Researchers studying the flu vaccine in pregnancy have found a hint of a possible link between miscarriage early in pregnancy and the flu vaccine in women who received a certain version of the vaccine two years in a row.
It’s the first study to identify a potential link between miscarriage and the flu vaccine and the first to assess the effect of repeat influenza vaccination and risk of miscarriage. The findings suggest an association, not a causal link, and the research is too weak and preliminary, experts said, to change the advice, which is based on a multitude of previous studies, that pregnant women should get a flu vaccine to protect them from influenza, a deadly disease that may cause serious birth defects.
But the study is likely to raise questions about the safety of the vaccine as flu season gets underway.
‘‘I think it’s really important for women to understand that this is a possible link, and it is a possible link that needs to be studied and needs to be looked at over more [flu] seasons,’’ said Amanda Cohn, senior adviser for vaccines at the Centers for Disease Control and Prevention, which funded the study.
‘‘We need to understand if it’s the flu vaccine, or is this a group of women [who received flu vaccines] who were also more likely to have miscarriages,’’ she said.
Health officials say they understand that the information may be of concern to pregnant women. They advised pregnant women to talk to their health care providers for the most accurate information and to determine the best timing for a flu shot.
The CDC, the American College of Obstetricians and Gynecologists, and the study authors continue to recommend that pregnant women get a flu vaccine during any stage of pregnancy because of the danger influenza poses to women and their developing babies. Vaccination during pregnancy is also the most effective strategy to protect newborns, experts say, because the flu vaccine is not approved for use in infants younger than six months.
Many previous studies have shown that flu vaccines can be given safely during pregnancy, including numerous studies that found no link between flu vaccination and miscarriage.
The new findings were part of an observational study published Wednesday in the journal Vaccine. The researchers who conducted the study emphasize that it is not a reason to avoid the flu vaccine, even for pregnant women.
Scientists at Wisconsin’s Marshfield Clinic compared 485 pregnant women, ages 18 to 44, who had a miscarriage to 485 pregnant women of similar ages who had normal deliveries during the flu seasons of 2010-2011 and 2011-2012. Of the women who miscarried, 17 had received flu vaccine in the 28 days before the miscarriage, and had also been immunized the prior flu season. Most miscarriages occurred in the first trimester, but several occurred during the second trimester. The median age of the fetus at time of miscarriage was 7 weeks.
By comparison, of the women who had normal deliveries, four who had received the flu vaccine in the preceding 28 days had also been vaccinated during the previous flu season.
‘‘We only saw the link between vaccination and miscarriage if they had been vaccinated in the season before,’’ said James Donahue, an epidemiologist and lead author.