The H1N1 influenza vaccine did not raise the risk of fetal death when given to pregnant women, according to a report published online Jan. 16 in the New England Journal of Medicine.
In a study analyzing data from Norwegian national health registries following the 2009 pandemic, "we found no evidence that influenza vaccination of pregnant women increased the risk of fetal death. However, influenza virus itself posed a major risk; among pregnant women who received a clinical diagnosis of influenza, the risk of fetal death nearly doubled," said Dr. Siri E. Hâberg of the Norwegian Institute of Public Health, Oslo, and her associates.
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A recent report says that the H1N1 influenza vaccine did not raise the risk of fetal death when given to pregnant women.
Moreover, "vaccination appeared to provide some protection against excess fetal mortality during the pandemic," they noted.
"Our study adds to growing evidence that vaccination of pregnant women during an influenza pandemic does not harm – and may benefit – the fetus."
At the time of the pandemic the vaccine was considered safe and was recommended for pregnant women, who were at particular risk from H1N1. Early anecdotal reports of fetal losses in pregnant women, however, including 30 cases in Norway, raised concerns.
Dr. Hâberg and her colleagues used data from nationwide health registries to examine the issue.
There were 113,331 singleton births among women who became pregnant in Norway during the 2009-2010 flu season. A total of 492 fetal deaths occurred, for an overall rate of 4.3 fetal deaths per 1,000 births. Among 99,539 women who delivered outside the pandemic window, there were 410 fetal deaths, for a rate of 4.1 deaths per 1,000 births.
Just over half (54%) of women pregnant during the pandemic received the flu vaccine. As expected, vaccination substantially reduced the risk that they would contract influenza, they said.
There were 78 fetal deaths among the 25,976 (0.3%) women who were vaccinated during pregnancy and 414 among the 87,335 (0.5%) who were unvaccinated.
Compared with the reference group of women who were pregnant either before or after the pandemic, those who were pregnant during the pandemic and acquired influenza showed a markedly increased risk of fetal death, with an adjusted hazard ratio of 1.91. In contrast, the risk of fetal death was slightly lower in women who were pregnant during the pandemic and were vaccinated, with an adjusted HR of 0.88, the investigators said (N. Engl. J. Med. 2013 Jan. 16 [doi:10.1056/NEJMoa1207210]).
These findings remained robust in several further analyses, including a substudy that included women with multiple births; another that excluded cases in which the vaccine was administered during the first trimester; and another analysis that adjusted for variables such as maternal diabetes, chronic illness, body mass index, and smoking status.
"We also considered nonfatal birth outcomes (preterm delivery, low birth weight at term, and low Apgar score at term) and found no evidence of an association between vaccination and these outcomes," Dr. Hâberg and her associates wrote.
"We found no basis for withholding influenza vaccination from pregnant women in their second or third trimester – an important group, given that these women can be particularly vulnerable to the severe effects of influenza virus infection," they said.
Recent studies in Denmark and Canada "have likewise shown no evidence that influenza vaccination during the 2009 pandemic increased the risk of stillbirth or other adverse birth outcomes," the researchers added.
This study was supported by the Norwegian Institute of Public Health and the U.S. National Institute of Environmental Health Sciences. Disclosures for Dr. Hâberg and her associates were not available. The makers of the vaccines assessed in this study had no role in the study design, implementation, or funding.