Getting the flu appears to nearly double a pregnant woman’s risk of having a miscarriage or stillbirth, data from Norway during the 2009-2010 global flu pandemic show. But getting vaccinated during pregnancy greatly reduces a woman’s risk of flu, researchers report online January 16 in the New England Journal of Medicine.
The study also finds that getting a flu shot during pregnancy is safe. Anecdotal reports had suggested that flu vaccination during gestation might have adverse effects on the fetus, but the new study — as well as two previous reports, from Canada and Denmark — now show no such connection.
“I think this is a strong finding,” says Lone Simonsen, an epidemiologist at George Washington University in Washington, D.C. “It’s good to see a carefully done, large study like this.”
Physician Camilla Stoltenberg of the Norwegian Institute of Public Health in Oslo and her colleagues scanned Norway’s national registry of medical information and identified more than 100,000 pregnancies during late 2009. Pregnant women who received the flu vaccine were one-third as likely to get the flu as were unvaccinated pregnant women.
“Our results confirm findings from other recent studies that have found no association between [flu] vaccination and stillbirth or other adverse events in pregnancy,” Stoltenberg says.
Then the study team tabulated miscarriages and stillbirths that occurred during the second and third trimesters of pregnancy. Among children born to roughly 26,000 vaccinated mothers, there were 78 fetal deaths; 414 fetal deaths occurred among 87,000 unvaccinated mothers. The researchers calculate that women who got the flu were almost twice as likely — 91 percent — to lose the fetus than were uninfected women.
Miscarriages during the first trimester, which experts note are very difficult to calculate, weren’t included in the study. “Miscarriages very early in pregnancy are sometimes not recognized by the women, and many are not treated in the health care system,” Stoltenberg says.
Influenza hits pregnant women harder than other women, and flu shots have been recommended for pregnant women for decades, says Deshayne Fell, an epidemiologist at Better Outcomes Registry & Network Ontario in Ottawa. Yet many pregnant women don’t get the shot. “One misconception women have is that it’s no big deal — it’s just the flu,” she says. “But it can be a big deal.”
The risk to the fetus shown in the new study might also apply to flu that strikes in the first trimester, other work has suggested. While analyzing medical records dating from the 1918 flu pandemic in the United States and Scandinavia, Simonsen and her colleagues found a sharp decline in births in May 1919. That would have coincided with a rash of first-trimester flu infections six or seven months earlier — in the autumn of 1918, the peak of the pandemic, they reported in 2011 in the Journal of Infectious Diseases.The mechanism connecting flu infection and increased fetal death risk is unknown, but scientists suspect high fever and systemic inflammation in the mother, Fell says.
S. Haberg et al. Risk of fetal death after pandemic influenza virus infection or vaccination. New England Journal of Medicine. doi: 10.1056/NEJMoa1207210.
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