Safety Outcomes of Influenza Vaccine in Pregnancy

Birth weight and week's gestation of infants at birth are not affected by maternal influenza vaccination during any trimester of pregnancy.

Birth outcomes, specifically birth weight and weeks’ gestation of the infant at birth, are not affected by the influenza vaccine during any trimester of pregnancy, according to a study published in Vaccine.

Lisa McHugh, MAE, from the National Centre for Epidemiology and Population Health at the Australian National University in Canberra, and colleagues conducted a nested retrospective cohort study of “FluMum” participants to investigate the effects of influenza vaccination during any trimester of pregnancy with respect to 2 critical birth outcomes: infant birth weight and weeks’ gestation at birth of the infant.

Participants were recruited between April 1, 2012, and December 31, 2014, from 6 cities in Australia: Sydney, Melbourne, Perth, Darwin, Adelaide, and Brisbane. There was an even distribution between the 6 participating Australian sites.

In accordance with World Health Organization guidelines, the study classified infants born before 37 completed weeks’ gestation as preterm, 35 weeks as very preterm, and 30 weeks as extremely preterm. Infants with a birth weight less than 2500 g were classified as low birth weight, less than 1500 g as very low birth weight, and less than 1000 g as extremely low birth weight.

Of the 7126 women who gave birth to a live singleton infant, 34% received the influenza vaccine during pregnancy. McHugh and colleagues found no statistically significant differences between infants born to vaccinated vs unvaccinated women. Infants born to vaccinated women had a mean gestation of 38.7 weeks vs 38.8 weeks in the unvaccinated group (P =.051). Mean birth weight of infants born to vaccinated women was 3337 g vs 3352 g in the unvaccinated group (P =.29).

Most of the infants (83%) were born during the Australian influenza season, and no relationship between season of birth and either low birth weights (relative risk [RR], 1.04; 95% CI 0.87-1.24; P =.67) or preterm births (RR, 0.94; 95% CI, 0.84-1.06, P =.34) were observed.

Of the 1705 women with an identified influenza vaccination date, 14% received the vaccine in the first trimester, 51% received the vaccine in the second trimester, and 35% received the vaccine in the third trimester.

Compared with the unvaccinated group, there were statistically significant differences in mean gestational ages of the infant at birth for each trimester of pregnancy a vaccine was given:

  • Vaccinated in first trimester: 38.4 weeks (95% CI, 38.1-38.7; P =.003)
  • Vaccinated in second trimester: 38.6 weeks (95% CI, 38.4-38.7; P =.005)
  • Vaccinated in third trimester: 38.9 weeks (95% CI, 38.8-39.1; P =.016)

Compared with the unvaccinated group, there were no statistically significant differences in mean birth weights of infants by trimester of pregnancy:

  • Vaccinated in first trimester: 3293 g (95% CI, 3215-3372; P =.11)
  • Vaccinated in second trimester: 3345 g (95% CI, 3307-3384; P =.74)
  • Vaccinated in third trimester: 3383 g (95% CI, 3342-3425; P =.20)

“Physicians can reassure women who will be pregnant during the flu season that there is no increased risk of having a preterm baby or low birth weight baby if they get an influenza vaccine during any stage of their pregnancy,” said McHugh in an email interview with Infectious Disease Advisor. “This is especially important for pregnant women who have comorbidities or other risk factors which may affect their immunity and capacity to cope with an infectious disease in pregnancy such as influenza,” she explained.

“There are still a wide range of birth outcomes that need to be analyzed, particularly now with the introduction of pertussis-containing vaccines recommended in pregnancy, and these are planned as part of our ‘next steps’ with ongoing research,” concluded Ms McHugh in the interview. “We will be analyzing our data with respect to congenital anomalies, small for gestational age, and preterm and low birth weight babies who have had both pertussis and influenza vaccines.”


McHugh L, Andrews RM, Lambert SB, et al. Birth outcomes for Australian mother-infant pairs who received an influenza vaccine during pregnancy, 2012-2014: the FluMum study. Vaccine. 2017;35:1403-1409. doi: 10.1016/j.vaccine.2017.01.075

This article originally appeared on Infectious Disease Advisor