Pregnant women who received vaccines containing the A/H1N1pdm2009 (pH1N1) antigen for 2 consecutive influenza seasons may have increased risk for spontaneous abortion 28 days postvaccination, according to recent findings published in Vaccine.[1]

Researchers from 7 US research institutes and the Centers for Disease Control and Prevention conducted a case-control study of the 2010-2011 and 2011-2012 influenza seasons by gathering data from the Vaccine Safety Datalink on 485 women, aged 18 to 44 years, who were pregnant and vaccinated with the pH1N1 antigen. Researchers compared women who had spontaneous abortions with those who had live births or stillbirths during the 2 seasons to evaluate spontaneous abortion risk after receipt of the vaccine. Because immune response to the influenza vaccine peaks in the first 4 weeks after vaccination, and immunity is naturally suppressed during pregnancy to protect the fetus, the researchers were especially interested in spontaneous abortions that occurred 1 to 28 days after vaccination.

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Among women who received pH1N1-containing influenza vaccine in the previous season, the adjusted odds radio (aOR) in the first 28 days was 7.7 (95% CI, 2.2-27.3) vs 1.3 in women who had not been vaccinated (95% CI, 0.7-2.7).

In spite of these findings, the researchers commented that the weight of evidence points to the influenza vaccine being safe for pregnant women, especially after the first trimester. Also, it is important to note that this study establishes only an association. However, the implications do warrant further research.

Update: The Centers for Disease Control and Prevention updated their guidelines after these results were released to state that pregnant women should continue receiving the influenza vaccine. This study was a case-control study, and therefore “cannot be used to estimate the probability of miscarriages for pregnant women who received an H1N1-containing flu vaccinations two years in a row.”[2]

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  1. Donahue JG, Kieke BA, King JP, et al. Association of spontaneous abortion with receipt of inactivated influenza vaccine containing H1N1pdm09 in 2010-11 and 2011-12. [published online September 13, 2017]. Vaccine. 35(40):5314-53.
  2. Flu vaccination & possible safety signal [press release]. CDC. Updated September 13, 2017. Accessed October 3, 2017.