Influenza is a serious disease for anyone, but especially for pregnant women who may experience high rates of hospitalization and death after infection. The best way to prevent infection with influenza is by getting vaccinated.
The inactivated influenza vaccine has been recommended for women in any stage of pregnancy since 2004. While the evidence for safety is substantial in the 2nd and 3rd trimesters, it is more limited in early pregnancy especially for the newer vaccines that protect against the virus that caused the 2009 influenza pandemic (pH1N1). To assess the vaccine’s safety, investigators at the Marshfield Clinic, Center for Clinical Epidemiology and Public Health, the Centers for Disease Control and Prevention (CDC), and other researchers in the Vaccine Safety Datalink (VSD) sought to determine if the vaccine containing pH1N1 antigens was associated with miscarriage, the most common serious adverse event of pregnancy. The VSD is a collaborative project between the CDC and 8 health care organizations designed to monitor the safety of vaccines in the U.S.
The investigators compared nearly 500 women in VSD who had a miscarriage with the same number of those who did not have a miscarriage during the 2010-11 and 2011-12 flu seasons. The study found a significant association between miscarriage and receipt of flu vaccine in the prior 28 days, but there was no association if they were vaccinated more than 28 days before the miscarriage. In addition, this association was seen only among women who had also been vaccinated in the previous flu season (i.e., vaccinated for influenza in two consecutive years). This finding was unexpected and differed with an earlier study which found no association between miscarriage and vaccination. The previous investigation was also a VSD study led by the Marshfield team; the main difference with the current investigation is that it studied women who were pregnant during the 2005-06 and 2006-07 flu seasons, i.e., before the 2009 pandemic.
For obvious reasons, the findings from this study may be concerning for pregnant women. However, these results do not demonstrate a causal relationship between the vaccine and miscarriage, but rather they represent a safety signal that requires additional investigation. One such investigation is already underway in VSD with results expected in late 2018 or early 2019. Because many other studies have found the vaccine to be safe in pregnant women, the recommendation from the CDC is clear and unchanged: Pregnant women should get the influenza vaccine because it is the best way to prevent an infection that is dangerous to women and their babies. Women who are pregnant should talk to their doctor about the best time to receive the flu vaccine based on their individual circumstance. See the following link for more information: https://www.cdc.gov/flu/professionals/vaccination/vaccination-possible-safety-signal.html
Marshfield Clinic investigators included Jim Donahue DVM PhD, Burney Kieke MA, Jennifer King MPH, Maria Mascola MD MPH, and Ed Belongia MD.