Seasonal Influenza Vaccine May Halve Risk for Stillbirth

Ricki Lewis, PhD

March 31, 2016

Pregnant women in Western Australia who received seasonal trivalent influenza vaccine had less than half of the risk for stillbirth than unvaccinated pregnant women, according to results of a population-based retrospective cohort study published online March 30 in Clinical Infectious Diseases.

“During the 2009 H1N1 pandemic, we saw a similar reduction in stillbirths following vaccination,” said lead author Annette K. Regan, MPH, from the School of Pathology and Laboratory Medicine, University of Western Australia, Crawley, and Western Australia Department of Health, Shenton Park, in a news release. “Our results are particularly exciting since they show we can get the same protection during seasonal epidemics, which occur every winter. Unfortunately, we know that about 40 percent of pregnant women go unvaccinated, missing out on these benefits.”

Regan and colleagues conducted their investigation to follow up on the possibility that the reduction in influenza among pregnant women who were vaccinated during the 2009 H1N1 pandemic might have led to lowering of stillbirth rate. Because studies have shown that stillbirth rate increases during influenza pandemics, as well as during seasonal epidemics, the World Health Organization gives its highest priority for seasonal influenza vaccination to pregnant women.

The researchers analyzed midwives' records for seasonal influenza vaccination and stillbirth among 58,008 women who delivered during the 2012 and 2013 winter influenza seasons in the southern hemisphere. Stillbirth was defined as birth at 20 or more weeks' gestation with an Apgar score of zero at 1 and 5 minutes.

Of 5076 (8.8%; 6.9% in 2012 and 10.2% in 2013) pregnant women who received trivalent influenza vaccine, 377 had stillbirths (6.5 per 1000 births). The 52,932 unvaccinated women had 5.0 stillbirths per 100,000 pregnancy days compared with 3.0 for vaccinated women. Stillbirth was more common among women with diabetes, women with hypertension, and women who smoked. Indigenous women were about twice as likely to experience stillbirth (odds ratio [OR], 2.04; 95% confidence interval [CI], 1.47 - 2.83), as were other women.

After adjusting for maternal smoking, indigenous status, and propensity for vaccination (accounting for higher-risk pregnancies, in which women are more likely to be vaccinated), the researchers found that stillbirth was 51% less likely among women who had been vaccinated compared with those who had not been vaccinated (adjusted hazard ratio [aHR], 0.49; 95% confidence interval [CI], 0.29 - 0.84). The largest relative reduction in stillbirths corresponded to births immediately after peak influenza season (aHR, 0.33; 95% CI, 0.12 - 0.88).

"These the safety of antenatal administration of seasonal [trivalent influenza vaccine]," the authors write.

"Given the growing body of evidence supporting the health benefits to mother and infant, concerted efforts are needed to improve seasonal influenza vaccine coverage among pregnant women," the researchers conclude. They call for additional research to confirm the reduction in stillbirth attributed to vaccination.

The authors also note that given the methods they used for the analysis, the 51% reduction in risk for stillbirth may be an underestimate of the true benefit.

Limitations of the investigation include reliance on providers to report vaccination and generalizability to pregnant women in developing nations.

The researchers have disclosed no relevant financial relationships.

Clin Infect Dis. Published online March 30, 2016. Abstract


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