It’s been long observed by doctors that the flu hits pregnant women harder than it does other healthy adults. Researchers from Stanford in conjunction with Lucile Packard Children’s Hospital have found why: The pregnant body sends out an unusually strong immune response to the influenza virus, offering mother and baby better protection but subsequently also increasing symptom severity. The study’s release coincides with the arrival of flu season, further emphasizing the importance of a flu shot.

The study, which is published online in the Proceedings of the National Academy of Sciences, is the first to examine the reactions of immune cells taken from pregnant women when exposed to the various influenza viruses, a press release reported. Researchers took cells from 21 pregnant women and 29 non-pregnant women, all of which were reported as being in good health, and exposed them to pandemic H1N1 and a strain of seasonal influenza H3N3 while in a lab.

Traditionally, doctors believed that the immune response was weakened by pregnancy to prevent the woman’s body from rejecting her fetus. What researchers observed contradicted this. Instead, the pregnancy was able to actually enhance the immune response to H1N1 of two types of white blood cells: natural killers and T cells. Compared against the cells of the non-pregnant women, the cells of pregnant women helped to attract other immune cells to the site of the infection.

The study’s findings could potentially change treatment options for pregnant women with the flu. Currently, pregnant women with the flu are given drugs to slow down the virus’s replication. "We now understand that severe influenza in pregnancy is a hyperinflammatory disease rather than a state of immunodeficiency,” Dr. Catherine Blish, the study’s senior author, explained in the press release. Blish went on to explain that this suggests that treating the flu in pregnant women may have “more to do with modulating the immune response than worrying about viral replication.”

According to the Centers for Disease Control and Prevention, the flu is particularly dangerous in pregnant women. Not only are its symptoms often more severe, but women who contract the flu when with child have increased chances of life-threatening problems for mother and baby. Lead author Dr. Alexander Kay explained that his team’s findings will potentially lead to new drug developments of "immune-modulating treatment approaches in the setting of severe influenza, especially in pregnant women,"

Although pregnant women are rightfully wary about putting things into their body, the flu vaccines have been proven to be safe for use in all stages of pregnancy.” All pregnant women should get the vaccine because it's 100 percent safe in pregnancy," maternal-fetal medicine Dr. Erin Burnett said told News 4 JAX.

The CDC describes a flu shot as the first and most important step in influenza protection. Burnett gave one extra reason for moms-to-be to get vaccinated: "I don't think a lot of moms realize it, but when they form antibodies, they actually get transmitted to the baby, so the baby then actually is then safe and decreases their risk of getting the flu, which is really important because babies less than 6 months can't get the flu vaccine.”