Study Reveals Why Flu Is Dangerous in Pregnancy

In a surprise finding, researchers revealed that severe influenza in pregnancy is a hyperinflammatory disease and not a state of immunodeficiency, as previously believed.

Research published online in the Proceedings of the National Academy of Sciences focused on the reactions of immune cells taken from pregnant women and then exposed to the influenza virus. The researchers looked at two strains of the virus: H1N1, which caused the 2009 flu pandemic, and the seasonal H3N2 strain.

Key Findings

- Severe influenza in pregnancy is a hyperinflammatory disease and not a state of immunodeficiency.

- Researchers believe treatment of influenza during pregnancy could be revised if the finding of enhanced immune response to the virus holds up in future studies.

The study was conducted using a small sample of just 21 pregnant women and 29 nonpregnant women. All the women were healthy. The immune response was tested by collecting blood samples from the women before they received flu vaccines and then again, a week after being vaccinated. Cells taken from the pregnant women six weeks after their babies were delivered were also tested.

The results could help inform treatment of influenza during pregnancy, said Catherine Blish, MD, PhD, assistant professor of infectious diseases and the study's senior author, by focusing more on modulating the immune response than on worrying about viral replication.

Specifically, the researchers found a stronger immune response when a pregnant women’s blood was exposed to the virus, compared with the samples from the nonpregnant women. The H1N1 strain caused a pregnant woman’s natural killer cells and T cells to produce more cytokine and chemokine molecules to the infection than what was observed with the samples from the nonpregnant women.

These robust cellular immune responses could drive pulmonary inflammation, explaining increased morbidity and mortality among pregnant women with the flu.

"If the chemokine levels are too high, that can bring in too many immune cells," Blish explained in a news release. "That's a bad thing in a lung where you need air space." It could explain why when pregnant women get the flu, especially during pandemics, they have a heightened risk for pneumonia and death, she said.