Study: Flu shots in pregnant women benefit newborns

Sep 18, 2008 (CIDRAP News) – A study conducted in Bangladesh suggests that an influenza shot during pregnancy lowers the risk of influenza both for the woman and for the baby in the first 6 months of life.

The study by a team from Bangladesh and the United States showed that vaccination lowered the risk of lab-confirmed flu in babies up to the age of 24 weeks by 63%, according to the report, published online yesterday by the New England Journal of Medicine. The immunization also appeared to reduce the risk of febrile respiratory illness by 29% in the infants and 36% in their mothers.

"Our data show that a single dose of maternal influenza vaccine provides a considerable two-for-one benefit to both mothers and their young infants," says the report by K. Zaman of the International Centre for Diarrheal Disease Research in Dhaka, Bangladesh, and colleagues from that center and the Johns Hopkins Bloomberg School of Public Health in Baltimore.

"The study is the first to demonstrate that the inactivated influenza vaccine provides protection to both mother and newborn," Johns Hopkins Bloomberg officials said in a news release.

In the United States, pregnant women have been advised to get an annual flu shot since 1997, but a survey by the Centers for Disease Control and Prevention (CDC) showed that only about 13% did so in 2007. The shots are not recommended or approved for babies younger than 6 months, who have a high risk of hospitalization if they contract flu. The CDC has recommended flu vaccination for 6- to 23-month-olds since 2003.

63% flu reduction in babies
In the randomized, blinded trial, the researchers recruited 340 healthy, unvaccinated pregnant women who had no history of pregnancy complications. The women were randomly assigned to receive either inactivated flu vaccine or a 23-valent pneumococcal vaccine. After the women gave birth, their babies were assigned to receive either a pneumococcal conjugate vaccine or a haemophilus influenzae type b (Hib) vaccine.

The mothers and infants were observed from August 2004 though October 2005. Most of the women were followed from 2 weeks after immunization through delivery, and all were interviewed weekly to check for illnesses from the time of childbirth until the babies were 24 weeks old. Of the 340 women recruited, 316 mother-infant pairs completed the 24 weeks of surveillance.

Among the 159 infants whose mothers received flu shots, 6 had lab-confirmed flu, versus 16 of the 157 infants whose mothers were in the control (pneumococcal vaccine) group. That signified a 63% reduction in risk of flu (95% confidence interval [CI], 5% to 85%), the report says.

In addition, febrile respiratory illnesses occurred in 110 infants in the flu-vaccine group and 153 infants in the control group, signaling a 29% reduction in risk (95% CI, 7% to 46%). Similarly, 50 of the immunized mothers had a febrile respiratory illness, compared with 77 of those in the control group, indicating a 36% lower risk (95% CI, 4% to 57%).

Further, flu vaccination was associated with improvements in other clinical outcomes in the infants, including a 42% reduction in clinic visits for respiratory illness with fever and a 49% reduction clinician testing for influenza, the report states.

Although the confidence intervals were wide, the estimated reductions in confirmed flu and other clinical outcomes were similar to those reported in flu-immunization trials in infants older than 6 months, the authors write.

They observe that the pneumococcal vaccination of control-group mothers might have reduced the rate of pneumococcus-related respiratory illness in the babies, which would have reduced the apparent effectiveness of maternal flu vaccination in preventing overall febrile respiratory ailments.

The authors conclude that every 100 flu shots in pregnant women prevented febrile respiratory illness in 14 infants and 7 mothers. "In other words, five pregnant women would need to be vaccinated to prevent a single case of respiratory illness with fever in a mother or infant," they write. The results also indicate that it took fewer than 16 maternal flu shots to prevent one confirmed flu case in their babies.

Applicable to temperate climes?
Janet Englund, MD, an associate professor in pediatric infectious disease, allergy, and rheumatology at the University of Washington in Seattle, called the study "an interesting paper with an expected outcome" and one that "shows the potential utility of flu vaccine in pregnant women." She expressed a hope that it will spur efforts to increase the vaccination rate among pregnant women.

Englund, who serves on the CDC's Advisory Committee on Immunization Practices, said one caveat is that the study was conducted in a tropical country where flu circulates year-round, unlike the United States, with its winter flu season.

"The fact that this shows a potential benefit could potentially be related to the fact that the study was conducted year-round and the flu virus was there year-round," she told CIDRAP News. She suggested it might be difficult to demonstrate the same benefits in a US study: "You'd have to have a bigger study that was restricted in time, which would be potentially difficult to do."

Zaman K, Roy E, Arifeen SE et al. Effectiveness of maternal influenza immunization in mothers and infants. N Engl J Med 2008 (early online publication) [Full text]

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