Even so, the CDC pointed out limitations in the study and is still recommending that pregnant women get the flu vaccine this season.
The study was published on September 13 in the journal Vaccine and the conclusions were based on 485 women who miscarried and 485 women who did not have a miscarriage.
The researchers found that women who experienced miscarriages between 2010 and 2012 were more likely to have received flu shots two years in a row.
Women who got a flu vaccine containing swine flu (H1N1) protection had an increased risk of miscarriage within 28 days of the shot — but only if they got the same shot in the previous season.
The study raises the possibility that pregnant women had a particular immunological response to taking an identical H1N1 vaccine on multiple occasions, but there isn’t enough data to know for sure.
Most of the miscarriages occurred in the first trimester, but a few occurred in the second trimester.
The CDC said the study does not prove that flu vaccines actually caused the miscarriages and “imprecise results” are possible because the conclusions were based on such a small number of women.
The findings are still concerning, however. There is not very much data on the safety of flu vaccines in the first trimester of pregnancy, which was what prompted the study in the first place.