Pam Harrison

April 04, 2014

CAPE TOWN, South Africa — When pregnant women receive the flu vaccine, their risk of developing influenza is halved, as is the risk to their newborns in the first 24 weeks of life, new research shows.

"This is the first randomized controlled study to actually confirm that vaccination is efficacious in pregnant women, and only the second randomized controlled trial to show protection of the infant," said Shabir Madhi, MD, PhD, professor of vaccinology at the University of Witwatersrand in South Africa.

"Our data support the recent WHO recommendation in terms of prioritizing pregnant women for influenza vaccination, not just for the protection of the mother, but protection of the infant as well," he told Medscape Medical News.

Dr. Madhi presented the results here at the 16th International Congress on Infectious Diseases.

The study was conducted in Soweto, South Africa in 2011 and 2012, and was specifically targeted at pregnant women not infected with HIV.

At a mean of 26 weeks into their pregnancy, approximately 1000 women were randomly assigned to receive the trivalent inactivated influenza vaccine and another 1000 were assigned to receive saline placebo.

The trivalent inactivated influenza vaccine did not change over the 2 flu seasons covered by the study.

Once infants were born, they were screened weekly for influenza-like illness. When influenza was suspected, nasopharyngeal aspirates were collected for influenza A and B testing with real-time PCR.

Pregnancy outcomes, including miscarriages and stillbirths, were similar in the vaccinated and placebo groups, Dr. Madhi reported. Rates of preterm deliveries (10.0%) and low-birth-weight infants (12.4%) were also similar in the 2 groups.

There were fewer cases of influenza in infants of mothers in the vaccination group than in infants of mothers in the placebo group (19 vs 37). The vaccine efficacy rate was approximately 49%.

In addition, infants were less likely to develop influenza at the same time as their mothers if their mothers were vaccinated than if their mothers were not (5% vs 50%).

Women were vaccinated at 20 to 36 weeks of gestation "because we wanted to know whether protection of the infant was antibody-related," Dr. Madhi explained.

"Our data suggest that the protective effect is more likely due to reduced transmission of the flu virus from the mother to newborn, rather than reduced antibody transfer. But the WHO recommendation is that pregnant women can be vaccinated at any stage of pregnancy from the first trimester on because there are no safety concerns in terms of fetal outcome."

Dr. Madhi and colleagues previously determined that influenza immunization during pregnancy was 70% effective in HIV-infected women and was 50% effective in the infants of those mothers.

Pregnancy and Severity of Illness

It has long been clear that pregnant women are at risk for more serious influenza than the general population, said Eskild Petersen, MD, associate professor of infectious diseases and internal medicine at Aarhus University Hospital in Denmark.

"If you go back to the Spanish flu days from 1918, pregnant women clearly had a higher mortality rate," he told Medscape Medical News.

Similarly, infants younger than 6 months of age are at greater risk of developing more severe illness than older infants, and of requiring hospitalization.

"You immunize pregnant women partly because they have more severe influenza and partly because you can provide partial protection to the child through the passive transfer of IgG antibodies from the mother to the child," Dr. Peterson explained.

This study was funded by the Bill and Melinda Gates Foundation. Dr. Madhi has disclosed no relevant financial relationships.

16th International Congress on Infectious Diseases (ICID): Abstract 14.001. Presented April 3, 2014.

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