Study Confirms Benefits of Flu Vaccine for Pregnant Women

While everyone is being urged to get the flu vaccine as soon as possible, some pregnant women avoid it in the belief that it may harm their babies. A large new study confirms that they should be much more afraid of the flu than the vaccine.

Norwegian researchers studied fetal death among 113,331 women pregnant during the H1N1 flu pandemic of 2009-2010. Some 54,065 women were unvaccinated, 31,912 were vaccinated during pregnancy, and 27,354 were vaccinated after delivery. The scientists then reviewed hospitalizations and doctor visits for the flu among the women.

The results were published on Thursday in The New England Journal of Medicine.

The flu vaccine was not associated with an increased risk for fetal death, the researchers found, and getting the shot during pregnancy reduced the risk of the mother getting the flu by about 70 percent. That was important, because fetuses whose mothers got the flu were much more likely to die.

Unvaccinated women had a 25 percent higher risk of fetal death during the pandemic than those who had had the shot. Among pregnant women with a clinical diagnosis of influenza, the risk of fetal death was nearly doubled. In all, there were 16 fetal deaths among the 2,278 women who were diagnosed with influenza during pregnancy.

Dr. Marian Knight, a professor at the perinatal epidemiology unit of the University of Oxford, who was not involved in the research, called it “a high-quality national study” that shows “there is no evidence of an increased risk of fetal death in women who have been immunized. Clinicians and women can be reassured about the safety of the vaccine in the second and third trimesters of pregnancy.”

The Norwegian health system records vaccinations of individuals and maintains linked registries to track effects and side effects. The lead author, Dr. Camilla Stoltenberg, director of the Norwegian Institute of Public Health, said that there are few countries with such complete records.

“This is a great study,” said Dr. Denise J. Jamieson, an obstetrician and a medical officer at the Centers for Disease Control and Prevention, who was not involved in the work. “It’s nicely done, with good data, and it’s additional information about the importance of the flu vaccine for pregnant women. It shows that it’s effective and might reduce the risk for fetal death.”

In Norway, the vaccine is recommended only in the second and third trimesters, so the study includes little data on vaccination in the first trimester. The C.D.C. recommends the vaccine for all pregnant women, regardless of trimester.

“We knew from other studies that the vaccine protects the woman and the newborn,” Dr. Stoltenberg said. “This study clearly indicates that it protects fetuses as well. I seriously suggest that pregnant women get vaccinated during every flu season.”