Karen Jacobson, MD, discusses maternal COVID-19 vaccination and lower infant hospitalization risk

Editor’s note: Watch the video above with our exclusive interview with lead study author Karen Jacobson, MD, MPH.

Maternal SARS-CoV-2 vaccination during pregnancy, particularly during the third trimester, was associated with lower risks of COVID-19 infection and hospitalization among infants younger than 6 months, according to a large retrospective cohort study published in Pediatrics.

Investigators from Kaiser Permanente Northern California evaluated 78,644 infants born between July 1, 2021, and June 30, 2023, to assess whether the timing of maternal messenger RNA (mRNA) COVID-19 vaccination influenced infant outcomes during the first 6 months of life.

Among the cohort, 43.6% of infants were born to mothers vaccinated during pregnancy, 29.6% to mothers vaccinated before pregnancy, and 26.8% to unvaccinated mothers. Overall, 3648 infants (4.6%) developed COVID-19 infection, and 76 infants experienced COVID-19–related hospitalization before age 6 months.

The authors reported that “maternal vaccination during the third trimester was protective against infant COVID-19 infections,” and that “maternal vaccination during pregnancy, particularly during the third trimester, was effective against infant COVID-19–related hospitalizations.”

Protection against infection and hospitalization

After adjustment for maternal and infant factors, vaccine effectiveness (VE) for maternal vaccination during pregnancy was 7.5% (95% CI, −2% to 16.2%) against infant COVID-19 infection through age 6 months. However, vaccination during the third trimester demonstrated greater protection, with VE of 19.2% (95% CI, 8.6%-28.6%) against infant infection.

Investigators also observed stronger protection against severe disease. Maternal vaccination during pregnancy was associated with 52.9% VE (95% CI, 11.1%-75.1%) against COVID-19–related hospitalization among infants younger than 6 months. Third-trimester vaccination increased VE against hospitalization to 64.6% (95% CI, 12.3%-85.7%).

Protection was greatest during early infancy. In secondary analyses, maternal vaccination during pregnancy was 37.8% effective against infant COVID-19 during the first 2 months of life.

The study also found continued benefit during more recent SARS-CoV-2 variant periods. Third-trimester vaccination remained protective during BQ/XBB predominance, with VE of 78.5% (95% CI, 22.2%-94.0%) against infant infection in the first 2 months of life.

Prepregnancy vaccination did not protect infants

Researchers found no evidence that maternal vaccination before pregnancy reduced infant COVID-19 risk, regardless of how recently vaccination occurred before conception. VE estimates for prepregnancy vaccination ranged from −14.9% to 23.6%, with confidence intervals crossing zero in all intervals examined.

The authors wrote, “Maternal vaccination before pregnancy onset was not significantly effective in preventing COVID-19 in infants aged 0 to 6 months, regardless of time interval from vaccination to pregnancy onset, time period of infant follow-up, or period of variant predominance.”

Investigators suggested waning antibody levels may explain the lack of infant protection from prepregnancy vaccination. The study noted that most transplacental antibody transfer occurs during the third trimester and that vaccine-induced antibodies decline over time.

Clinical implications for pediatricians

The authors emphasized that infants younger than 6 months remain ineligible for COVID-19 vaccination and continue to face elevated hospitalization risk compared with older pediatric populations.

Among infants hospitalized for COVID-19, 81.6% presented with fever, 31.6% experienced respiratory distress, and 28% required supplemental oxygen.

Researchers concluded that “maternal vaccination during pregnancy is likely the most effective way to protect infants against COVID-19.”

Reference

Jacobson KB, Merchant M, Fireman B, et al. SARS-CoV-2 vaccination before and during pregnancy and prevention of infant COVID-19 infection. Pediatrics. 2026;157(5):e2025073000. doi:10.1542/peds.2025-073000