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Flu is short for influenza, an infectious disease caused by the influenza virus. In healthy adults the flu normally isn’t serious 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. It is recommended that pregnant women get the flu vaccine during their pregnancy to protect them and give passive immunity to their baby. There are many flu vaccines available and it can be a challenge 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 pros and cons and considerations to think about to make an informed decision on whether you should get the flu vaccine during your pregnancy.

Article and Resources

When I am evaluating 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 ingredients are included 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 the status of my current health, the likelihood of exposure which would include where I live, places I am planning 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. There are professionals who spend their lives dedicated to researching and studying vaccines and their safety. This article is intended as 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. This is not the vaccine podcast and there are a lot of other really important 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 especially challenging. It is easy to find yourself on a website that appears to be unbiased, only to discover there is an agenda and it is strongly pro or con vaccine. My agenda 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. It is impossible to 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. You should be able to make the choice as to whether or not you receive any vaccine, or medical intervention. I trust if you are reading this article you are making 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 come into contact with 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 skin. Please wash your hands often and ty to avoid touching your face.

Symptoms of the flu include fever, runny nose, sore throat, muscle pains, headache, coughing, and fatigue. You can tell the difference between a cold and the flu by the sudden onset of symptoms and fatigue that comes along with the flu. In general, symptoms show up 2 days after you are infected and last about a week. It is possible that a lingering cough lasts longer. Complications of the flu can be pneumonia and sinus infections. There are a couple antiviral drugs that can be used to treat the flu. Antibiotics will not help treat the flu, because it is caused by a virus and not a bacteria.

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 (like when you are pregnant) you are at a much higher risk for complications. It is recommended by the CDC that babies older than 6 months get a flu shot every year.

According to the CDC, each year between 7,000 and 26,000 children younger than 5 years are hospitalized 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 186 deaths.

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. Which is exactly what you want your body to do, so it can 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 was an unexpected finding that may explain why pregnant women get sicker from the flu than other healthy adults. It is suggested that physiological changes such as immunological changes, increased cardiac output and oxygen consumption, as well as lung tidal volume might increase the susceptibility to influenza complications if infection occurs during pregnancy.

Types of Flu

There are 3 types of the flu: A, B, and C. A is the most serious in humans. Type A is commonly carried by birds. 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 isn’t carried by a lot of animals 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 include 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 hemagluttinin of which there are 16 types and the N stands for neuremanidase of which there are 9 types. These H’s and N’s are protein spikes on the surface of the virus that help the virus invade cells. In total there are 144 different subtypes.

How the Flu Virus Mutates

A good example of how these viruses mutate I’m borrowing from an NPR article that explains this in really simple 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 off 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 then mixed together 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 has something to do with spread 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 it isn’t one simple virus an easy vaccine can be created for. Every year the flu vaccine is formulated for 3 or 4 viruses. The World Health Organization predicts which strains of the flu are going to 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 specific strains are chosen it takes 6 months to make and produce the vaccine. Each year there are actually 2 vaccines because there are 2 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 huge 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 principal. Part of a bacteria, virus, or pathogen is introduced into your body and an immune response is triggered. Your body produces antigens to fight off the bacteria or virus or pathogen, and then has the ability to 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 2 weeks to develop antibodies. When you are pregnant and your body creates antibodies they are passed to your baby through the placenta. This is called passive immunity. This is the goal when you are getting a vaccine during pregnancy, that both you and your baby will be protected.

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 newborns immune system and help to keep them healthier.

When should you get the vaccine?

The first trimester is one that is so critical to development, some expecting mothers and some care providers may choose to avoid a flu shot during this trimester for that reason. Talk to your doctor or midwife about the timing.

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 timing of the flu vaccine is whether to get it at the same time as the Tdap vaccine. Tdap is for Tetanus, Diptheria, and Pertusis (aka whooping cough). According to CDC, you can get Tdap and flu shot at the same time. Adacel is the Tdap vaccine and in the insert for this they state that in studies when Adacel vaccine was administered at the same time 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 is one of the things that helps you develop immunity for pertussis, or whooping cough. If you are planning to get both the Tdap and the flu shot during your pregnancy talk with your doctor or midwife about the timing of those two vaccines.

Types of Flu Vaccines

Some vaccines, like the Tdap are fairly easy to compare manufacturers because there are only 2. This is a lot harder with the flu. For the 2019-2020 flu season there are 9 vaccines available You can compare the vaccine options on the CDC website. This lists mercury content, whether the vaccine contains latex, and whether it is single or multidose.

Single vs. Multi-Dose

A single dose vaccine comes in a vial with enough vaccine for one dose for one person. A multi dose vial contains multiple doses that will be used for multiple people. Of course, new sterile needles are used for each vaccination, but multi-dose vaccines require an additional preservative to prevent bacteria and fungi from growing.

Thimerisol (mercury) is an ingredient that is added to multi-dose vaccine vials to keep them bacteria and fungus free. Multi-dose vaccines have 25 micrograms of thimerosal and single dose vaccines are mercury free. According to 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. 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 one of the top ten chemicals or groups of chemicals of major public health concern.

You can request the single dose vaccine, which does not contain mercury.

Ingredients in Flu Vaccines

Different flu vaccines contain different ingredients. Depending on the type and brand of vaccine it could contain trace amounts of eggs. Some vaccines contain formaldehyde which is used in growth mediums and processing, and may be 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 which helps make 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. This is then given to private companies who inject it into fertilized hens eggs so it can multiply. Liquid is taken from the eggs containing the virus 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.

Cell Based Vaccines

Cell based vaccines were approved in 2012. It started off 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 are that they have more control over mutations, and hopefully as a result, 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. In an epidemic scenario cell based vaccines could be produced faster because they do not rely on a big 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, which is three times what most other flu vaccines contain.

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

Flu Vaccines Available for the 2019-20 Flu Season

The CDC doesn’t recommend nasal spray vaccine during pregnancy, which uses a live attenuated vaccine. If you are pregnant and opt in to the flu vaccine the good news is that you will be getting an inactivated vaccine so the virus is not living, it has been killed. The bad news is that since the nasal spray is not an option you will be getting an injection.

There were a lot of changes to the flu vaccines for the 2019-2020 flu season. Flu vaccines are either trivalent (3 components) – two A strains and one B strain, or they are quadrivalent, and have one additional B strain. All of the flu vaccines available for the 2019-20 season are quadrivalent, there are no trivalent vaccines. This means they contain 2 A strains and 2 B strains.

The Research on the Safety of Vaccines During Pregnancy

Inflammatory response

There is a study that shows that the trivalent influenza virus vaccination elicits a measurable inflammatory response among pregnant women. Although for the 2019-20 season only quadrivalent vaccines are available, I think research on previous year vaccines is still applicable. While the strains of the flu change every year, many of the other ingredients do not. What is interesting about 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. The researchers of this study noted that an inflammatory response elicited by vaccination is substantially milder and more transient than seen in infectious illness, arguing for the clinical value of vaccination. However, further research is needed to confirm that the mild inflammatory response elicited by vaccination is benign in pregnancy.

There are some animal studies that activation of the maternal immune activation during pregnancy can have a negative impact on brain development of offspring. I really want to stay away from linking autism to vaccines. I haven’t seen any solid evidence that there is a link or a causal relationship. Also keep in mind that autism is a spectrum disorder. 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 strongly believe their child developed autism following a vaccination.

With that being said, there have been some animal studies involving rodents and monkeys that could raise some questions about how a maternal immune response affects a baby in utero. One study on mice shows that maternal immune activation affects 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 responses 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 click bait headlines and talk about what this study found and what it could mean for you. The conclusion of the study is that first trimester miscarriage was associated with the flu vaccine when the vaccine was given in the preceding 28 days, and 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 and cannot establish a causal relationship between repeated influenza vaccination and miscarriage, but further research is warranted.

Of course the headlines that there is a link between the flu vaccine and miscarriage gets everyone in a panic. Organizations like the CDC and ACOG were quick to come out with a response that ultimately still recommends the flu shot for expecting mothers.

The American College of Obstetricians and Gynecologists came out with a statement saying, “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 CDC also states that the flu shot is safe during pregnancy.

Really, 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 completely your choice whether you opt in to having the flu shot.

There is really limited research on vaccines during pregnancy. Even when you read through package inserts which have a lot of legal language and required disclosures, there is limited information. In one package insert for Flublok, one of the available flu vaccines it states: Pregnancy outcomes in women who have been exposed to Flublok Quadrivalent during pregnancy are being monitored. Sanofi Pasteur Inc. is maintaining a prospective pregnancy exposure registry to collect data on pregnancy outcomes and newborn health status following vaccination with Flublok Quadrivalent during pregnancy. Healthcare providers are encouraged to enroll women who receive Flublok Quadrivalent during pregnancy in Sanofi Pasteur Inc.’s vaccination pregnancy registry.

Possible Side Effects of Flu Vaccines

The most common side effects experienced by pregnant women are the same as those experienced by other people. They are generally mild and include: soreness, redness, and/or swelling from the shot, fainting, headache, fever, muscle aches, Nausea, and fatigue. Side effects 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 is depends on both efficacy and effectiveness. Efficacy is measured by randomized studies where a group of people are given either the flu shot or a placebo then that is compared to whether they get the flu. Effectiveness is measured by how many people get the flu and visit a doctor.

The CDC  states recent studies show vaccine 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 the flu vaccine is designed to protect against. Data on effectiveness, which dates back to 2004 is published by the CDC each year. Overall it is a big range from 10-60% depending on the year. For the 2018-2019 flu season it was 29%. Obviously this number is higher in years that they accurately predicted the right flu strains that were 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 that was not included in the vaccine. Second, it takes 2 weeks for your body to produce antibodies, and this leaves you vulnerable during that 2 week timeframe.

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 brand of vaccine, or want 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.

Tips to Avoid the Flu

Whether or not you get the flu vaccine you want to avoid the flu and stay healthy. The simplest and most important thing you can do to avoid the flu virus is to wash your hands often. You also want to avoid touching your face because that takes a virus on your hands and introduces it directly into your body. If a friend or family member is sick stay away. As always, the basic things you can do to stay healthy during your pregnancy are to eat well, stay hydrated, and get enough sleep.


Thank you to Zahler for their support of this episode.

Zahler makes a high quality prenatal vitamin that has the active form of folate plus omega 3s and DHA. This is my favorite prenatal vitamin, the one I take everyday and the one I recommend to all expecting moms. Zahler is offering an exclusive discount to listeners of the Pregnancy Podcast. To check out the vitamin and find out how you can save 25% when you buy a one month supply on Amazon click this link Zahler Prenatal + DHA.

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