HealthDay News — A vaccine against pandemic influenza A (H1N1) infection administered during the 2009 season in Norway did not increase risk for fetal death, suggesting there is no reason to withhold flu vaccines from pregnant women, researchers said.

Vaccination was associated with a significantly lower risk for influenza (HR=0.30; 95% CI: 0.25-0.34) —  infection with which nearly doubles the risk for fetal death (HR=1.91; 95% CI: 1.07-3.41), Siri E. Håberg, MD, of the Norwegian Institute of Public Health in Oslo, and colleagues reported in New England Journal of Medicine.

“Given the danger posed by maternal influenza infection for fetal survival, our study adds to growing evidence that vaccination of pregnant women during an influenza pandemic does not harm — and may benefit — the fetus,” the researchers wrote.

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Two additional studies that assessed health outcomes of pregnant women after vaccination during the 2009 H1N1 pandemic yielded similar results, showing no increases in stillbirths or other adverse birth outcomes.

Pregnant women have elevated risk of flu-related complications, and are considered a priority group for vaccination. Data on the safety of the AS03-adjuvanted monovalent pandemic vaccine (Pandemrix) used in Norway during the 2009 pandemic in pregnant women were lacking at the time, but animal studies “showed no reproductive toxicity, and it was considered safe for use during pregnancy,” the researchers wrote.

To better evaluate the safety of influenza vaccine in pregnant women, Håberg and colleagues examined information from Norwegian national registries and medical consultation data for 117,347 pregnant women who gave birth from 2009 through 2010.

A total of 54% were vaccinated in their second or third trimester. Overall fetal mortality was 4.9 deaths per 1,000 births.

After adjusting for age, parity, marital status, use of nutritional supplements during pregnancy, smoking during pregnancy, previous fetal death, and chronic medical conditions, vaccination was not significantly associated with the risk of fetal death, the researchers found.

Additionally, there was no associations between vaccination and other adverse birth outcomes, including preterm delivery, low birth weight at term and a low Apgar score at term.


  1. Håberg S et al. N Engl J Med 2013; doi: 10.1056/NEJMoa1207210.