Infants whose mothers were vaccinated while pregnant better protected from severe COVID-19

Infants aged younger than 6 months were better protected from serious COVID-19 outcomes if their mothers received two doses of the vaccine while pregnant, according to researchers.

Maternal vaccination during pregnancy reduced the infant’s risk for COVID-19-related hospitalization by more than 50%, Natasha B. Halasa, MD, MPH, a professor of pediatrics at Vanderbilt University, and colleagues reported in The New England Journal of Medicine.

Previous research has shown that vaccinated mothers pass anti-spike immunoglobin G antibodies to their babies, and that infants retain antibodies at 6 months. The study by Halasa and colleagues takes things a step further, showing the protective effect of maternal COVID-19 vaccination against hospitalization among infants.

“Benefits of maternal vaccination with respect to illness among infants during the first 6 months of life have previously been shown for other vaccine-preventable diseases, such as pertussis and influenza,” they noted.

In their case-control, test-negative study, the researchers compared “the odds of full maternal vaccination during pregnancy among symptomatic infants younger than 6 months of age who were hospitalized for COVID-19” and infants hospitalized without COVID-19. The researchers enrolled 1,049 infants between July 1, 2021, and March 8, 2022 — 537 case infants and 512 control infants.

Overall, the effectiveness of vaccination during pregnancy against COVID-19-related hospitalizations among infants was 52% (95% CI, 33-65), but the numbers shifted depending on the predominant variant. For example, during the delta period, effectiveness was 80% (95% CI, 60-90), and during the omicron wave, it was 38% (95% CI, 8-58).

Effectiveness also varied depending upon when the pregnant person received the vaccine. When women received the vaccine during the first 20 weeks of pregnancy, effectiveness was 38% (95% CI, 3-60). After 20 weeks of pregnancy, effectiveness rose to 69% (95% CI, 50-80).

In a related editorial, Sonja A. Rasmussen, MD, a professor of pediatrics and epidemiology at the University of Florida, and Denise J. Jamieson, MD, MPH, the James Robert McCord Professor and chair of the department of gynecology and obstetrics at Emory University, said the study provides “compelling evidence that maternal vaccination is effective” but also raises the question about optimal timing of vaccination during pregnancy.

“Determining the appropriate timing is difficult because the benefits of maximizing infant protection must be balanced against the maternal risks of delaying vaccination, given the increased risk of severe COVID-19 during pregnancy,” they wrote. “Further study is needed to assess whether an additional booster dose given in later pregnancy would increase infant protection.”