Flu vaccine not associated with adverse pregnancy outcomes

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1. In a large cohort study, trivalent inactivated influenza vaccination during pregnancy was not associated with adverse obstetric events. 

2. Women who were vaccinated against influenza during pregnancy were found to have a significantly lower incidence of gestational diabetes compared with controls, which might represent a manifestation of significant baseline differences between exposure groups. 

Evidence Rating Level: 2 (Good)            

Study Rundown: This retrospective cohort study identified no association between the trivalent inactivated influenza vaccine and adverse obstetric outcomes. Women who received the vaccine had a slightly (insignificantly) higher incidence of chorioamnionitis and a significantly lower incidence of gestational diabetes compared with those who did not. These results add to the growing body of evidence that supports the safety of influenza vaccination in pregnancy.

The strength of this study is a large sample size and multi-site cohort design. One limitation of this study is significant differences existed between the two study groups such as preexisting comorbidities and complications, though attempts were made to control for poverty and access to care. For instance, the association between gestational diabetes and influenza vaccination is more likely to represent a baseline differences in cases and controls than a biologically plausible, causal association. Future studies might employ a prospective design and control for confounders to further characterize adverse outcomes, including the associations identified in this investigation.

Click to read the study in Obstetrics & Gynecology

Relevant Reading: Uptodate: Influenza and pregnancy

Study Author, Dr. Elyse Olshen Kharbanda, MD, MPH, talks to 2 Minute Medicine: Health Partners Institute for Education and Research, Minneapolis, MN

“Our study demonstrates in a large, diverse cohort, that receipt of influenza vaccine during pregnancy was not associated with increased risk for a wide range of adverse obstetric events.  This study should reassure pregnant women and providers that it is safe to receive influenza vaccine during pregnancy.   In future work our team will evaluate potential risks associated with vaccination and preterm delivery, low birth weight and birth defects.

In-Depth [retrospective cohort study]: This retrospective cohort study included pregnant women over a seven year period from seven Vaccine Safety Datalink sites across the U.S. Women who received the trivalent inactivated influenza vaccines during pregnancy (n=74,2929) were identified and matched on age, site, and pregnancy start date in a ratio of one-to-two to unvaccinated pregnant females (n=144,597). Risk of 13 different adverse obstetric events was assessed. Models were adjusted for receipt of first trimester medical care, hospitalization before vaccination, poverty, and other outcome specific factors and given the multiple comparisons, a p-value of <0.005 was considered significant.

There was no difference in the incidence of hyperemesis, chronic hypertension, gestational hypertension, proteinuria, or UTI within 42 days of vaccination. Throughout pregnancy and delivery, vaccination was not associated with a significant increased incidence of proteinuria, UTI, gestational hypertension, preeclampsia/ eclampsia, puerperal infection, venous complications, pulmonary embolism, or peripartum cardiomyopathy. Vaccination was associated with reduced incidence of gestational diabetes (aHR=0.88, p<.001).

By Maren Shapiro and Leah Hawkins, MD, MPH

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