Kaiser Permanente study illustrates “bonus” protection of infants, who can’t get influenza vaccine until they are 6 months old
Pregnant patients are encouraged to get vaccinated against influenza because the illness can be more severe in pregnancy. New research from Kaiser Permanente shows prenatal inoculation also benefits infants during their first months of life.
The bonus protection for babies is a secondary but important reason for flu vaccination in pregnancy, said study lead author Ousseny Zerbo, PhD, a research scientist with the Kaiser Permanente Division of Research. “Our study emphasizes that when you are vaccinating yourself in pregnancy, you are protecting yourself, but you are also protecting your infant at a crucial time,” Zerbo said.
The study, published in Obstetrics & Gynecology, followed 245,498 infants born at Kaiser Permanente Northern California from birth to age 6 months, over 10 flu seasons (2011 to 2022). The researchers found that 46% of the children were born to mothers who were vaccinated against flu during pregnancy. Those children were 44% less likely to be diagnosed with laboratory-confirmed influenza than those born to unvaccinated mothers.
The study found protection against flu was strongest for the infants during the first 2 months of life, though protection extended through 6 months. It also found the strongest protection when mothers get vaccinated in the second and third trimesters of pregnancy.
The difference in protection by pregnancy trimester should not influence when patients get vaccinated in pregnancy, said senior author Nicola Klein, MD, PhD, who heads Kaiser Permanente’s Vaccine Study Center. “The primary goal is protecting the mother from serious outcomes from the flu, so it’s important for her to get the flu shot,” Klein said.
However, only about half of pregnant patients get a flu shot, which is less than some other vaccines, and that percentage has gone down since the Covid-19 pandemic. Since the vaccination provides double coverage of both the mother and baby, flu vaccination rates should be higher, Klein said. Previous research shows getting a flu shot in pregnancy reduced the risk of being hospitalized by 40%. “In this study we found that it might also reduce the risk of infant hospitalization by 48%,” Klein said.
The study was funded by the National Institute of Allergy and Infectious Disease.
Additional co-authors were Sharareh Modaressi, MD, MPH, and Kristin Goddard, MPH, of the Vaccine Study Center (VSC) and Bruce Fireman, MA, formerly of the VSC.
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About the Kaiser Permanente Division of Research
The Kaiser Permanente Division of Research conducts, publishes, and disseminates epidemiologic and health services research to improve the health and medical care of Kaiser Permanente members and society at large. KPDOR seeks to understand the determinants of illness and well-being and to improve the quality and cost-effectiveness of health care. Currently, DOR’s 720-plus staff, including 73 research and staff scientists, are working on nearly 630 epidemiological and health services research projects. For more information, visit divisionofresearch.kp.org or follow us @KPDOR.
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