a new study shows.
Previous research has confirmed that COVID-19 neutralizing antibodies following maternal vaccination or maternal COVID-19 infection are present in umbilical cord blood, breast milk, and infant serum specimens, wrote Sarah C.J. Jorgensen, PharmD, MPH, of the University of Toronto, and colleagues in their article published in The BMJ.
In the study, the researchers identified maternal and newborn pairs using administrative databases from Canada. The study population included 8,809 infants aged younger than 6 months who were born between May 7, 2021, and March 31, 2022, and who underwent testing for COVID-19 between May 7, 2021, and September 5, 2022.
Maternal vaccination with the primary COVID-19 mRNA monovalent vaccine series was defined as two vaccine doses administered up to 14 days before delivery, with at least one of the doses after the conception date.
Maternal vaccination with the primary series plus one booster was defined as three doses administered up to 14 days before delivery, with at least one of these doses after the conception date.
The primary outcome was the presence of delta or omicron COVID-19 infection or hospital admission of the infants.
The study population included 99 COVID-19 cases with the delta variant (with 4,365 controls) and 1,501 cases with the omicron variant (with 4,847 controls).
Overall, the vaccine effectiveness of maternal doses was 95% against delta infection and 45% against omicron.
The effectiveness against hospital admission in cases of delta and omicron variants were 97% and 53%, respectively.
The effectiveness of three doses was 73% against omicron infant infection and 80% against omicron-related infant hospitalization. Data were not available for the effectiveness of three doses against the delta variant.
The effectiveness of two doses of vaccine against infant omicron infection was highest when mothers received the second dose during the third trimester of pregnancy, compared with during the first trimester or second trimester (53% vs. 47% and 53% vs. 37%, respectively).
Vaccine effectiveness with two doses against infant infection from omicron was highest in the first 8 weeks of life (57%), then decreased to 40% among infants after 16 weeks of age.
Although the study was not designed to assess the mechanism of action of the impact of maternal vaccination on infants, the current study results were consistent with other recent studies showing a reduction in infections and hospitalizations among infants whose mothers received COVID-19 vaccines during pregnancy, the researchers wrote in their discussion.
The findings were limited by several factors including the potential unmeasured confounders not available in databases, such as whether infants were breastfed, the researchers noted. Other limitations included a lack of data on home test results and the inability to assess the waning impact of the vaccine effectiveness against the delta variant because of the small number of delta cases, they said. However, the results suggest that the mRNA COVID-19 vaccine during pregnancy was moderately to highly effective for protection against omicron and delta infection and infection-related hospitalization – especially during the first 8 weeks of life.