September 14, 2017
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Early miscarriage potentially linked to influenza vaccination

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Researchers discovered a potential link between miscarriage early in pregnancy and receipt of the influenza A(H1N1)pdm09-containing vaccine in the preceding 28 days, especially among women who had received the same vaccine during the previous influenza season.

“Since 2004, the Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices (ACIP) and other organizations have recommended routine influenza vaccination for pregnant women regardless of gestational age,” James G. Donahue, PhD, MPH, from the Marshfield Clinic Research Institute, and colleagues wrote in Vaccine.

“Influenza in pregnancy can cause serious, life-threatening illness in both the mother and fetus, as demonstrated during the 2009 pandemic,” the researchers wrote. “Numerous studies of influenza vaccine during pregnancy have not identified serious safety concerns, but relatively few investigations have evaluated vaccination in the first trimester, a period when the embryo is highly vulnerable to teratogens and other factors.”

Donahue and colleagues performed a case-control study during two influenza seasons (2010-2011 and 2011-2012) to investigate the relationship between receipt of vaccine containing A(H1N1)pdm2009 (pH1N1) and miscarriage.

Using data from the Vaccine Safety Datalink, researchers identified 485 eligible women aged 18 to 44 years who had a miscarriage (cases) and 485 women who had live births or stillbirths (controls), and matched them on site, date of last menstrual period and age. Participants who were vaccinated with the inactivated influenza vaccine before the miscarriage date were considered exposed. Primary exposure was between one and 28 days before miscarriage.

During the primary exposure window, the overall adjusted OR for vaccine receipt was 2 (95% CI, 1.1-3.6). Specifically, the aOR was 3.7 (95% CI, 1.4-9.4) for the 2010 to 2011 season and 1.4 (95% CI, 0.6-3.3) for the 2011 to 2012 season. The post hoc analysis revealed a significant effect modification by influenza vaccination in the prior season (P = .02) for each season.

Vaccine receipt in the 1- to 28-day period was associated with a significantly increased aOR (7.7; 95% CI, 2.2-27.3) for miscarriage in women who were also vaccinated in the previous season. Regardless of current influenza vaccination status, women who did not receive the vaccine in the previous season were not at an increased risk for miscarriage (aOR = 1.3; 95% CI, 0.7-2.7).

At the time of miscarriage, the median gestational age was 7 weeks. There were no statistically significant associations between miscarriage and receipt of inactivated influenza vaccine during any other exposure windows.

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“It is important to note that this study does not and cannot confirm a causal association, but the validity of the major findings is supported by the effect modification across two influenza seasons and the observation of elevated odds ratios in the 1 to 28 day exposure window only,” Donahue and colleagues concluded. “More research is needed regarding the immunologic effects of influenza vaccination during pregnancy. A follow-up study funded by CDC is currently underway to evaluate the risk of [miscarriage] after repeated influenza vaccination during the 2012-13, 2013-14 and 2014-15 influenza seasons; results are expected by late 2018.”

In response to these findings, the CDC, ACIP and the American College of Obstetrics and Gynecology (ACOG) reaffirmed their recommendations that women receive routine influenza vaccine during pregnancy regardless of trimester.

“In evaluating all of the available scientific information, there is insufficient information to support changing the current recommendation,” Haywood L. Brown, MD, president of ACOG, said in a press release.

“Influenza vaccination is an essential element of prenatal care because pregnant women are at an increased risk of serious illness and mortality due to influenza,” he said. “In addition, maternal vaccination is the most effective strategy to protect newborns because the vaccine is not approved for use in infants younger than 6 months.” – by Alaina Tedesco

Disclosures: Donahue reports no relevant financial disclosures. Please see the study for all other researchers’ relevant financial disclosures.