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Flu is an infectious disease caused by the influenza virus. In healthy adults, the flu typically isn’t severe, and symptoms usually subside in about a week. When you are pregnant, the inflammatory response your body has to the flu during pregnancy puts you at a higher risk for some complications. The medical community recommends pregnant women get the flu vaccine during their pregnancy to protect them and give passive immunity to their baby. The COVID-19 pandemic is also bringing some additional changes and challenges to the current flu season. There are many flu vaccines available, and it can be challenging to weigh the risks and benefits to decide whether getting the flu vaccine during pregnancy is right for you. This episode dives into all of the evidence and considerations to make an informed decision on whether you should get the flu vaccine during your pregnancy.

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For the 2020-2021 flu season, we should see a big push for people to get the flu vaccine. This is especially true for expecting mothers who are having regular appointments with a doctor or midwife. The CDC website states, “For the upcoming flu season, flu vaccination will be very important to reduce flu because it can help reduce the overall impact of respiratory illnesses on the population and thus lessen the resulting burden on the healthcare system during the COVID-19 pandemic.” There have been some questions this year about the accuracy of information from the CDC. This organization drives many policies adopted by medical organizations, like the American College of Obstetricians and Gynecologists. Plus, the CDC is the most extensive database in the United States for statistics on influenza. This is the source your care provider is likely looking for information on the flu vaccine.

When I evaluate a vaccine or any medical intervention, I run through a series of questions and a lot of research. This article follows a similar format. Some of the questions I ask are:

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

As you can imagine, this involves a lot of research. I also look at the particulars of the situation. With the flu vaccine, I would evaluate my current health status and the likelihood of exposure, including where I live, places I plan to travel to, the time of year, etc. You get the point. There are a lot of questions, and this involves lots of research.

The Controversy of Vaccines

Vaccines are a very controversial and complex topic. Some professionals spend their lives dedicated to researching and studying vaccines and their safety. This article is an overview of the flu vaccine to give you a good base on the information available. It is impossible to analyze every single study done because this is not the vaccine podcast, and there are many other essential topics I want to share with you. I have spent a significant number of hours and days and weeks on this topic, and I hope that you find the information in this episode to be helpful as you are making choices about whether or not to get the flu vaccine during your pregnancy.

Researching vaccines is incredibly challenging. It is easy to find yourself on a website that appears to be unbiased, only to discover an agenda, and it is firmly pro or con vaccine. My goal with this article is to help you sift through all of the data out there to educate yourself and make an informed decision that is right for you.

These are decisions that you should be making with your doctor or midwife. I cannot possibly cover every facet of this topic, and if you have further questions or concerns, I urge you to bring them up with your care provider. I am not for or against the flu vaccine, and I firmly believe that you should be able to choose whether or not you receive any vaccine or medical intervention. If you read this article, you can make an informed decision and not one based on fear.

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 influenza virus is extremely contagious because it can be airborne and on surfaces. It becomes airborne when someone who is infected coughs or sneezes. This releases half a million particles into the air. You can also contract the flu virus by touching a contaminated surface, 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 only about 5 minutes on the skin. Please wash your hands often and try to avoid touching your face.

The flu’s symptoms include fever, runny nose, sore throat, muscle pains, headache, coughing, and fatigue. The sudden onset of symptoms and exhaustion that comes with the flu can help you distinguish it from a cold. In general, symptoms show up two days after you are infected and last about a week. A lingering cough may last longer. Complications of the flu can be pneumonia and sinus infections. There are a couple of antiviral drugs to treat the flu. Antibiotics will not help treat the flu because a virus and not a bacteria cause it.

If you are a generally healthy adult, the flu usually isn’t a big deal. If you are very young, elderly, or have a weakened immune system, you are at a much higher risk for complications. Pregnancy does lower your immune system.

The Flu and Young Children

The CDC recommends that babies older than six months get a flu shot every year. According to the CDC,  between 7,000 and 26,000 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 CDC during regular flu seasons have ranged from a low of 37 to a high of 188 deaths. Children under six months of age do not receive a flu shot. Part of the push to get expecting mothers to get the vaccine is to transfer some of that immunity, to protect your baby until they can receive their vaccine at six months.

How the Flu Affects Your Body 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, which we see as redness, swelling, pain, or heat. Think about sore muscles and fever when you have the flu. The positive side of inflammation is that it creates an immune response. You want your body to elicit an immune response to fight off whatever is causing the inflammation.

A 2014 Stanford University study found that pregnant women have an unusually strong immune response to influenza. This unexpected finding may explain why pregnant women get sicker from the flu than other 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.

Types of Flu

There are three types of flu: A, B, and C. A is the most serious in humans. Birds commonly carry type A. All the major flu pandemics documented in our history were type A, including the bird and swine flu. Type B is less common than A. Type B mutates 2-3 times slower. The only animals it affects are seals and ferrets, and almost exclusively affects humans. Because type B evolves much slower and few animals carry it, we rarely have epidemics of type B. Type C affects humans, dogs, and pigs and is the least common, and tends to be the mildest. Vaccines usually include type A and B and do not have type C.

Within the different types of flu, there are subtypes. You will see the letter H followed by a number, and the letter N followed by a number. The H stands for hemagglutinin, of which there are 16 types, and the N stands for neuraminidase, of which there are nine types. These Hs and N’s are protein spikes on the virus’s surface that help the virus invade cells. In total, there are 144 different subtypes.

How the Flu Virus Mutates

An excellent example of how these viruses mutate I’m borrowing from an NPR article explains this straightforwardly. 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 talks about the 2009 flu. The virus had some segments from H3N2 circulating in humans from the 1968 pandemic. It had genes coming in from birds, an H1 coming from pigs, and an N1 coming in from a completely different virus. All of these are then mixed in pigs. You can see this gets messy and complicated very quickly.

Flu Season

Flu season tends to be during the winter months. Part of this is due to the lack of humidity and low UV light. During the winter, more people are indoors, so it spreads more easily. Plus, the virus survives longer on surfaces at colder temperatures. There is a theory that vitamin D levels being lower in winter help spread this virus during winter months. I’m not going to examine this in-depth here, and if you want to dig deeper into vitamin D, you can listen to this episode.

The Flu Vaccine

The influenza virus is a very rapidly evolving virus. For this reason, we cannot make a one-time vaccine. Every year the flu vaccine is formulated for 3 or 4 viruses. The World Health Organization predicts which strains of the flu will be the most common the following year, and that is what they use to create the vaccine. Vaccine virus selection has been conducted by the WHO Global Influenza Surveillance and Response System (GISRS) since 1973. From the time the WHO chooses specific strains, it takes six months to make and produce the vaccine. Each year there are two vaccines because there are two flu seasons—one during winter in the upper hemisphere and one during winter in the lower hemisphere. There is a lot of research around flu and big motivation to create a better vaccine, not to mention billions of potential dollars in profit to the company who figures it out.

How the Flu Vaccine Works

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 the bacteria or virus or pathogen and then recognize them and fight them off in the future. Ultimately the goal is to become immune, which is why we also call vaccines immunizations.

Once you get the flu vaccine, it takes your body about two weeks to develop antibodies. When you are pregnant and your body creates antibodies, they pass to your baby through the placenta. This is called passive immunity. The goal when you get a vaccine during pregnancy is to protect both you and your baby. Remember that your baby cannot get a flu vaccine until they are at least six months old.

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

When should you get the vaccine?

Some expecting mothers and care providers may choose to avoid a flu shot during this trimester because this period is so critical to development. I encourage you to talk to your doctor or midwife about the timing of a vaccine. The Centers for Disease Control says that 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.

Combining the Flu and Tdap Vaccines

Another consideration with the flu vaccine’s timing is whether to get it simultaneously with the Tdap vaccine. Tdap is for Tetanus, Diptheria, and Pertussis (aka whooping cough). According to the CDC, you can get Tdap and flu shot at the same time. Adacel is the Tdap vaccine. In the insert, for this, they state that in studies when the Adacel vaccine was administered simultaneously as a trivalent inactivated influenza vaccine, lower antibody response was observed for pertactin antigen compared to when the Adacel vaccine was administered alone. The pertactin antigen is one of the things that help you develop immunity for pertussis or whooping cough. If you plan to get the Tdap and the flu vaccines during your pregnancy, talk with your doctor or midwife about the timing. ACOG does state that receiving the Tdap and flu vaccinations at the same time is safe and effective.

Types of Flu Vaccines

Some vaccines, like the Tdap, are relatively easy to compare manufacturers because there are only 2. This is a lot harder with the flu. There are ten vaccines available for the 2020-2021 flu season. You can compare the vaccine options on the CDC website. The CDC lists what ages it is approved for, whether the vaccine is egg or cell-based, and whether it is single or multidose with mercury content.

Single vs. Multi Dose

A single dose vaccine comes in a vial with enough vaccine for one dose for one person. A multidose vial contains multiple doses for multiple people. Of course, each vaccination uses a new sterile needle, but multidose vaccines require an additional preservative to prevent bacteria and fungi from growing.

Thimerosal (mercury) is an ingredient added to multidose vaccine vials to prevent bacteria and fungus. Multidose vaccines have about 25 micrograms of thimerosal, and single-dose vaccines are mercury-free. According to the WHO, mercury use in some pharmaceuticals, such as thimerosal (ethyl mercury), which is used as a preservative in some vaccines, is very small by comparison with other mercury sources. There is no evidence that suggests a possible health hazard resulting from the amounts of thimerosal currently used in human vaccines.

The WHO also states, “Mercury is a naturally occurring element that is found in air, water, and soil. Exposure to mercury – even small amounts – may cause serious health problems and is a threat to the development of the child in utero and early in life. Mercury may have toxic effects on the nervous, digestive, and immune systems and on lungs, kidneys, skin, and eyes. Mercury is considered by WHO as one of the top ten chemicals or groups of chemicals of major public health concern.”

You can request a single-dose vaccine, which does not contain mercury. The CDC states that for the 2020-2021 season, only multidose vaccines contain thimerosal. Approximately 87% of the projected vaccine supply, or 171 million doses, produced for the 2020-2021 season will be thimerosal-free or thimerosal-reduced. To see which vaccines contain mercury, you visit the CDC website.

Ingredients in Flu Vaccines

Flu vaccine ingredients vary depending on the type and brand. They can contain trace amounts of eggs. Some vaccines contain formaldehyde, used in growth mediums and processing, or added to the final product. The amount of formaldehyde is much less than is naturally contained in your blood. Vaccines could also contain gelatin made from animal collagen, making it stable during storing 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 go to private companies who inject it into fertilized hens’ eggs so it can multiply. The liquid is taken from the eggs containing the virus, and then it is killed or inactivated. There is a concern about this type of vaccine if you have an egg allergy. The final vaccine has such trace amounts that some people who have an allergy don’t have any trouble. There are other options, and if you do have an egg allergy, please be sure to bring it up with your care provider. According to the CDC, approximately 81% of the projected vaccine supply produced for the 2020-2021 flu season will be produced using egg-based manufacturing technology.

Cell-Based Vaccines

Cell-based vaccines were approved in 2012. It started in eggs and is now entirely egg-free, but it is grown in animal cells. 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. Cell-based vaccines are also not dependent on egg supply. Hundreds of thousands of eggs are used each year in flu vaccine production. Manufacturers produce cell-based vaccines faster in an epidemic scenario because they do not rely on a large supply of eggs.

Recombinant Vaccines

Recombinant flu vaccines were approved in 2013. These are made using insect cells and are 100% egg-free. These types of vaccines do not contain thimerosal. The product contains 45 micrograms (mcg) of hemagglutinin for each of the three targeted flu strains, three times what most other flu vaccines contain. The only recombinant vaccine approved for the 2020-21 season is Flublok Quadrivalent, made by Sanofi Pasteur.

If you are vegan – It is complicated. There is no vegan flu shot, which could be a consideration if that is important to you. 

Flu Vaccines Available for the 2020-2021 Flu Season

Of the ten total vaccine options for this season, three are approved only for those over 65. One of the ten is a nasal spray that uses a live attenuated vaccine. The CDC does not recommend this vaccine during pregnancy. If you are pregnant and opt into the flu vaccine, the good news is that you will be getting an inactivated vaccine, so the virus is not living. The bad news is that you will be getting an injection since the nasal spray is not an option.

Flu vaccines are either trivalent (3 components) – two A strains and one B strain, or they are quadrivalent and have one additional B strain. For 2020-2021 there is only one trivalent  (3-component) vaccine available, approved for use in adults over 65. All of the other flu vaccines available are quadrivalent (4-component).

For the 2020-21 season, there are six vaccine options you could receive during pregnancy.

Requesting a Specific Vaccine

The American College of Obstetricians and Gynecologists’ official statement is that pregnant women should receive any licensed, recommended, age-appropriate inactivated influenza vaccine. The Centers for Disease Control and Preventions’ Advisory Committee on Immunization Practices and ACOG do not preferentially recommend a specific formulation of the influenza vaccine.

The vaccine choices available to you may be limited by the vaccine carried by your doctor or midwife. Medical offices, pharmacies, and hospitals have to order vaccines ahead of time, and cost plays a significant role in which vaccines they purchase. If you prefer a specific brand of vaccine, please talk with your doctor or midwife. You can likely find the brand you want for a low cost at a pharmacy or other place in your neighborhood. 

The Research on the Safety of Vaccines During Pregnancy

Inflammatory response

A study shows that the trivalent influenza virus vaccination elicits a measurable inflammatory response among pregnant women. Although for the 2020-21 season, only quadrivalent vaccines are available for expectant mothers, research on previous year vaccines is still applicable. While the strains of the flu change every year, many of the other ingredients do not. Interestingly, the flu or the flu vaccine creating an inflammatory response is that preeclampsia and preterm birth have an inflammatory component. When you are pregnant, you want to limit inflammation when possible. This study noted that an inflammatory response elicited by vaccination is substantially milder and more transient than seen in infectious illness, arguing for vaccination’s clinical value. However, further research is needed to confirm that the mild inflammatory response elicited by vaccination is benign in pregnancy.

Some animal studies show maternal immune activation during pregnancy can harm the brain development of offspring. I want to stay away from linking autism to vaccines. I haven’t seen any concrete evidence that there is a link or a causal relationship. Also, keep in mind that autism is a spectrum disorder. So that tells us that since it is diagnosed on a spectrum, with varying degrees, it is unlikely there is one cause, and more likely, there are many causes. I acknowledge that there are parents who firmly believe their child developed autism following vaccination.

Some animal studies involving rodents and monkeys could raise questions about how a maternal immune response affects a baby in utero. One study on mice shows that maternal immune activation affects the brain development of a fetus. Ultimately a flu shot does create an immune response, but so does the flu. The bottom line is that there is a lot we do not know about immune reactions during pregnancy and how that potentially affects a baby in utero.

Flu Vaccine and Miscarriage

There was a recent study that made headlines about the flu vaccine and miscarriage. I want to go beyond the clickbait headlines and talk about what this study found and what it could 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 vaccinated in the previous influenza season. This study looked at women who received both the 2010-2011 season vaccine and the 2011-2012 vaccine. 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 that link the flu vaccine and miscarriage get everyone in a panic. Organizations like the CDC and ACOG were quick to respond that they ultimately still recommend the flu shot for expecting mothers. ACOG and the CDC state that the flu shot is safe during pregnancy. The study doesn’t tell us much, other than more research is needed. If you have any concerns over the flu shot or when to get the flu shot during your pregnancy, please bring it up with your doctor or midwife. Remember, this is not mandatory. It is entirely your choice whether you opt into having the flu shot. Like any intervention, this should be an informed consent decision.

Vaccine Safety During Pregnancy

Five of these six vaccine options available to you have some disclaimer variation that there are insufficient data on the vaccine in pregnant women to inform vaccine-associated risks. We know that pregnant women are underrepresented in vaccine trials. There is one exception, and that is the Flucelvax vaccine. The manufacturer, Seqirus, performed a reproductive and developmental toxicity study in rabbits. The rabbits received three doses (one pre-pregnancy and two during gestation) approximately 15 times the human dose based on body weight. They observed no adverse effects on mating, female fertility, pregnancy, embryo-fetal development, or post-natal development. There were also no vaccine-related fetal malformations or other evidence of teratogenesis. The results of this study are good news. The package insert also states that because animal reproduction studies are not always predictive of human response, this vaccine should be used during pregnancy only if clearly needed.

Possible Side Effects of Flu Vaccines

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

If side effects occur, they usually begin soon after the shot is given and generally last for 1-2 days. Rarely, flu shots can cause serious problems like severe allergic reactions. Please let your doctor or midwife know if you are experiencing any side effects. 

Efficacy and Effectiveness of Flu Vaccines

How successful the flu vaccine depends on both efficacy and effectiveness. The CDC measures efficacy with randomized studies where a group of people gets either the flu shot or placebo, compared to whether they get the flu. The CDC measures effectiveness by how many people get the flu and visit a doctor.

According to the CDC, one study showed getting a flu shot reduced a pregnant woman’s risk of being hospitalized with flu by an average of 40 percent. This study looked at data collected across many different hospitals for over six years. Of 19,450 hospitalizations for acute respiratory or fever, 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 the women who tested negative were immunized. When the researchers adjusted the vaccine effectiveness for the site, season, season timing, and the presence of any high-risk medical condition, they came up with a figure of 40%. The reason I go through the data here is to put this conclusion into perspective.

The CDC states that recent studies show vaccines can reduce the risk of flu illness by about 40-60% among the overall population during seasons when most circulating flu viruses are like the flu vaccine viruses. The CDC has also been keeping track of effectiveness, which dates back to 2004. This published data shows the effectiveness rate for each year. Overall it is a big range from 10-60% depending on the year. For the 2018-2019 flu season, it was 29%. The preliminary data for the 2019-2020 season shows the vaccine was about 35% effective for those ages 18-49, which would typically include most mothers who are expecting. This number is higher in years that they accurately predict the right flu strains that are most common.

If you get the flu vaccine, it is still possible to get the flu. This happens for a couple of reasons. First, you could get infected by a strain of flu not included in the vaccine. Second, it takes two weeks for your body to produce antibodies, which leaves you vulnerable during that two-week timeframe.


The current COVID-19 pandemic introduces some additional considerations for the flu vaccine. There is a lot of speculation from scientists on how this flu season may differ from recent years. Measures like social distancing, mask-wearing, and children not attending schools could result in fewer flu cases. Vaccine rates may decrease this year as people are more hesitant to visit their doctor or a medical facility because of the risk of COVID-19.

Both SARS-CoV-2, which causes COVID-19, and influenza are respiratory viruses. They share a lot of the same symptoms and spread similarly. If you get a respiratory virus, you may not be able to tell whether it is SARS-CoV-2 or influenza without a diagnostic test. One difference is that many people experience the loss of taste or smell with the onset of COVID-19. The CDC has a breakdown of the differences between symptoms of these two viruses. If you are experiencing symptoms, please let your doctor or midwife know. Searching symptoms online can create a lot of anxiety, and the only definitive way to tell whether you have influenza or COVID-19 is with a diagnostic test. Since the flu and SARS-CoV-2 are different, it is possible that you can get both. A flu vaccine may protect you from some strains of the flu, but it cannot protect you from SARS-CoV-2.

Talk to Your Doctor or Midwife

Please talk to your doctor or midwife about the flu vaccine. If you have concerns, bring them up. If you want to consider a specific vaccine brand or wish to avoid a certain kind of vaccine, bring it up. They are your trusted partner during your pregnancy, and their opinion is worth hearing. Please discuss any questions, no matter how small or silly they may seem.


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