No increased risk associated with flu vaccines given in closely spaced pregnancies

Monitoring of about 45,000 pregnant people receiving flu vaccines in each of 2 successive pregnancies showed no increased risk of side effects or health issues, according to a Kaiser Permanente study published September 19, 2024, in JAMA Network Open.

“Our study’s findings support recommendations to give flu vaccinations to pregnant persons even if they received a flu vaccine during a previous pregnancy,” said lead author of the study, Darios Getahun, MD, PhD, a perinatal epidemiologist with the Kaiser Permanente Southern California Department of Research & Evaluation. “We found that there was no increased risk of poor pregnancy outcomes regardless of the length of time between the pregnancies or the type of flu vaccine the person received.”

This study was led by the Department of Research & Evaluation in partnership with the Centers for Disease Control and Prevention and other integrated health care organizations. It is the first study to assess the safety of seasonal flu vaccination when given in 2 successive pregnancies and investigate if a shorter interval between pregnancies changes the risk. It also was the first to investigate the association between trivalent or quadrivalent flu vaccine type and health issues in successive pregnancies.

Women who are pregnant and their infants are especially vulnerable to sickness and death due to flu. Although the Advisory Committee on Immunization Practices (ACIP) and the American College of Obstetricians and Gynecologists (ACOG) recommend that women receive flu vaccination during pregnancy, coverage has been substantially below 60% in recent years. Flu vaccination during pregnancy reduces the risk of getting the flu as well as flu severity and hospitalization for the vaccinated person. Maternal flu vaccination also protects babies against flu, flu severity, and flu hospitalizations for several months after birth, when they are too young to be vaccinated themselves.

Researchers examined electronic health records of about 82,000 pregnant people within 8 health systems across the country associated with the Vaccine Safety Datalink. The analysis covered a diverse group of pregnant people whose successive pregnancies resulted in live births between 2004 and 2018. About 45,000 women received seasonal flu vaccines during both pregnancies, and about 37,000 did not receive seasonal flu vaccines during both pregnancies. Specifically, researchers looked at health issues including:

  • Preeclampsia, which is a serious blood pressure condition during pregnancy, and eclampsia, which is more severe and can lead to seizures
  • Placental abruption, which is a serious, life-threatening condition in which the placenta separates from the wall of the uterus prematurely
  • Maternal fever
  • Preterm birth, which is when babies are born too early
  • Preterm premature rupture of membranes, a condition in which the amniotic sac — which contains the baby and the fluid surrounding it — breaks before the 37th week of pregnancy before labor starts
  • Chorioamnionitis, which is a bacterial, fungal, or viral infection occurring before or during labor, affecting the placenta and the amniotic fluid, as well as the membranes surrounding the baby
  • Babies who are born small for their gestational age

“These results provide further reassurance for patients and clinicians considering flu vaccination during pregnancy,” said senior author, Lei Qian, PhD, a biostatistician research scientist with the Department of Research & Evaluation. “Pregnant people can feel safe knowing they are protecting themselves and their babies from getting the flu, when they get the flu vaccine in consecutive pregnancies, no matter how closely it follows a previous pregnancy.”

The 8 participating Vaccine Safety Datalink sites include the lead site, Kaiser Permanente Southern California, as well as Denver Health, HealthPartners Institute, Kaiser Permanente Colorado, Kaiser Permanente Northern California, Kaiser Permanente Northwest, Kaiser Permanente Washington, and Marshfield Clinic Research Institute.

In addition to Dr. Getahun and Dr. Qian, authors on the paper include: In-Lu Amy Liu, MS, and Lina S. Sy, MPH, of the Kaiser Permanente Southern California Department of Research & Evaluation, Pasadena, CA; Jason M. Glanz, PhD, Institute for Health Research, Kaiser Permanente Colorado, Denver, CO; Ousseny Zerbo, PhD, Kaiser Permanente Vaccine Study Center, Kaiser Permanente Northern California, Oakland, CA; Gabriela Vazquez-Benitez, PhD, HealthPartners Institute, Minneapolis, MN; Jennifer C. Nelson, PhD, Kaiser Permanente Washington Health Research Institute, Seattle, WA; Joshua T. Williams, MD,, and Simon J. Hambidge, MD, PhD, of Denver Health Ambulatory Care Services, Denver, CO; Huong Q. McLean, PhD, Marshfield Clinic Research Institute, Marshfield, WI; Stephanie A. Irving, MHS, Kaiser Permanente Center for Health Research, Portland, OR; Eric S. Weintraub, MPH, Immunization Safety Office, Centers for Disease Control and Prevention, Atlanta, GA