Experts Concerned About Study that Links Flu Vaccine to Spontaneous Abortion

Pregnant women are still urged to get vaccinated at some time during their pregnancy, as per CDC recommendations
Pregnant women are still urged to get vaccinated at some time during their pregnancy, as per CDC recommendations

A study published in Vaccine found an association between spontaneous abortion with administration of a flu vaccine containing the H1N1pdm2009 antigen, however the authors stressed their findings do not establish a causal relationship and that more research is needed.

For pregnant women, the inactivated influenza vaccine is recommended at any stage but the data on safety in early pregnancy (1st trimester) is lacking. Study authors aimed to determine whether receiving vaccines containing pH1N1 was tied to spontaneous abortion. They performed a case-control study over the 2010–2011 and 2011–2012 flu seasons documented in the Vaccine Safety Datalink. Cases (spontaneous abortion) and controls (live or stillbirths) were matched on location, date of last menstruation, and age. 

A total of 485 cases and 485 matches were eligible for analysis. Exposure was defined as receipt of inactivated influenza vaccine before the spontaneous abortion date. The primary exposure window was established as 1–28 days before the spontaneous abortion.

For pregnant women receiving the vaccine in the 28-day exposure window, the overall adjusted odds ratio (aOR) was 2.0 (95% CI: 1.1–3.6). No association was seen in other exposure windows, the authors noted. Specifically, the aOR in the preceding 28 days was 3.7 (95% CI: 1.4–9.4) in the 2010–11 season, and 1.4 (95% CI: 0.6–3.3) in the 2011–12 season. 

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Findings from a post-hoc analysis further revealed that this association was seen only among women who received a pH1N1-containing vaccine the previous flu season. The aOR in the 1–28 days was 7.7 (95% CI: 2.2–27.3) among pregnant women who received the vaccine in the previous flu season vs. 1.3 (95% CI: 0.7–2.7) among those not vaccinated in the previous flu season. 

In a statement released today addressing the findings, Jay Butler, MD president of the Association of State and Territorial Health Officials (ASTHO) and chief medical officer of the Alaska Department of Health and Social Services said, “Given the large volume of data indicating the safety of influenza vaccination during pregnancy, ASTHO and its members support current recommendations for use of vaccines during pregnancy.”

“I want the CDC and researchers to continue to investigate this,” Dr. Laura Riley, a Boston-based obstetrician who leads a committee on maternal immunization, told the New York Times. “But as an advocate for pregnant women, what I hope doesn't happen is that people panic and stop getting vaccinated.”

In order to be prepared for the concern that will likely follow these findings, the Centers for Disease Control and Prevention (CDC) contacted the American Congress of Obstetrician and Gynecologists to inform them of the study. Pregnant women are still urged to get vaccinated at some time during their pregnancy, as per the CDC's recommendations.

"This study does not and cannot establish a causal relationship between repeated influenza vaccination and spontaneous abortion [SAB], but further research is warranted," concluded study authors. 

For more information visit sciencedirect.com.