Getting the flu is never a pleasant experience, but for pregnant women, the illness can be particularly bad. That's because pregnant women are considered one of the "high-risk" groups who are more likely to develop complications from the flu.
Despite this risk, last flu season, just 49.1 — less than half — of the pregnant women in the U.S. got a flu vaccine, according to the Centers for Disease Control and Prevention (CDC).
"When [pregnant] women come in and they say, 'Do I really need to get the flu shot,' my answer is yes," Dr. Laura Riley, Given Foundation professor and chair of the Department of Obstetrics and Gynecology at Weill Cornell Medicine, said at a news conference on Sept. 27.
The reason is that pregnancy changes the immune system, Riley said. In a healthy pregnancy, "your immune system isn't working exactly the way we need it to work" to fight off the virus. So, when pregnant women get the flu, they get sicker than nonpregnant individuals. And as the pregnancy progresses into the second and third trimesters, the odds of more severe illness and even death increase as well. [Flu Shot Facts & Side Effects (Updated for 2018-2019)]
A changed immune system isn't the only thing that makes pregnant women more vulnerable to the flu. Because a woman's lung volume decreases as the pregnancy progresses (and the fetus grows and takes up more space), it becomes more difficult to clear respiratory infections, Riley said.
And catching the flu endangers not only the mother but also the unborn child: If the mother has a prolonged fever due to the flu, it can lead to birth defects, Riley said. But another, more common problem is that women who get the flu during pregnancy are more likely to have a preterm birth than pregnant women who don't get the flu. This is a problem "we see every year," Riley said, and problems associated with preterm births can become a "lifelong issue" for some babies.
The flu vaccine, Riley said, leads to the creation of antibodies in the body that then cross the placenta into the fetus. These antibodies protect the baby from the flu in the infant's first six months of life.
Indeed, the CDC and the American College of Obstetricians and Gynecologists recommend that every pregnant woman get a flu shot. They can do so at any trimester, Riley said.
Women's immune systems remain in this altered state for a couple of weeks after giving birth, so it's recommended that postpartum women get vaccinated if they haven't already done so, she added.
Studies have shown that pregnant women who received a flu vaccination were 40 percent less likely to be hospitalized during pregnancy than pregnant women who weren't vaccinated, Riley said. "Pregnant women naturally want to protect their babies," she said, and the vaccine is "effective" and "safe."
Last year's flu season was the worst in at least four decades, with around 80,000 deaths and 900,000 hospitalizations, according to the CDC.
Originally published on Live Science.
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Yasemin is a staff writer at Live Science, covering health, neuroscience and biology. Her work has appeared in Scientific American, Science and the San Jose Mercury News. She has a bachelor's degree in biomedical engineering from the University of Connecticut and a graduate certificate in science communication from the University of California, Santa Cruz.