A new study published in the journal Vaccine making the rounds Wednesday is the first to suggest a link (i.e., a correlation, not causation) between pregnant women taking a certain form of the flu vaccine multiple years in a row and miscarriages.
The research, conducted by Centers for Disease Control (CDC) scientists and others from the Marshfield Clinic in Wisconsin and Kaiser Permanente, analyzed the immunizations of 485 pregnant women who had regular baby deliveries as well as 485 women who had miscarriages during flu seasons in 2010-2011 and 2011-2012. In the group of women who miscarried, there was a small but significant number who received vaccines to protect against the H1N1 flu virus strain two years in a row, including one shot within 28 days prior to their miscarriages; this link was only apparent in the women who got the vaccinations in consecutive seasons.
Researchers stressed that this observational data isn't nearly strong enough to warrant changes to vaccine recommendations. "This study does not and cannot establish a causal relationship between repeated influenza vaccination and [miscarriage], but further research is warranted," wrote the study authors.
The CDC is clear in its guidelines that pregnant women in any trimester of their pregnancies should get the flu vaccine. "Flu is more likely to cause severe illness in pregnant women than in women who are not pregnant. Changes in the immune system, heart, and lungs during pregnancy make pregnant women (and women up to two weeks postpartum) more prone to severe illness from flu, including illness resulting in hospitalization," states the public health agency.
The new research, despite its very real limitations and caveats, is still a call for more research into this arena, CDC officials said. For instance, it's possible that pregnant women had a particular immunological response to taking an identical H1N1 vaccine on multiple occasions. But there simply isn't enough information at this time to draw any sort of definitive conclusion.
An unfortunate outcome, said the study authors, would be if the preliminary findings fueled anti-vaccine hysteria and stopped pregnant women from getting the vaccinations that could protect both them and their babies—and which have been proven safe and effective in numerous previous studies.