Receiving the influenza A (H1N1) vaccination during pregnancy did not increase the risk for fetal mortality, according to study results published in The New England Journal of Medicine.
The vaccine also reduced the mother’s risk for influenza during pregnancy and may have reduced the risk for influenza-related fetal death.
“It is reassuring that we do not find any indications of a harmful effect on the fetus of vaccination during pregnancy,” Siri Håberg, MD, PhD, of the Norwegian Institute of Public Health, told Infectious Disease News. “In addition, our results indicate that influenza infection itself may pose a risk to the unborn child. These findings support the recommendation that pregnant women should be vaccinated against influenza.”
Using national health registry data and physician reimbursement data, the researchers identified 113,331 women who were pregnant in 2009 or 2010 in Norway. Among these pregnancies, 492 ended in fetal death, a rate of 4.3 deaths per 1,000 births. Among the 25,976 women who received the vaccination, there were 78 fetal deaths. Among the 87,335 women who were unvaccinated, there were 414 fetal deaths.
There were 46,491 women in the second or third trimester during the pandemic wave and 54% were vaccinated. The risk for influenza was significantly reduced with vaccination (HR=0.30; 95% CI, 0.25-0.34). Women who developed influenza had a higher risk for fetal death (HR=1.91; 95% CI, 1.07-3.41).
Håberg said because many countries already recommend that pregnant women receive influenza vaccines, these data do not change current recommendations. There are still many unanswered questions regarding the effects of influenza.
“We would like to know a lot more about immunological processes in pregnant women who get infected and which mechanisms may harm the fetus when the mother has influenza,” Håberg said. “We would also like to know more about vaccine response in pregnancy and if these are different in pregnant women.”
Siri Håberg, MD, PhD, can be reached at Norwegian Institute of Public Health, EPFO, PO Box 4404, Nydalen, 0403 Oslo, Norway; email: Siri.firstname.lastname@example.org.
Disclosure: The researchers report no relevant financial disclosures.