Infants born to mothers who received flu immunization during pregnancy were 70% less likely to contract lab-confirmed influenza and 81% less likely to be hospitalized for flu before age 6 months, compared with infants of unimmunized mothers, a study reaffirmed.

Yet just one in ten pregnant women received the vaccine, a proportion that has steadily risen since the 2009-2010 H1N1 flu season.

“The results of this large retrospective study support the conclusions of prospective studies regarding the protective benefit of maternal influenza immunization during pregnancy,” reported Dr. Julie H. Shakib of the University of Utah, Salt Lake City, and her associates (Pediatrics. 2016 May 3. doi: 10.1542/peds.2015-2360). “Interventions that target both healthy pregnant women and those with chronic conditions are needed to increase vaccine uptake,” they wrote.

The researchers analyzed self-reported seasonal influenza immunization uptake in the 245,386 women who gave birth between December 2005 and March 2014 in the Intermountain Healthcare facilities in Utah and Idaho. Although 10% of the women overall received flu vaccinations, just 2.2% of the women who delivered before the 2009-2010 H1N1 pandemic had received them. That number rose after the pandemic to 21% (P < .001). More than half (52%) of the women giving birth during the 2013-2014 flu season reported getting the seasonal flu vaccine.

Among the women’s 249,387 infants, 866 had at least one influenza-like illness (ILI), including 32 born to vaccinated women and 834 born to unvaccinated women. The infants born to women receiving the flu vaccine during pregnancy were 64% less likely to develop an ILI, with illnesses in 1.34 per 1,000 born to vaccinated women and 3.7 per 1,000 born to unvaccinated women (relative risk, 0.36).

The rates of laboratory-confirmed influenza in the 658 children who contracted it were 0.84 per 1,000 children born to vaccinated mothers and 2.83 per 1,000 children born to unvaccinated mothers, translating to a 70% lower risk of flu in those born to vaccinated mothers (RR, 0.30). Similarly, infants born to vaccinated mothers were 81% less likely to be hospitalized for lab-confirmed influenza (RR, 0.19, P = .005). Just 3 of 151 hospitalized infants had been born to mothers who received the flu vaccine, for a rate of 0.13 per 1,000 for children of vaccinated mothers and 0.66 per 1,000 for children of unvaccinated mothers.

Pregnant women with public insurance or no insurance were less likely to report getting the seasonal vaccine than were privately insured women, but those with chronic conditions were more likely to be vaccinated. Uptake also was lower among women with incomes below the federal poverty level and among women living in either rural or frontier areas or in the Urban South or Southwest Intermountain regions.

“The Intermountain Urban South region includes Utah County, 1 of 30 U.S. counties with the largest estimated numbers of unvaccinated children from 1995-2001 CDC National Immunization Surveys (NIS) data,” the authors wrote. “It is possible that factors leading to parental vaccine hesitancy in children may similarly affect pregnant women considering maternal immunization during pregnancy.”

Because of widespread testing for respiratory viruses at Intermountain facilities, the researchers also could determine that flu vaccine receipt among the mothers did not affect incidence of RSV.

“Our study strengthens the evidence that maternal immunization provides passive protection against influenza to infants during the vulnerable period before they are old enough to receive active immunization,” Dr. Shakib and her associates wrote.

The research was funded by the National Institutes of Health, the University of Utah Children’s Health Research Center and the Pediatric Clinical and Translational Scholar Program, the H.A. and Edna Benning Presidential Endowment, and the University of Utah Center for Clinical and Translational Science through the National Center for Research Resources and the National Center for Advancing Translational Sciences. The authors reported no disclosures.


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