See more of the story

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 preliminary, experts said, to change the advice, which is based on a multitude of previous studies, that pregnant women should get a flu vaccine. 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 … that needs to be studied," 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 advised pregnant women to talk to their health care providers 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 fetuses.

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 Marshfield Clinic Research Institute in Wisconsin 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.

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.

If the flu vaccine did somehow make miscarriage more likely during the years in the study, a possible explanation could lie in the makeup of flu vaccine. As a result of the 2009 H1N1 swine flu pandemic, vaccine manufacturers developed vaccines to protect against the new H1N1 strain, which was different from viruses before 2009.

Flu vaccinations of pregnant women increased substantially during and after the pandemic. The authors speculate that the association they observed — if it is real — may be related to an immunological response to having been vaccinated in two consecutive influenza seasons with the same vaccines.