[NIH] – Norwegian pregnant women who received a vaccine against the 2009 H1N1 influenza virus showed no increased risk of pregnancy loss, while pregnant women who experienced influenza during pregnancy had an increased risk of miscarriages and still births, a study has found. The study suggests that influenza infection may increase the risk of fetal loss.

Scientists at the U.S. National Institutes of Health and the Norwegian Institute of Public Health (NIPH) published their findings online Jan. 17 in the New England Journal of Medicine. The research was conducted following the H1N1 influenza pandemic that took place between spring 2009 and fall 2010. The researchers at the NIH were from the National Institute of Environmental Health Sciences (NIEHS).

Norwegian public health officials had urged pregnant women to be vaccinated. However, media reports of pregnancy losses after flu shots caused some expectant mothers to forgo vaccination.

First author Siri Haberg, M.D., Ph.D., of the NIPH and colleagues initiated the study to help address the question of vaccine safety, by taking advantage of Norway’s excellent registries and medical records system. Haberg spent one year of her postdoctoral fellowship in the NIEHS Epidemiology Branch before returning home to Norway during the pandemic.

NIEHS researcher and co-author Allen Wilcox, M.D., Ph.D., said the NIPH researchers combined data from obstetrical visits, birth records, and vaccination registries to investigate whether the influenza vaccination posed a risk to pregnancy. The study found that influenza infection increased the risk of fetal loss by up to twofold. Influenza vaccination did not increase the risk of loss. Instead, the results suggest that vaccination reduces the risk of fetal loss.

“Most important is that vaccinations protect pregnant women against influenza illness, which could be harmful for both the mother and the baby,” Wilcox said. “If pregnant women are worried about their fetus, then getting a flu shot is a good thing to do.”

Haberg added, “Pregnant women should find it reassuring that we found no harmful effects on the fetus associated with H1N1 vaccination.”

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