The flu is a contagious disease caused by the influenza virus. In most healthy adults, flu symptoms are typically mild and resolve within about a week. However, during pregnancy, the body’s heightened inflammatory response can increase the risk of complications. To reduce these risks and provide passive immunity to babies, healthcare professionals recommend that pregnant women receive the flu vaccine during flu season. Deciding whether to get vaccinated during pregnancy is a challenging task, as it involves weighing all of the potential risks and benefits. Additionally, you have choices regarding the type of flu vaccine and the timing of administration. This episode reviews the research and considerations for making an informed decision about the flu vaccine during pregnancy.

Listen Now

This episode is made possible with support from our sponsors. I appreciate your support for the brands that help power this podcast.

Article and Resources

Vaccine Basics

Vaccines have been in use since the 1790s, beginning with the development of the smallpox vaccine. Let’s first cover some basics to better understand how vaccines work, their risks, and their benefits.

What is a Vaccine?

Traditionally, vaccines use a weakened or inactivated version of a disease-causing microorganism or its components, such as toxins or surface proteins. When given, typically through an injection, vaccines trigger your immune system to produce antibodies against the disease without causing illness. Essentially, the vaccine teaches your body to recognize and fight the disease if you encounter it in the future. In some cases, immunity lasts a lifetime. In others, booster shots may be necessary to maintain protection.

Types of Vaccines

Different types of vaccines employ various methods to stimulate the immune system. Inactivated vaccines, like those for the flu, use microorganisms that have been killed through heat, radiation, or chemicals. There are also attenuated vaccines, such as the live attenuated nasal spray for the flu, which contain live but weakened microorganisms. These are strong enough to trigger an immune response but not enough to cause illness. Toxoid vaccines, like Tdap, use inactivated toxins from bacteria. Subunit vaccines, like the HPV vaccine, utilize specific proteins or fragments of the pathogen to stimulate an immune response. mRNA vaccines, a relatively new technology used in COVID-19 vaccines, work differently. Instead of using part of the virus or bacteria, these vaccines instruct your cells to make a protein that triggers an immune response. This teaches your immune system to recognize and fight the virus if you are exposed to it in the future.

Vaccine Ingredients

Vaccines often contain additional ingredients to ensure safety and effectiveness. Adjuvants, such as aluminum salts, help boost the immune response to the vaccine. Stabilizers maintain the vaccine’s potency during storage and transport. Preservatives in multidose vials prevent contamination. Thimerosal, a mercury-containing preservative, is in some multidose vaccines. Lipid nanoparticles (tiny fat particles) are in mRNA vaccines to protect the fragile mRNA and help it enter your cells. Other vaccines may contain trace amounts of antibiotics to prevent bacterial growth, as well as formaldehyde to inactivate toxins or kill viruses and bacteria during the production process. If the virus grows in eggs, as is the case for certain flu vaccines, the final product contains traces of egg protein.

How Immunity Works

Individual immunity occurs when a single person develops immunity after receiving a vaccine. Passive immunity happens when a pregnant mother passes antibodies to her baby through the placenta or later through breast milk, providing the baby with temporary protection. The passive immunity a baby receives in utero is a significant driver of recommending vaccines during pregnancy. Herd immunity occurs when a significant portion of the population is vaccinated, thereby reducing the spread of disease. Herd immunity is crucial to protect individuals who cannot get vaccinated, such as those with compromised immune systems, allergies to vaccine ingredients, or babies too young to be vaccinated.

Evaluating Vaccines

I follow a systematic approach and conduct thorough research when evaluating any vaccine. I ask several key questions:

• What are the risks of the illness the vaccine is targeting?
• What is the history of the vaccine?
• What ingredients are in the vaccine?
• How effective is it?
• What are the risks and benefits?

In addition to these general questions, I also consider personal factors such as my current health status, the likelihood of exposure, where I live, places I may travel to, and the time of year. Answering these questions requires a significant amount of research and critical thinking.

The Controversy Surrounding Vaccines

Some professionals dedicate their careers to researching and studying the safety and efficacy of vaccines. Vaccines are a complex, controversial topic that many people feel very strongly about. As a result, researching vaccines is a challenging endeavor. It is easy to come across information that appears unbiased but may ultimately push a pro- or anti-vaccine agenda. This episode aims to provide an objective overview of the evidence, enabling you to make an informed decision.

The Role of the CDC and the WHO

In recent years, concerns have grown about the reliability of information from organizations such as the CDC and WHO, as well as the potential influence of pharmaceutical companies on these institutions. However, it’s important to note that the CDC and WHO maintain some of the most extensive centralized databases for disease statistics.

These organizations focus on protecting public health on a national and global scale. Their guidelines influence recommendations by bodies like the American College of Obstetricians and Gynecologists (ACOG), which, in turn, impact your healthcare provider’s advice. Understanding the information that informs your doctor’s or midwife’s recommendations will equip you to have an educated conversation with them and help you make the best decision for you and your baby.

What is the Flu?  

Flu is short for influenza. This is an infectious disease caused by the influenza virus. Like any virus, the flu replicates in living cells. The flu is highly contagious because it can be transmitted through the air and on surfaces. It becomes airborne when someone infected with the virus coughs or sneezes, releasing half a million particles into the air. You can also contract the flu virus by touching a contaminated surface and then touching your mouth, nose, or eyes. The flu virus can live 1-2 days on hard, nonporous surfaces, like a doorknob, 15 minutes on dry paper tissues, like a dollar bill, and about 5 minutes on the skin. An easy way to minimize your risk is to wash your hands frequently and avoid touching your face.

Flu symptoms include fever, runny nose, sore throat, muscle pains, headache, coughing, and fatigue. The sudden onset of symptoms and exhaustion with the flu can help you distinguish it from a cold. Symptoms generally appear two days after infection and last about a week. A lingering cough may last longer. Complications of the flu can include pneumonia and sinus infections.

The flu usually isn’t a big deal if you are a generally healthy adult. You are much more likely to have complications if you are very young, elderly, or have a weakened immune system. Pregnancy increases your risk of complications from the flu. While most cases without complications resolve on their own, there are a couple of prescription antiviral drugs available to treat the flu.

Types of Flu  

Three types of flu affect humans: A, B, and C. A is the most serious in humans and is commonly carried by birds. All the major flu pandemics documented in our history have been type A. Type B is less common and mutates approximately 2-3 times slower. The only animals it affects are seals and ferrets, and it almost exclusively affects humans. We rarely have epidemics of type B. Type C affects humans, dogs, and pigs, is the least common, and tends to be the mildest. Vaccines usually include types A and B and do not have type C.

Within the various types of flu, there are subtypes. You will see the letters ‘H’ and ‘N’ followed by a number. These Hs and Ns are protein spikes on the virus’s surface that help the virus invade cells. The H stands for hemagglutinin, of which there are 16 types, and the N stands for neuraminidase, of which there are nine.

How the Flu Virus Mutates  

An excellent example of how these viruses mutate I’m borrowing from an NPR article that explains this in straightforward terms. Imagine a bird gets infected with two viruses: H1N1 and H2N2. When the two viruses reproduce inside the bird’s cells, H1N1 can grab accessories from H2N2. This creates a new strain that looks like H2N2 but can now infect people (because it started as just H1N1). This article specifically discusses the 2009 flu pandemic. The virus contained segments from H3N2 that had circulated in humans since the 1968 pandemic. It had genes from birds, an H1 from pigs, and an N1 from a completely different virus. All of these are then mixed in pigs. You can see this gets messy and complicated very quickly.

How the Flu Affects You During Pregnancy  

Influenza causes your body to release large amounts of cytokines and chemokines. These are cells produced by your immune system. This causes inflammation, characterized by redness, swelling, pain, or heat, resulting in symptoms such as sore muscles and fever. The positive aspect of inflammation is that it triggers an immune response to combat the underlying cause of the inflammation. A study by Stanford University found that pregnant women have an unusually strong immune response to the influenza virus. This may explain why pregnant women get sicker from the flu than healthy adults. Physiological changes such as immunological changes, increased cardiac output, oxygen consumption, and lung tidal volume might increase the susceptibility to influenza complications if infection occurs during pregnancy.

The Flu and Your Baby

A significant driver for pregnant mothers to get vaccinated is to transfer immunity to protect their babies. The CDC recommends that babies older than six months receive a flu shot every year. According to the CDC, between 6,000 and 25,000 children younger than five years are hospitalized annually because of influenza complications. Since the 2004-2005 flu season, flu-related deaths in children reported to the CDC during regular flu seasons have ranged from a low of 37 to a high of 199 deaths per year. During the 2023-2024 flu season, 70 percent of children 0 to 17 years old hospitalized with flu had at least one underlying health condition, such as asthma, neurologic disease, obesity, or cardiovascular disease.

The Flu Vaccine  

The influenza virus is a very rapidly evolving virus. For this reason, we cannot create a one-time vaccine. Every year, the flu vaccine is formulated to protect against 3 or 4 viruses. Vaccine virus selection has been conducted by the WHO Global Influenza Surveillance and Response System (GISRS) since 1973. The World Health Organization predicts which flu strains will be the most common the following year. Once the WHO chooses specific strains, it takes six months to produce the vaccine.

Flu Season  

There are two vaccines each year because of two flu seasons: one during winter in the northern hemisphere and one during winter in the southern hemisphere. Flu season tends to be during winter, partly due to the lack of humidity and low UV light. During the winter, more people are indoors, making it easier for the flu virus to spread. Additionally, the virus persists longer on surfaces at lower temperatures. There is a theory that lower vitamin D levels may contribute to the spread of this virus during the winter months.  I won’t examine this in-depth here, and if you’d like to delve deeper into vitamin D, you can listen to this episode.

How the Flu Vaccine Works  

Historically, vaccines all work on a similar principle. They introduce some part of a bacteria, virus, or pathogen into your body to trigger an immune response. Your body produces antigens to fight off bacteria, viruses, or pathogens and then recognizes and fights them off in the future. Ultimately, the goal is to become immune, which is why we also refer to vaccines as immunizations.

Once you receive the flu vaccine, it takes your body approximately two weeks to develop immunity through the production of antibodies. When you are pregnant, you can pass antibodies to your baby through the placenta, thereby creating passive immunity for your baby. Remember that your baby cannot get a flu vaccine until six months old. This is one of the reasons healthcare providers recommend this vaccine during pregnancy.

After your baby is born, you continue to pass antibodies to them through your breast milk. I cannot stress enough the importance of breastfeeding, especially during the first few months of your baby’s life. Even if your baby is getting formula and every meal isn’t breast milk, those antibodies you pass when you breastfeed are amazing. Breastfeeding will significantly improve your newborn’s immune system and help to keep them healthier.

When to Get the Flu Vaccine   

The Centers for Disease Control states you can get a flu shot during any trimester. One study showed that the influenza vaccination at any time during the second and third trimester of pregnancy, but at least 15 days before delivery, creates passive immunity. Most vaccine research excludes women in the first trimester because this is a critical development period, and the period in which miscarriage is the highest. For these reasons, some expecting mothers and care providers may avoid a flu shot during the first trimester.

Combining Flu and Other Vaccines  

An additional consideration regarding the timing of the flu vaccine is whether to get it simultaneously with other vaccines. According to the American College of Obstetricians and Gynecologists, if the timing of the influenza vaccine aligns with other inactivated or non-virus-containing vaccines recommended in pregnancy, such as the Tdap, RSV, or COVID-19 vaccine, it is safe and effective to administer these vaccines during the same visit.

In May 2025, Robert F. Kennedy, Jr., the Secretary of Health and Human Services, announced that the CDC would no longer recommend the COVID-19 vaccine for healthy children and pregnant women. As a result, the CDC recommends vaccination based on individual-based decision-making, with an emphasis that the risk-benefit of vaccination is most favorable for individuals who are at an increased risk for severe COVID-19 disease and lowest for individuals who are not at an increased risk. Although the CDC changed its position, ACOG did not and continues to recommend COVID-19 vaccines during pregnancy.

In one of the clinical trials disclosed in the Adacel Tdap vaccine package insert, they found that when the Adacel vaccine was administered simultaneously as a trivalent inactivated influenza vaccine, a lower antibody response was observed for pertactin antigen compared to when the Adacel vaccine was administered alone. The pertactin antigen helps you develop immunity to pertussis. If you plan to receive Tdap and flu shots during pregnancy, discuss the timing with your a doctor or midwife. However, they will likely align with ACOG in recommending you get them during the same visit.

Types of Flu Vaccines 

Some vaccines, like the Tdap, are relatively easy to compare manufacturers because there are only two. This is much more challenging with the flu because nine vaccines are available for the 2025-26 flu season. You can compare the vaccine options on the CDC website, and we will provide more details about comparing your options.

Ingredients in Flu Vaccines   

The ingredients of flu vaccines vary depending on the type and brand. They can contain trace amounts of egg. Some vaccines contain formaldehyde in growth media and processing or in the final product. The amount of formaldehyde is much less than you naturally contain in your blood. Vaccines may also contain gelatin derived from animal collagen, which helps stabilize them during storage and shipping.

Egg-Based Vaccines  

Egg-based vaccines have been around for 70 years. To make these, a lab partner of the WHO (or the CDC in the U.S.) provides candidate vaccine viruses grown in eggs. The viruses are sent to private companies, which inject them into fertilized chicken eggs, allowing them to multiply. They remove the liquid from the eggs containing the virus and then kill or inactivate the virus . There is a concern about this type of vaccine if you have an egg allergy. The final vaccine contains such trace amounts that some people with allergies experience no issues. There are other options; if you have an egg allergy, please discuss it with your healthcare provider.

Cell-Based Vaccines  

Cell-based vaccines were approved in 2012. These originally began in eggs and are now entirely egg-free. Cell-based vaccines use Madin Darby Canine Kidney (MDCK-kidney cells from dogs). The benefit of cell-based vaccines is that manufacturers have more control over mutations and, hopefully, a more effective vaccine. Manufacturers can produce cell-based vaccines faster in an epidemic scenario because they do not rely on a large supply of eggs. Hundreds of thousands of eggs are used each year in flu vaccine production.

Recombinant Vaccines  

Recombinant flu vaccines were approved in 2013. These are made using insect cells and are 100% egg-free. The product contains 45 micrograms (mcg) of hemagglutinin for each of the targeted flu strains, three times what most other flu vaccines contain. The only recombinant vaccine approved for the 2025-26 season is Flublok.

If you are vegan, it is complicated. There is no vegan flu shot, which could be a consideration if you are uncomfortable with animal-derived ingredients.

Mercury in Flu Vaccines

Flu vaccines come in two forms. A single-dose vial containing enough for one dose for one person, and a multidose vial containing multiple doses for multiple people. Each vaccination uses a new sterile needle, but multidose vaccines contain Thimerosal (mercury) as an additional preservative to prevent bacteria and fungi from growing. There have long been concerns about mercury in vaccines, manufacturers no longer use this additive except in influenza multi-dose vaccines.

In June 2025, the Advisory Committee on Immunization Practices (ACIP) made a new recommendation that children aged ≤18 years, pregnant women, and all adults receive seasonal influenza vaccines only in single-dose formulations that are free of thimerosal as a preservative. For the 2025-26 season, all available flu vaccines in the United States should only be available in single-dose vials and should not contain thimerosal. That may vary in other countries. For example, Canada still uses multidose vials and those do contain thimerosal. You can always request a single-dose vaccine if you would like to avoid thimerosal.

Flu Vaccines for the 2025-26 Flu Season  

For the 2025-26 flu season, all vaccines are trivalent (3-component) and contain two A and one B strains. Two of this season’s nine vaccine options are only for individuals 65 and older. One is a nasal spray that is a live-attenuated vaccine. The CDC does not recommend the nasal spray vaccine during pregnancy. There are six vaccine options you may receive during pregnancy. Below is the list, accompanied by a link to all the filings with the FDA, including the package insert, which contains in-depth information about each vaccine.

• Afluria (manufactured by Seqirus)
• Fluarix (manufactured by GlaxoSmithKline)
• FluLaval (manufactured by GlaxoSmithKline)
• Fluzone (manufactured by Sanofi Pasteur)
• Flucelvax (manufactured by Seqirus) 
• Flublok (manufactured by Sanofi Pasteur)

Research on the Safety of Vaccines During Pregnancy

Pregnancy adds an additional layer of caution for any intervention. A considerable amount of research is available on the safety and risks associated with the flu vaccine during pregnancy. Let’s examine some of these studies.

Inflammatory response 

A study shows that the trivalent influenza virus vaccination elicits a measurable inflammatory response among pregnant women. While the strains of the flu change annually, most of the other ingredients remain the same, and research on vaccine formulations from previous years is still applicable. Interestingly, the flu or the flu vaccine triggers an inflammatory response, and preeclampsia and preterm birth also have an inflammatory component. When you are pregnant, it is best to limit inflammation whenever possible. This study noted that the inflammatory response elicited by vaccination is substantially milder and more transient than that of infectious illness, suggesting the clinical value of vaccination. However, further research is needed to confirm that the mild inflammatory response elicited by vaccination is benign in pregnancy.

Some animal studies show that maternal immune activation during pregnancy can harm the brain development of offspring. Some animal studies involving rodents and monkeys may raise questions about how a maternal immune response affects a developing fetus in utero. A study on mice has shown that maternal immune activation affects the brain development of the fetus. Ultimately, a flu shot creates an immune response, just as the flu does. The bottom line is that there is a lot we do not know about immune reactions during pregnancy and how they potentially affect a baby in utero.

The data we have on humans reflects decades of research showing the safety of the flu vaccine during pregnancy. A systematic review examined 40 studies and confirmed the safety of the flu vaccine in pregnancy. They also found evidence to support a protective effect against preterm birth and low birth weight. The researchers looked at specific outcomes, including preterm birth, low birth weight, small for gestational age newborns, congenital abnormalities, spontaneous abortion, and stillbirth.

The vast majority of studies on humans regarding the safety and effectiveness of the flu vaccine are observational studies. These studies only monitor outcomes without requiring any group to receive an intervention or placebo. There has been criticism that observational studies overestimate vaccine effectiveness and safety due to the healthy-vaccinee bias. This concept suggests that individuals in better health are more likely to receive the flu vaccine. Despite researchers calling for more randomized controlled trials, most studies rely on observational data and may reflect a healthy-vaccinee bias.

Flu Vaccine and Miscarriage   

One study made headlines regarding the flu vaccine and miscarriage. I want to delve beyond the clickbait headlines and discuss what this study revealed and what it may mean for you. The study concluded that first-trimester miscarriage was associated with the flu vaccine when the vaccine was given in the preceding 28 days. The association was significant only among women who had been vaccinated in the previous influenza season. This study examined women who received the 2010-2011 and 2011-2012 vaccines. The researchers noted that this study does not establish a causal relationship between repeated influenza vaccination and miscarriage, but further research is warranted. 

Of course, the headlines linking the flu vaccine and miscarriage make everyone panic. ACOG acknowledged this study and responded: “In evaluating all of the available scientific information, there is insufficient information to support changing the current recommendation, which is to offer and encourage routine flu vaccinations during pregnancy regardless of the trimester of pregnancy.

The study doesn’t provide much insight beyond the need for further research. If you have any concerns over the flu shot or when to get it during your pregnancy, please bring it up with your doctor or midwife. Remember, this is not mandatory. It is entirely your choice whether to opt for the flu shot and when to receive it. Like any intervention, this should be a decision based on informed consent.

Pregnancy Warnings

Some vaccine manufacturers maintain a pregnancy exposure registry that monitors outcomes in pregnant women who have received a vaccine. Four (Afluria, Fluarix, Fluecelvax, and Flublok) of the six vaccine options available have human data from either an exposure registry or post-exposure surveillance study showing the rates for miscarriage and major birth defects for those who received a vaccine were consistent with estimated background rates. In the U.S. general population, the estimated background risk of major birth defects is 2-4% and the rate of miscarriage in clinically recognized pregnancies is 15-20%.

Two (FluLaval and Fluzone) of the six vaccine options available do not have human data available and, as a result, carry the disclaimer that “available data with FluLaval/Fluzone use in pregnant women are insufficient to inform vaccine-associated risk of adverse developmental outcomes.”

All six vaccines have undergone reproductive and developmental toxicity studies in animals for the current vaccine or a previous version approved for a past flu season. These studies exposed rats or rabbits to the vaccine before and during pregnancy. There were no adverse effects on fertility or development in animal studies.

Flu Vaccine Side Effects

If side effects occur, they usually begin soon after the shot and generally last for 1-2 days. Rarely can flu shots cause serious problems, such as severe allergic reactions. The most common side effects experienced by pregnant women are the same as those experienced by other adults. They are generally mild and include:   

• Soreness, redness, or swelling from the shot   
• Fainting   
• Headache   
• Fever   
• Muscle aches   
• Nausea   
• Fatigue 

Flu Vaccine Efficacy and Effectiveness

The success of the flu vaccine depends on both efficacy and effectiveness. The CDC measures efficacy with randomized studies where people get either the flu shot or a placebo, compared to whether they get the flu. The CDC measures effectiveness by the number of people who get the flu and visit a doctor. These are not perfect metrics because they do not include individuals who contracted the flu but did not visit a doctor or undergo testing.

According to the CDC, one study showed that getting a flu shot reduced a pregnant woman’s risk of being hospitalized with flu by an average of 40%. This study examined data collected across multiple hospitals over a six-year period. Of 19,450 acute respiratory or fever hospitalizations, only 1,030 pregnant mothers were tested for influenza. Of those, 598 tested positive for the flu. 13% of the women who had the flu were vaccinated, and 22% of those who tested negative were vaccinated. When the researchers adjusted the vaccine effectiveness for the site, season, season timing, and the presence of any high-risk medical conditions, they arrived at a figure of 40%.

The CDC states that during seasons when flu vaccine viruses are similar to circulating flu viruses, the flu vaccine has been shown to reduce the risk of having to go to the doctor with flu by 40 – 60%. The CDC has tracked effectiveness since 2003 and publishes effectiveness rates for each year. Overall, the range is 10-60%, depending on the year. Vaccines are based on the influenza viruses predicted to be the most prevalent for the current flu season. If those predictions are accurate, the vaccines are more effective.

Below, you can review the effectiveness figures for the past several years.

• 2021-22 36%
• 2022-23 30%
• 2023-24 44%
• 2024-25 56%

Choosing a Flu Vaccine

You could easily spend a lot of time comparing these six vaccine options. You could also be comfortable getting whatever vaccine is available. If you are allergic to eggs, request a vaccine that does not use eggs (Flucelvax or Flublok). While vaccines in the United States should be only single-dose vials, that may vary in other countries. If you would like to avoid thimerosal, you can request a single-dose vial instead of a multidose. If you prefer a vaccine that has some human data on safety during pregnancy, you may want to opt for Afluria, Fluarix, Flucelvax, or Flublok. Otherwise, all six options are very similar.

The American College of Obstetricians and Gynecologists does not preferentially recommend a specific formulation or brand of vaccine over another. The official statement is that pregnant individuals should receive any licensed, recommended, age-appropriate, inactivated, or recombinant influenza vaccine during any trimester.

Requesting a Specific Vaccine

Medical offices, pharmacies, and hospitals must order vaccines well in advance of flu season. The vaccine choices available to you may be limited by the vaccine your doctor or midwife carries. If you prefer a specific brand of vaccine, please talk with your doctor or midwife, or contact local pharmacies.

Talk to Your Doctor or Midwife  

As with any intervention during pregnancy, you have options. The flu vaccine should be accompanied by informed consent. This means you understand the risks and benefits and can opt in or out. If you choose to get a flu vaccine, you have options regarding timing, opting for a specific brand, or type of vaccine. Discuss any questions or concerns about the flu vaccines with your doctor or midwife.

Thank you to the brands that help power this podcast.