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Pregnant women must get their flu shots to help avoid future influenza epidemics

The flu virus treats the altered immune systems of pregnant individuals as its training ground to get stronger, making vaccination an imperative

Marnie Willman


University of Manitoba Bannatyne and National Microbiology Laboratory

The international healthcare and economic burden of influenza in the United States alone is approximately $11.2 billion annually. Our best defense, the flu shot, takes almost a full year of planning to produce, made by groups such as the World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC). This flu shot is generated months in advance of flu season (which runs from October to March in North America), based on the three "biggest risk" strains in southern hemisphere countries that have already experienced their flu season for the year.

Contrary to popular belief, the flu shot does not have any live influenza virus in it. Exposure to dead flu virus particles stimulates an immune response, and makes immune cells and antibodies able to fight off the virus. These antibodies are "remembered" by the immune system, so that if your body comes in contact with that particular flu strain again, it has specialized immune cells waiting in the wings to quickly combat the invader. Some people may feel slightly ill or warm after getting their flu shot, but this is the immune system building up its defense for future fights against this bug, not an actual flu infection itself. 

Given this potential protection, it is surprising that the proportion of people who get the flu shot in a given season is low in North American countries, where the shot is readily available. During the U.S. 2017-2018 influenza season, just 45-50% of the U.S. population reported getting their flu shot. Vaccination rates in Canada are even lower, at approximately 37% of the population. 

Vaccination rates matter because those of us who do get our flu shots protect those who are unable to get the shot for medical reasons, like immunocompromised individuals. This is called "herd immunity", meaning that enough people in a population are vaccinated to protect the unimmunized from getting the flu. The vaccination rate required to achieve herd immunity for flu is about 80%, far greater than current vaccination rates in the U.S. and Canada. 

There are two predominant vaccine formulations available in North America: a live, weakened version of the virus (for example FluMist, available as a nasal spray), and a killed, injectable vaccine (the flu shot). While women are not encouraged by healthcare professionals to get the live vaccine during pregnancy, it is safe for them to receive the killed vaccine. Because their bodies need to protect the growing fetus as well as themselves, pregnant women have altered immune responses compared to non-pregnant individuals that make them more susceptible to severe influenza infection.

In a study published in the journal Cell Host and Microbe at the end of the 2016-2017 flu season, a team of German scientists led by Géraldine Engels set out to understand how pregnant women's bodies respond to flu infections. They infected pregnant mice with the H1N1 flu strain (commonly known as Swine Flu) to test their immune responses to the virus. They measured amounts of several immune cells and the amount of virus detected in the lung tissue of the pregnant mice after infection. They found that that pregnant mice tended to harbor more virulent (i.e. more harmful) flu viruses than non-pregnant mice. In other words, the viruses are able to take advantage of a "distracted" host immune system and turn the body into their playground, becoming more pathogenic and capable of causing severe disease as they replicate.

a smiling baby getting a shot

Getting a flu shot is safe and important for healthy people of all ages

Heather Hazzan, SELF Magazine 

The results of this study make the problem of low vaccination rates even more alarming. The CDC reported a vaccination rate of approximately 54% for pregnant women in the U.S., similar to the national average, but according to Engels' research, the remaining 46% of unvaccinated pregnant women may be at higher-than-average risk of passing more dangerous flu viruses to others.

A large reason why many people in developed countries do not vaccinate against seasonal influenza is because they believe it is a low threat virus. We vaccinate for chicken pox, measles, mumps, and polio because they are life-threatening illnesses, but are less fearful of the flu because it has a low death rate among healthy people. But practice makes perfect, and viruses get a lot of practice, randomly swapping out different genetic information across generations to get the perfect combination to infect as many hosts as possible. The flu this year is different from the flu of last year, and some strains of flu can lead to high death rates even in healthy people.

Some propaganda says the influenza vaccine is harmful to pregnant mothers and developing fetuses. But many clinical studies have shown that these side-effects are not linked to the seasonal flu shot. The CDC advises women at any stage of their pregnancy to receive the seasonal influenza vaccination, due to the many adverse effects that influenza infection can have on a fetus, such as neural tube defects, a serious and often fatal developmental problem.

Prevention is key when it comes to influenza infections, particularly seasonal influenza. Even if you got your flu shot last year, you should know that the dominant seasonal flu strains change from year-to-year, so each year is a fresh slate. Getting your vaccination is as important for those who cannot be vaccinated as it is for you. The flu shot is low-cost, readily available, and safe for pregnant and non-pregnant individuals alike. The power to prevent the next deadly flu epidemic — and protect the next generation — could be in your (pharmacist's) hands. 

Comment Peer Commentary

We ask other scientists from our Consortium to respond to articles with commentary from their expert perspective.

Christa Trexler


UC San Diego

I really loved how this article taught me new things while being written in such an understandable and conversational way. I didn’t know how low the rates of flu vaccination actually are in North America and as someone about to have their first child next month, it left me a little unsettled for sure!

I particularly liked the different analogies you used to convey your points like “…so that if your body comes in contact with that particular flu strain again, it has specialized immune cells waiting in the wings to quickly combat the invader.” & " In other words, the viruses are able to take advantage of a “distracted” host immune system and turn the body into their playground, becoming more pathogenic and capable of causing severe disease as they replicate."

Thanks for a great read!

I enjoyed reading this article. It was clear, well informed, and focused on a topic I’ve never thought about–pregnant women’s susceptibility to the flu. It also can spark behavioral change and make for a safer world for pregnant and immunocomprimised people.

The only question I had pertained to the degree of applicability of mice research to humans. How often is what happens to mice translatable to humans? I think touching on this quickly would bring home your already compelling message.

Marnie Willman responds:

Hi Marisa! Thanks for taking time to provide peer commentary. To answer your question, it is debatable how directly this relates to flu directly, in terms of the mouse versus human immune system, but the trend is likely the same. Humans experience the same altered immune system during pregnancy as mice, as well as many of the same immune cell proportion changes. That said, there’s no doubt the mouse immune system is far from identical to humans. The dampened pregnant human immune situation would likely have a similar effect when viruses are introduced as the pregnant mice in the study. There’s a fair bit of room for research on this topic, and I’m interested to see what this research group finds next.