(HealthDay News) – Vaccinating pregnant women against the influenza virus appears to have a significant positive effect on infant birth weight, according to a study published online Feb. 21 in CMAJ, the journal of the Canadian Medical Association.

Mark C. Steinhoff, MD, of Johns Hopkins University in Baltimore, and colleagues performed a secondary analysis of 340 pregnant women in Bangladesh participating in the Mother’s Gift project, a randomized study designed to test the effectiveness of inactivated influenza and 23-valent pneumococcal polysaccharide (control) vaccines during pregnancy. Outcomes of maternal influenza immunization were assessed when influenza virus was not circulating and when influenza virus was circulating.

During the period with no circulating influenza virus, the researchers found that there were no differences in the incidence of respiratory illness with fever per 100 person-months among mothers and infants in the two groups. The number of small-for-gestational-age infants and the mean birth weight were similar between groups. There was a significant reduction seen in the incidence per 100 person-months of respiratory illness with fever among the mothers and infants who had received the influenza vaccine during the period with circulating influenza virus. During this period, in the influenza vaccine group, there was a significantly lower proportion of small-for-gestational-age infants, and the mean birth weight was higher.

“During the period with circulating influenza virus, maternal immunization during pregnancy was associated with a lower proportion of infants who were small for gestational age and an increase in mean birth weight,” the authors write.

One of the authors disclosed a financial relationship with Merck.

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