Flu is an infectious disease caused by the influenza virus. The flu typically isn’t severe in healthy adults, and symptoms usually subside in about a week. When you are pregnant, your body’s inflammatory response to the flu during pregnancy puts you at a higher risk for some complications. The medical community recommends that pregnant women get the flu vaccine to protect them and give their babies passive immunity. 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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Evaluating Vaccines

When I evaluate a vaccine or any medical intervention, I run through a series of questions and a lot of research. Thisarticle 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 available information. 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, days, and weeks on this topic, and I hope that you find the information in this episode 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. This article aims 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. I firmly believe that you should be able to choose whether or not you receive any vaccine or medical intervention. If you listen to this podcast, you can make an informed decision and not one based on fear.  

The Role of the Centers for Disease Control and the World Health Organization

In recent years, many people have raised questions about the accuracy of information from organizations like the CDC and the World Health Organization. There have also been questions about the influence pharmaceutical companies have over these organizations. As a data source, the CDC and the WHO have the most extensive centralized database for statistics on disease.

It is essential to keep in mind that national and international health organizations are interested in protecting the population as a whole. Plus, their recommendations influence more specific policies by organizations like the American College of Obstetricians and Gynecologists, which directly impacts your care provider’s recommendation on topics like vaccines. Your main concern, understandably, is the safety and efficacy of a vaccine for you and your baby. Understanding the information informing your doctor or midwife will help you decide whether or not you want to get the flu vaccine and help arm you with the knowledge to have a conversation with your doctor or midwife.

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 highly 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 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 much more likely to have 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 199 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 their baby until they 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 the 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 it 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 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 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 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 lower vitamin D levels in winter help spread this virus during the 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. Once the WHO chooses specific strains, it takes six months to make and produce the vaccine. There are two vaccines each year 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 that figures it out. 

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 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 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 the first 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. Most of the research on vaccines has not been done on women in the first trimester because this is a critical development period.

Combining Flu and Other Vaccines 

An additional consideration with the timing of the flu vaccine is whether to get it simultaneously as other vaccines.

When COVID-19 vaccines first became available, the CDC recommended waiting 14 days between the COVID-19 vaccine and any other vaccine as a precautionary measure. The CDC has since changed their recommendation and states that you can receive the COVID-19 vaccine simultaneously as other vaccinations. The CDC does say it is not known if the reactogenicity of COVID-19 vaccines is increased with coadministration, including with other vaccines known to be more reactogenic, such as adjuvanted vaccines.”

According to CDC, you can get Tdap and flu shots at the same time. 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, 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. If you plan to get both the Tdap and flu shots during pregnancy, talk with your doctor or midwife about the timing.

Types of Flu Vaccines 

Some vaccines, like the Tdap, are relatively easy to compare manufacturers because there are only two. This is a lot more challenging with the flu because there are nine vaccines available for the 2021-2022 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 World Health Organization, 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. Mercury exposure – 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, immune systems, lungs, kidneys, skin, and eyes. The WHO considers mercury 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. For the 2021-2022 season, only multidose vaccines contain thimerosal. Approximately 87% of the projected vaccine supply produced for the 2021-2022 season will be thimerosal-free or thimerosal-reduced. 

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 them into fertilized hens’ eggs so they 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 have an egg allergy, please be sure to bring it up with your care provider. According to the CDC, approximately 82% of the projected vaccine supply produced for the 2021-2022 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 targeted flu strains, three times what most other flu vaccines contain. The only recombinant vaccine approved for the 2021-22 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 2021-2022 Flu Season 

Of the nine total vaccine options for this season, two are approved only for those over 65. One of the nine is a nasal spray that uses a live attenuated vaccine. The CDC does not recommend the nasal spray vaccine during pregnancy. If you are pregnant and opt for 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 nasal spray is not an option.  

For the 2021-2022 flu season, all vaccines are quadrivalent (4-component) and contain two A strains and two B strains. 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 influenza vaccine formulation. 

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 purchasing vaccines. 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  

study shows that the trivalent influenza virus vaccination elicits a measurable inflammatory response among pregnant women. Although only quadrivalent vaccines are available for the 2021-22 season 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 that 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 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 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 the flu shot is safe during pregnancy, and they ultimately still recommend the flu shot for expecting mothers.  

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 it 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. 

Pregnancy Warnings

All of the six vaccine options available to you have some variation of the disclaimer that there is insufficient data on the vaccine in pregnant women to inform vaccine-associated risks. We know that pregnant women are underrepresented in vaccine trials.

For the previous 2020-2021 flu season, one manufacturer did perform a reproductive and developmental toxicity study in animals. Rabbits received three doses (one pre-pregnancy and two during gestation) of the trivalent Flucelvax vaccine. The dosage is 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. It would be ideal to have more data on this season’s vaccine, but the results of this study are still good news.

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. 

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 people get 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. These are not perfect metrics because they do not include people who got the flu but did not visit a doctor or get tested.

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 those 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 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 viruses in the flu vaccine. The CDC has also been keeping track of effectiveness, which dates back to 2004. The CDC publishes this data and shows the effectiveness rate for each year. Overall it is a big range from 10-60% depending on the year. For the 2019-2020 flu season, it was 39%. The CDC did not estimate the effectiveness of the 2020-2021 influenza vaccine due to low influenza virus circulation.

Influenza and COVID-19 

We have drastically changed our daily lives and how we operate due to COVID-19, and these changes have led to far fewer flu cases. Measures like social distancing, mask-wearing, businesses, and closed schools all limit the spread of the flu virus. We may see more flu cases this season as people are socially distancing less.

One consideration for the flu vaccine is that if you have any cold or flu symptoms, you will need to isolate and get tested for 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 for symptoms online can create a lot of anxiety. 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 COVID-19. 

Talk to Your Doctor or Midwife 

Please talk to your doctor or midwife about the flu vaccine. If you have concerns, bring them up. Discuss if you want to consider a specific vaccine brand or avoid a certain kind of vaccine. Your care provider is 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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