Pregnant women’s immune systems overreact to the flu

There’s a commonly held assumption that a pregnant woman’s immune system fades into a weak shell of its former self to keep from attacking the growing fetus. According to the pregnancy advice website What to Expect, “Your immune system runs at low speed when you’re pregnant.” Baby Center agrees. The Cleveland Clinic weighs in, writing that the immune system is lowered slightly during pregnancy, rendering a pregnant woman more susceptible to coughs, colds and flu.

A new study offers a compelling exception to that assumption: When it comes to flu, the immune system of a pregnant woman is supercharged.

The damage from flu doesn’t come from an immune system that’s too weak, the new study suggests. In fact, the opposite might be true: Flu is especially dangerous to pregnant women because their immune reaction is too strong, scientists write September 22 in the Proceedings of the National Academy of Sciences. And dangerous it is: When pregnant women get the flu, they really get the flu. During the 2009 H1N1 pandemic, for instance, pregnant women made up five percent of deaths, even though they accounted for about one percent of the U.S. population. 

In the new study, Stanford University researchers took blood samples from pregnant and non-pregnant women, and then they exposed that blood to the influenza A virus. Immune cells in the pregnant women’s blood showed a more vigorous response to the virus than cells in blood of non-pregnant women, the team found. Two types of immune cells, natural killer and T cells, appeared supercharged, producing more molecules involved in a response. This outsized inflammatory response may “tip the balance from a healthy immune response into one that is destructive, resulting in increased disease severity,” the authors write.

That destruction probably happens in the lungs, where inflamed tissue produces mucus that’s especially hard for pregnant women to clear. It is amazingly hard to cough effectively when a fetus is forcing your lungs up into your throat.

Of course, the study comes with caveats. It was small, and it didn’t test women who were actually infected with influenza, instead relying on blood samples’ responses in dishes. Nonetheless, I think the results are worth talking about. They’re unexpected, and they might ultimately lead to better ways to treat a severe bout of flu in pregnant women. Tamping down the immune response might prove more effective than fighting the virus itself, for instance.

The results also make a strong case for a seasonal flu vaccine during pregnancy, which only half of pregnant women get. Not only do flu shots keep influenza at bay during pregnancy, but they protect newborns too young to get the shot themselves. Babies born to moms who were vaccinated while pregnant were less likely to get the flu themselves, scientists have found. That means that a flu shot during pregnancy may be one way to proactively keep young babies flu-free, instead of having to rely on everyone else to be healthy. Flu vaccinations during pregnancy have also been linked to lower rates of premature birth or low birth weights.

There’s a lot that scientists don’t know about how the immune system changes during pregnancy. But what they do know hints that the answer is a lot more complicated than simply dialing it up or down.

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