Late-Pregnancy Flu Vaccination Improves Infant Protection

Ingrid Hein

August 26, 2016

Infants are 33% less likely to be hospitalized for respiratory illness in the first 6 months of life when their mothers are vaccinated for influenza during the third trimester of pregnancy, new research shows.

This offers mothers extra incentive to get vaccinated, said lead researcher Annette Regan, PhD candidate, from the Communicable Disease Control Directorate in the Department of Health in Western Australia.

"Mothers are less likely to get vaccinated to protect themselves, but if the vaccination will protect their babies, they are more interested," she told Medscape Medical News.

Because infants cannot be vaccinated for seasonal influenza before they are 6 months old, maternal vaccination is the only protection they have against the virus, she explained.

Previous studies have shown that maternal vaccination during pregnancy has a protective effect against respiratory illness in newborns; however, "this is the first study in which we were able to look specifically at when the women received the vaccine and how well it worked to protect the baby," said Regan.

She presented the results at the Options for the Control of Influenza 2016 Conference in Chicago.

Regan and her colleagues used probabilistic matching to look at the state vaccination records of 31,028 infants born from March 2012 to December 2013 and their mothers to determine how many infants had maternal protection from the virus.

The mothers of 3169 of these infants received a seasonal flu vaccine 14 days or more before delivery and the mothers of 27,859 infants were not vaccinated during pregnancy.

The 96 mothers who received the vaccine in their first trimester were excluded from the analysis, leaving the 1520 mothers vaccinated during their second trimester and the 1553 vaccinated during their third trimester.

Of the 732 infants with a respiratory hospital admission documented during their first 6 months, 69% had bronchiolitis, and 8% of these were attributed to laboratory-confirmed influenza.

There were fewer hospitalizations for respiratory illness among children of vaccinated mothers than among children of unvaccinated mothers (21.0 vs 30.2 per 100,000 infant-days).

During influenza season, children of vaccinated mothers were 25% less likely to be hospitalized for an acute respiratory illness than children of unvaccinated mothers (adjusted hazard ratio [aHR], 0.75; 95% confidence interval [CI], 0.56 - 0.99; P = .04).

What's more, infants born to mothers vaccinated during their third trimester were 33% less likely to be hospitalized than those born to unvaccinated mothers (aHR, 0.67; 95% CI, 0.47 - 0.95; P = .03).

Timing the vaccination can be a challenge, Regan noted. "There are many factors to consider when vaccinating. It depends on when a woman is pregnant, the year, and when the baby is going to be born. If she's late in her last trimester, you'll still want to vaccinate her."

Other studies looking at the burden of influenza have focused on pandemic infection, she explained. Her team wanted to put seasonal influenza vaccines under the microscope because young infants are at increased risk for hospitalization for influenza and respiratory disease.

Vaccination adherence in pregnant mothers is still relatively low, despite evidence showing benefits to both mother and child. In the dataset used by the Regan's team, only about 10% of pregnant women were vaccinated.

However, vaccine coverage is improving, Regan reported. Flu shot uptake during pregnancy increased from 6.4% in 2012 to 13.6% in 2013 in Australia.

She said she hopes the data from this study will help improve adherence. "Mothers are not as concerned about their own health, but when they find out it could help their baby, they could be more likely to get vaccinated," she pointed out.

This study confirms data published earlier this year, which showed that before the age of 6 months, babies born to vaccinated mothers are about 64% less likely to develop flu symptoms than infants born to mothers not vaccinated during pregnancy, and 70% less likely to have confirmed infections (Pediatrics. 2016;137:e20152360). But that study did not look at vaccination during different trimesters.

In another recent study, researchers assessed the success of maternal vaccination in South Africa, and found that babies born to vaccinated mothers got the best protection in their first 8 weeks of life, but that the protection diminished after 8 weeks (JAMA Pediatr. Published online July 5, 2016).

This study adds to "the increasing evidence showing no harmful fetal effects of immunizing pregnant woman against influenza," said Deshayne Fell, MSc, from the Children's Hospital of Eastern Ontario Research Institute in Ottawa, Ontario, Canada.

"Moreover, this population-based study showing effectiveness in reducing influenza among newborn babies of mothers who were immunized concurs with clinical trial evidence," Fell pointed out. "These are important messages for pregnant women and their care providers."

Ms Regan and Ms Fell have disclosed no relevant financial relationships.

Options for the Control of Influenza 2016 Conference: Abstract 076. Presented August 26, 2016.

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