monitoring continue to show that Tdap vaccination during pregnancy is safe and effective for both mother and baby. This is why Tdap remains strongly recommended to protect newborns from pertussis, a potentially deadly disease. These findings are supported by large cohort studies and post-marketing surveillance efforts.
Sidebar on Tdap - There are three things we need to clarify here. One is, if you think you recall hearing something about the Tdap vaccine being dangerous, you're probably thinking about the TDP (Diphtheria, Tetanus, and Pertussis) vaccine scare in the 1980s that led to a decline in vaccination rates and increased cases of whooping cough. This was primarily due to concerns about potential side effects, including neurological problems, and negative media coverage highlighting these concerns. This led to reduction in vaccine uptake and serious fatal outbreaks. Concerns about the side effects of the whole-cell pertussis vaccine also led to the development of acellular vaccines, which have fewer side effects and are now widely used. In response to the vaccine scare and the increased number of lawsuits, Congress passed the NCVIA in 1986, creating a no-fault system to compensate victims of vaccine-related injuries, which I give you all the info on in my episode on the Vaccine Court.
The second thing is that the abbreviations for these vaccines can differ depending on the country. In the U.S. and many places globally, ‘Tdap’ and ‘DTP’ refer to the tetanus, diphtheria, and pertussis vaccines, with DTP containing the whole-cell pertussis component. However, in the UK and some European countries, you might see ‘dTP’ or ‘dT/IPV’ used to describe booster vaccines that contain diphtheria, tetanus, and polio. This can understandably be confusing, but it’s a good reminder to check what’s in the actual vaccine product in your region.
And finally, it’s important to understand the difference between Tdap and DTaP vaccines. These are not just interchangeable abbreviations for the same vaccine. They refer to two distinct vaccines. DTaP is used for babies and young children as part of their primary vaccine series—it has higher doses of the diphtheria and pertussis components to help build strong immunity from the start. On the other hand, Tdap is specifically designed as a booster vaccine for adolescents and adults. It contains lower doses of diphtheria and pertussis antigens, which makes it ideal for safely boosting immunity in older populations, including pregnant individuals. That’s why during pregnancy, it’s the Tdap vaccine that is recommended to protect both the mother and the baby from whooping cough.
Now let's get into what's in the two Tdap vaccines available in the US.
Tdap vaccines are toxoid (for diphtheria, tetanus, and pertussis ) and acellular subunit (pertussis) vaccines.
There are two Tdap vaccines available for pregnant women in the US: Adacel® and Boostrix®. Here are the package inserts:
Antigens:
Adacel® (Sanofi Pasteur) uses tetanus toxoid, diphtheria toxoid, and acellular pertussis antigens (individual pertussis proteins that will elicit an immune response including detoxified pertussis toxin, and three proteins that help pertussis stick to your cells: filamentous hemagglutinin, pertactin, and fimbriae)
Boostrix® (GlaxoSmithKline) contains tetanus toxoid, diphtheria toxoid, and inactivated pertussis toxin and formaldehyde-treated filamentous hemagglutinin and pertactin.
Adjuvants: enhance the immune response and improve the vaccine's effectiveness
Adacel®: aluminum phosphate
Boostrix®: aluminum hydroxide
Antigens are adsorbed onto the adjuvants in these vaccines as is done with the anthrax vaccine, which I talk about in the anthrax episode.
Stabilizers:
Adacel®: 2-phenoxyethanol
Boostrix®: aluminum hydroxide
Delivery Components:
Adacel®: None
Boostrix®: polysorbate 80 (Tween 80) is used as an emulsifier, to keep the ingredients mixed together.
Residuals: Only trace amounts remain after sequential filtration, ultrafiltration, and chromatography are used to purify the vaccine.
Adacel®:
formaldehyde from inactivating the diphtheria and tetanus toxins
glutaraldehyde to inactivate the pertussis toxin
filamentous hemagglutinin and pertactin are treated with formaldehyde
sodium chloride (salt) from salt precipitation during production.
Boostrix®:
formaldehyde from inactivating the diphtheria and tetanus toxins
glutaraldehyde and formaldehyde to inactivate the pertussis toxin.
filamentous hemagglutinin and pertactin are treated with formaldehyde
sodium chloride (salt) from salt precipitation during production
Preservative: Both Adacel® and Boostrix come in single dose vials or syringes and so do not need or contain preservatives.
Now let's move on to the COVID-19 Vaccine: ACOG says you should get the currently recommended COVID-19 vaccine if you are pregnant and not up to date on your COVID-19 vaccines.
COVID-19 is an infectious disease caused by the SARS-CoV-2 virus, which typically spreads between people who are in relatively close contact, through droplets and larger aerosols, and also over a bit longer distances through airborne transmission of smaller aerosols, especially in indoor, crowded, or poorly ventilated environments.
COVID-19 vaccines offer strong protection against severe disease and death. While it’s still possible to become infected with COVID-19 after vaccination, the illness is usually much milder or even asymptomatic in those who have been vaccinated. Multiple studies, including systematic reviews and large cohort studies, have shown that COVID-19 vaccination during pregnancy is safe and effective. They protect moms-to-be and infants from COVID disease. mRNA vaccines, such as Pfizer-BioNTech and Moderna, including the updated formulations targeting Omicron variants, are recommended by the CDC and WHO. Well, before RFKjr that is. The truth is, COVID019 vaccination in pregnant women reduces the risk of severe COVID-19 and also decreases the chance of preterm birth associated with maternal infection. The American College of Obstetricians and Gynecologists (ACOG) says, "If you are pregnant or postpartum, you have a higher risk for more severe illness from COVID-19 than people who are not pregnant. Stay up to date on COVID-19 vaccines." I have an entire episode on mRNA vaccines for COVID that explains how they work and why they are safe.
OK, so the fear mongering and disinformation campaigns against these vaccines get people killed. The antivax movement is a concerted effort by people with a financial stake in the movement. Please listen to, or read, my episode on Andrew Wakefield to understand how it all started and why it's all bullshit. If there were people dying from these vaccines we would have evidence of that. There is none. The evidence is that these vaccines save lives. You know what kills people and causes long-term illness that we are still figuring out the extent of? COVID-19. mRNA vaccine technology was a revolutionary achievement that saved millions of lives. We still had over 300 people dying from COVID each week in April 2025. Call your reps and demand that they fight to keep vaccines, including COVID-19 vaccines, accessible to all Americans. We all need them, but pregnant women are especially vulnerable to severe disease.
My episode on mRNA vaccines for COVID goes into the technology of the mRNA vaccine platform and explains how they work but not what's in the vials, so let's go over that here real quick. The info here is abbreviated so see my What's in Vaccines episode for detailed explanations of the purpose of each component
Pfizer-BioNTech: COMIRNATY (COVID-19 Vaccine, mRNA) is a sterile suspension for intramuscular injection. Here's the package insert:
Each dose contains:
Antigen - the part of the pathogen that elicits an immune response. This one uses messenger RNA (modRNA) encoding the viral spike (S) glycoprotein of SARS-CoV-2 Omicron variant lineage KP.2. This means our bodies will make the protein using the mRNA as a blueprint. The mRNA then degrades as all mRNA does. Our bodies have mRNA naturally, they are the blueprints for protein production. This technology is safe and effective but for the nitty-gritty details check out my mRNA vaccine episode.
Delivery components: these are used to formulate the lipid nanoparticles (LNPs) that protect the fragile mRNA and help deliver it into cells:
Lipids
2-(Polyethylene glycol 2000)-N,N-ditetradecylacetamide: A PEG-lipid conjugate used to stabilize the LNP structure and increase circulation time.
1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC): A phospholipid that helps form the lipid bilayer of the nanoparticle.
Cholesterol: Contributes to the fluidity and stability of the lipid nanoparticle membrane.
Stabilizers:
Tromethamine (Tris) and Tromethamine Hydrochloride (Tris HCl)
Buffers that maintain the pH of the vaccine formulation.
Sucrose
Acts as a cryoprotectant to stabilize the lipid nanoparticles during freezing and thawing.
Preservatives: COMIRNATY does not contain preservatives.
Moderna: SPIKEVAX (COVID-19 Vaccine, mRNA) is a sterile suspension for intramuscular injection. Here's the package insert:
Each dose (2024-2025 Formula) contains:
Antigen: messenger RNA (mRNA) encoding the pre-fusion stabilized Spike glycoprotein (S) of the SARS-CoV-2 Omicron variant lineage KP.2.
Delivery components: these are used to formulate the lipid nanoparticles (LNPs) that protect the fragile mRNA and help deliver it into cells:
Lipids:
SM-102: A proprietary ionizable lipid that binds to the negatively charged mRNA, forming stable LNPs and facilitating cellular uptake.
Cholesterol: Provides membrane stability and flexibility to the lipid nanoparticles.
1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC): A phospholipid that helps form the structural bilayer of the LNP.
PEG2000-DMG (polyethylene glycol 2000-dimyristoyl glycerol): A PEG-lipid that stabilizes the LNPs and extends their circulation time in the body.
Stabilizers
Tromethamine (Tris): A buffer to maintain an optimal pH.
Tromethamine hydrochloride (Tris HCl): Provides additional pH stabilization.
Acetic acid and Sodium acetate: These work together as a buffer system to help control pH.
Sucrose: Acts as a cryoprotectant, preventing damage during freezing and thawing.
Preservatives: SPIKEVAX does not contain a preservative.
All right, so let's move on to RSV Maternal Vaccine: RSV is a seasonal virus that can be dangerous for babies and young children and ACOG recommends this vaccine if you are 32 to 36 weeks pregnant, from September to January.
Respiratory syncytial virus, or RSV, is the leading cause of acute lower respiratory tract illness in infants under six months of age and remains a significant cause of death in this age group, especially in low- and middle-income countries. Severe RSV disease tends to peak during the first two to three months of life, even though babies are born with some maternal antibodies. RSV circulates in winter in temperate climates and during rainy seasons in tropical regions, although these patterns shifted somewhat during the COVID-19 pandemic.
In Europe, a 2023 study found that RSV was responsible for about half of hospitalizations due to respiratory illness in children under one year old, with the majority of these cases occurring in infants younger than three months.
As with the other infectious disease we've talked about today, an important way to protect these vulnerable infants is through maternal vaccination. By vaccinating pregnant people, higher levels of maternal antibodies can be transferred across the placenta, providing protection for babies in their first months of life.
The RSV maternal vaccine Abrysvo™ was approved for use as a maternal vaccine in the US in 2023 as well as in the EU, Canada, Japan, Australia, the UK, Iceland, Liechtenstein, and Norway. This is the only RSV vaccine approved for pregnant individuals at 32 (28 wks in some countries) through 36 weeks of gestation to protect infants from birth up to 6 months of age. Clinical trials show that this vaccine, when given during pregnancy, significantly reduces the risk of RSV-associated lower respiratory tract infections in infants in the first six months of life. In fact, to the right is a summary from the clinical trial linked above.
ABRYSVO (Pfizer) is a sterile solution for intramuscular injection. Here's the package insert:
Each dose contains:
Antigen:
RSV stabilized prefusion F proteins: preF A and preF B.
Stabilizers:
Tromethamine (Tris): help maintain pH of the vaccine formulation.
Tromethamine Hydrochloride (Tris HCl): help maintain pH of the vaccine formulation.
Sucrose: helps protect vaccine formulation during freezing or temperature fluctuations.
Mannitol: provides additional structural stability to the vaccine and can also prevent aggregation of proteins during manufacturing, freeze-drying, or storage.
Polysorbate 80 (Tween 80): surfactant and stabilizer that prevents protein aggregation and reduces surface tension, helping maintain uniformity of the vaccine components in the vial.
Sodium Chloride: Tonicity adjuster that helps ensure that the vaccine has an osmolarity similar to that of human cells, minimizing irritation and promoting safe delivery into the body.
Preservatives: ABRYSVO contains no preservatives.
Residuals:
During manufacturing, trace amounts of host cell proteins (≤0.1% w/w) and DNA (<0.4 ng/mg of total protein) may remain.
These residuals are present at very, very low levels and do not impact health in any way.
Vaccines that are safe during pregnancy and recommended on a case-by-case basis: ACOG also recommends talking to your OBGYN about whether or not you need hepatitis, pneumonia, or certain meningitis vaccines. Talk with your doctor about the vaccines you've had in the past. They may recommend these vaccines based on your medical history and occupation.
Hepatitis B vaccination is safe and highly effective — a crucial step in protecting both mothers and babies from this serious liver infection. All pregnant people who are at risk for hepatitis B infection and haven’t been vaccinated should start the vaccine series as soon as possible during pregnancy. CDC says: For pregnant women, after initial hepatitis B surface antigen (HBsAg) screen is drawn for current pregnancy, initiate vaccine series with Engerix-B, Recombivax HB, or Twinrix for those who have not previously been vaccinated. For newborns, it’s standard practice to begin hepatitis B vaccination right at birth, providing essential protection from day one. The two single-antigen vaccines, Engerix-B and Recombivax-HB, can be used starting at birth.
I'm going to go into less detail for these vaccines but the package inserts are here for you. Check out Section 11, Description, in the insert for information on what's in the vaccines. Hepatitis B vaccines come in two main forms: a single-antigen formulation and a combination formulation that includes protection against other diseases. Regardless of the formulation, all hepatitis B vaccines contain a small amount of yeast protein and an aluminum adjuvant, which helps boost the body’s immune response to the vaccine. For more info on aluminum as an adjuvant, listen to my episode on What's in Vaccines. Here are the package inserts:
Pneumococcal vaccines help protect against Streptococcus pneumoniae, a bacteria that can cause serious infections like pneumonia, bloodstream infections, and meningitis. While these vaccines aren’t routinely recommended for all pregnant people, they’re considered safe and may be recommended in certain situations — such as for pregnant people with underlying health conditions like diabetes, heart disease, or weakened immune systems. Some more info on maternal pneumococcal immunization:
Pneumococcal vaccines come in two forms: the polysaccharide vaccine (Pneumovax 23, AKA PPSV23) and the conjugate vaccine (Prevnar 13, AKA PCV13). For those who need protection during pregnancy, these vaccines can be safely administered based on individual risk factors. As always, the best approach is to talk with your healthcare provider about whether you need this vaccine during pregnancy to stay protected and healthy. Here are the package inserts. Again check Section 11, Description, for what's in it.
Meningococcal vaccines protect against Neisseria meningitidis, a dangerous bacteria that can cause life-threatening infections like meningitis and sepsis. Meningococcal vaccination isn’t recommended routinely during pregnancy, but it is considered safe if it’s needed. For example, pregnant people who are at increased risk — such as during an outbreak or if they’re traveling to areas where meningococcal disease is more common — may be advised to receive the vaccine. For all the details on the disease and exactly what is in the vaccines, see my episode on Invasive meningococcal Disease.
Two main types of meningococcal vaccines are available and they protect against different serogroups of meningococcal bacteria: the MenACWY conjugate vaccine and the MenB vaccine. In pregnancy, MenACWY is generally the one used if needed. Ultimately, decisions about meningococcal vaccination during pregnancy should be made in partnership with your healthcare provider, who can assess your individual risks and ensure the best protection for you and your baby.
MenACWY conjugate vaccines:
MenB vaccines:
NOTE: While there are several causes of bacterial meningitis, vaccines that can be safely used during pregnancy mainly cover meningococcal disease and pneumococcal disease. The Hib vaccine isn’t routinely used in adults, including during pregnancy, because most adults are already protected from childhood vaccination and the disease is rare in healthy adults. No other vaccines specifically target meningitis-causing bacteria in pregnancy.
Not mentioned by ACOG but one that is given during pregnancy if really needed is the anthrax vaccine. The anthrax vaccine adsorbed (AVA, BioThrax®) is an inactivated bacterial vaccine and has been used in military settings. The CDC advises that pregnant people at high risk of exposure (e.g., laboratory workers, certain military personnel) can receive the vaccine if the benefits outweigh the theoretical risks. Limited data, including studies from military populations, show that maternal vaccination with AVA causes no increase in adverse pregnancy outcomes. However, routine anthrax vaccination should be deferred during pregnancy unless there is a clear occupational or environmental risk, just to err on the side of caution. Check out my episode on anthrax!
Vaccines You're Not Supposed to Get While Pregnant
OK, so those are the vaccines recommended during pregnancy. Now let's talk about those that are not recommended during pregnancy and why that is. Live vaccines, which contain weakened forms of viruses or bacteria, are generally not given during pregnancy because there’s a theoretical risk that the weakened organisms could cross the placenta and infect the developing baby. While this risk is largely theoretical and hasn’t been shown to cause harm in many cases of accidental vaccination, it’s still safer to avoid live vaccines during pregnancy. Inactivated and subunit vaccines, by contrast, do not contain live organisms and are generally considered safe for pregnant people. but a few, like the HPV vaccine, aren’t recommended during pregnancy — not because they’re unsafe, but because there isn’t enough data to support routine use, and they can be safely given after pregnancy instead.
To make it easy, here’s a list of vaccines that are contraindicated or not given during pregnancy because they are live-attenuated vaccines:
🚫 Vaccines Contraindicated During Pregnancy (Live-Attenuated Vaccines)
✅Measles, Mumps, Rubella (MMR)
✅ Varicella (chickenpox) vaccine
✅ Live-attenuated influenza vaccine (LAIV, FluMist nasal spray)
✅ Oral polio vaccine (OPV) (no longer used in the U.S. but still relevant in some countries)
✅ BCG vaccine (for tuberculosis)
✅ Typhoid vaccine (oral live-attenuated Ty21a)
✅ Yellow fever vaccine (use only if travel or outbreak risk outweighs theoretical risk; otherwise deferred)
✅ Smallpox vaccine (vaccinia) (only used in very high-risk scenarios, like lab workers or outbreak response)
And here’s a list of non-live vaccines for which routine use is not recommended in pregnancy:
⚠️ Non-Live Vaccines Not Routinely Given During Pregnancy
✅ Human papillomavirus (HPV) vaccine
✅ Anthrax vaccine (used only if there’s high exposure risk)
✅ Japanese encephalitis vaccine (used only if travel or outbreak risk outweighs risk of deferral)
✅ Cholera (inactivated form) and inactivated typhoid vaccine (not typically recommended unless travel/outbreak risk outweighs deferral)
But let's talk more in-depth about two that are avoided during pregnancy that I've gotten questions about, MMR and polio.
Measles, Mumps, and Rubella (MMR) Vaccine: The MMR vaccine protects against measles, mumps, and rubella — three highly contagious diseases that can lead to severe complications:
Measles: Can cause pneumonia, brain swelling (encephalitis), and even death.
Mumps: Can lead to deafness, painful swelling of the testicles or ovaries, and brain inflammation.
Rubella (German measles): Particularly dangerous in pregnancy, causing miscarriage, stillbirth, or birth defects (congenital rubella syndrome).
And as we are seeing in real time, vaccination is important to prevents outbreaks of measles and mumps, which are highly contagious. Right now we have terrible outbreaks of measles in the US, Canada, Mexico, and elsewhere. In the US we have had 1,046 cases of measles and 3 deaths so far this year. That's almost four times as many cases as in all of 2024 (285) and it's only May.
So, the MMR vaccine is a live attenuated vaccine and is generally contraindicated during pregnancy. So let's talk about why. Well, the theoretical risk of fetal infection has led to the longstanding guidance to avoid live vaccines in pregnancy (ACOG, 2023). However, accidental administration of MMR during early pregnancy has been documented and has not been linked to increased rates of congenital anomalies or adverse outcomes, according to decades of surveillance data. Despite this reassuring data, MMR vaccination is recommended to be given at least one month prior to conception to ensure optimal safety. This is really because of the theoretical risk of rubella. Rubella itself can cause congenital rubella syndrome if a pregnant person is infected. However, the vaccine strain rubella virus has never been shown to cause congenital rubella syndrome. Ever.
So, if someone is inadvertently vaccinated during early pregnancy, the reassuring data indicate that there’s no need for intervention — just routine prenatal care. Obviously talk to your doctor to about your specific situation but there is no evidence this vaccine will cause problems if given in pregnancy.
And I know you're wondering... how does one accidentally give the vaccine to a pregnant woman? Well, it's because she doesn't know she's pregnant when she's vaccinated. Not because a nurse accidentally jabbed a known pregnant person with MMR.
OK, so if you miss the one-month-before-conception vaccine window that is recommended, women who are not immune to rubella or measles are advised to be vaccinated postpartum to reduce the risk of future infections in subsequent pregnancies. This is even more important now with measles outbreaks resurging. Get vaccinated or get boosted! To understand fully why we need to be protected against measles and everything you need to know about the vaccines, check out my measles episode. I give you all the tea!
Polio Vaccine: Poliomyelitis, or polio as we know it, is an extremely contagious viral disease that can cause lasting paralysis. Thanks to widespread vaccination campaigns, the global incidence of polio has dropped dramatically, pushing the disease closer to eradication. In many regions, the oral polio vaccine is still critical because it’s affordable and easy to deliver, though it carries a very rare risk of paralysis. By contrast, in countries with good sanitation infrastructure, such as the United States, the inactivated polio vaccine is used exclusively to avoid the risk of vaccine-derived poliovirus.
Routine polio vaccination is not recommended during pregnancy in most high-income countries because of the low risk of exposure. However, inactivated polio vaccine (IPV) may be considered for pregnant individuals traveling to areas with ongoing polio transmission or at increased risk of exposure (CDC; WHO). IPV is an inactivated vaccine and does not contain live virus, making it safe for use if needed during pregnancy (Haber et al., 2019). The live oral polio vaccine (OPV) is generally avoided in pregnancy due to the theoretical risk of fetal transmission, though inadvertent administration has not been shown to cause harm.
Polio Reversion Sidebar: The oral polio vaccine, or OPV, uses a live, weakened virus that’s excellent at stopping transmission in communities but carries a very rare risk of causing polio through viral reversion. What the heck is viral reversion?
Well, viral reversion is when a weakened virus in a vaccine mutates back to a form that can cause disease. The attenuated poliovirus used in OPV still replicates in the gut of vaccinated individuals. During replication, the virus undergoes random genetic changes. If certain mutations occur, the virus can regain its neurovirulence (ability to invade the nervous system and cause paralysis). This is a rare but real risk with the oral polio vaccine because the weakened poliovirus has a high mutation rate since it doesn’t proofread its RNA. This allows rapid accumulation of mutations during replication in the gut.
So why does reversion to a disease-causing form happen with polio but is not a concern with other live vaccines like MMR or varicella? You might ask?
Well, other live vaccines (e.g., MMR, varicella, rotavirus) use attenuated viruses that have been passaged in cells to be less likely to revert. Passaging a virus, in case you don't remember from some other episode I can't remember right now, is when you grow viruses repeatedly, and sequentially, in lab cell cultures. So, you infect a culture dish with some virus and let it grow and grow for a few days then you remove the virus and put it into a new dish of cells. Each time you remove the virus from one dish and put it into a new dish is called a passage. And you repeat that over and over. This forces the virus to adapt to the lab environment rather than the human body. These adaptations often reduce the virus’s ability to cause disease in people, and they also make it harder for the virus to revert to a dangerous form. It’s a deliberate, science-based strategy that’s been used to safely develop live vaccines like MMR and polio
Say thank you to the virologists working hard for you.
OK, so these cell culture-attenuated vaccine viruses either don’t replicate in the gut (measles or rubella vaccines) or don’t have the same risk of neurovirulence (varicella or rotavirus vaccines).
Many also have large, more complex genomes with lower mutation rates than poliovirus. But the key difference is poliovirus’s small genome and lack of proofreading during replication. Those create a perfect storm where reversion is more likely compared to other live attenuated vaccine viruses.
So if the oral polio vaccine has this risk of reversion that can actually give someone polio and cause paralysis, even if it's rare, why is it still used? Well, it's still used in areas with ongoing polio circulation and poor sanitation because it’s easier to administer and protects entire communities better than the inactivated polio vaccine. And THAT's because of how polio spreads. You see, poliovirus is spread primarily via the fecal-oral route. And yes that means exactly what you think it means. Fecal-oral. Poop to mouth. Yum. So, in places with poor sanitation, there's more poop particles everywhere. There just are. More poop means more virus can circulate in the community (e.g., through contaminated water or surfaces). In those settings, interrupting person-to-person transmission is essential to stopping outbreaks.
And this is where the oral polio vaccine really shines... or polishes the turd you could say. The oral polio vaccine (OPV) is swallowed and so the live, weakened virus replicates in the intestines, unlike the inactivated polio vaccine which is injected, and so does not go to the gut and even if it did go to the gut it's inactivated so it can't replicate. This means the oral polio vaccine virus, replicating in the gut, generates strong intestinal (mucosal) immunity, which is critical to blocking the virus from being shed in feces. In places with poor sanitation, reducing the amount of virus in feces, reduces community spread. This is important! Important enough that the high risk of catching polio and getting seriously ill, is such a greater risk than vaccine reversion, that they still use the oral vaccine and it saves millions of lives.
By contrast, the inactivated polio vaccine, or IPOL, uses killed virus, so it can’t cause infections.
It's also given by injection and so does not replicate in the gut. It provides excellent protection against paralysis (systemic immunity) but less gut-level immunity. So, while IPV prevents disease in the vaccinated individual, it’s less effective at stopping virus shedding and spread. That’s why IPV is the standard in countries with strong sanitation systems and no ongoing polio transmission.
IMPORTANT NOTE: Here is an example of a vaccine that has a very real, although very rare, risk of causing disease resulting in paralysis - a series adverse event. We are transparent about this. We don't hide this fact. We aren't hiding similar risks with other vaccines. We really aren't. The data we have for decades and decades of using vaccines is that they are safe and when we find one that has risks, we document it and talk about it. We weigh the risks vs benefits. There's no conspiracy here guys. Just antivax headliners with a financial stake in their disinformation and a eugenics-leaning agenda. Make no mistake, RFKjr and his kids and grandkids are fully and safely vaccinated. He's admitted as much in interviews. No matter who they take vaccines from, the wealthy and powerful will retain full access to vaccines and healthcare.
Now we've talked about the vaccines you should and shouldn't get while pregnant. And it's important to remember that for all the recommended vaccines I include links to studies showing they are safe and important in protecting moms and babies. We also have ongoing, continuous monitoring of the safety of these and all vaccines. Safety is continually monitored through systems like VAERS (Vaccine Adverse Event Reporting System), V-safe, and international pregnancy registries. These systems ensure that any safety signals are rapidly detected and investigated.
Common Myths
Let’s tackle some of the most common misconceptions in light of what we now know:
“I’ll wait until after birth to get vaccinated—it’s safer.”
A: Actually, it’s safer to get vaccinated during pregnancy. That’s when you’re most at risk for complications, and it’s also how you protect your baby in those critical early months.
“I’m worried about fertility or miscarriage risks.”
A: Multiple studies have shown no link between these vaccines and miscarriage or infertility. These claims have been thoroughly investigated and debunked by the CDC, ACOG, and major peer-reviewed studies. The statement from ACOG:
“As experts in reproductive health, we continue to recommend that the vaccine be available to pregnant individuals. We also assure patients that there is no evidence that the vaccine can lead to loss of fertility. While fertility was not specifically studied in the clinical trials of the vaccine, no loss of fertility has been reported among trial participants or among the millions who have received the vaccines since their authorization, and no signs of infertility appeared in animal studies. Loss of fertility is scientifically unlikely.”
“I already had COVID, so I don’t need the vaccine during pregnancy.”
A: Natural infection does provide some protection, but vaccination on top of prior infection (hybrid immunity) is even more effective at reducing severe outcomes, especially during pregnancy
Global & Health Equity Perspective
Unfortunately, access to vaccines during pregnancy is not equal worldwide. Barriers include healthcare infrastructure, vaccine availability, and cultural or linguistic differences. Efforts to build trust and provide culturally sensitive information are vital. Studies have shown that when healthcare providers communicate clearly and compassionately, vaccine acceptance improves. We must support these efforts globally and locally. We must also vote out antivax science deniers who insist on taking away our access to vaccines while they remain safely vaccinated.
So, guys, to wrap up:
Vaccines like flu, Tdap, RSV, and COVID-19 have robust safety data in pregnancy.
These vaccines protect both the pregnant person and the baby.
Misinformation can cause real harm, so rely on evidence-based sources.
Talk to a trusted healthcare provider — they’re your best ally in making these decisions.
If you found this episode helpful, please share it with friends, family, or colleagues who are pregnant or planning to be. Let’s work together to combat misinformation with kindness and facts.
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