Maternal COVID Vaccine Curbs Infant Infection From Delta Variant

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Maternal vaccination with two doses of the mRNA COVID-19 vaccine was 95% effective against infant infection from the delta variant, and 45% effective against infant infection from the omicron variant, 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, MD, 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.

Effectiveness is encouraging, but updates are needed

The effectiveness of maternal vaccination to prevent COVID-19 infection and related hospitalizations in infants is promising, especially since those younger than 6 months have no other source of vaccine protection against COVID-19 infection, wrote Dana Danino, MD, of Soroka University Medical Center, Israel, and Ilan Youngster, MD, of Shamir Medical Center, Israel, in an accompanying editorial also published in The BMJ.

They also noted that maternal vaccination during pregnancy is an established method of protecting infants from infections such as influenza and pertussis.

Data from previous studies show that most infants whose mothers were vaccinated against COVID-19 during pregnancy retained maternal antibodies at 6 months, “but evidence for protection against neonatal COVID-19 infection has been deficient,” they said.

The current study findings support the value of vaccination during pregnancy, and the findings were strengthened by the large study population, the editorialists wrote. However, whether the same effectiveness holds for other COVID-19 strains such as BQ.1, BQ.1.1, BF.7, XBB, and XBB.1 remains unknown, they said.

Other areas in need of exploration include the optimal timing of vaccination during pregnancy, the protective effects of a bivalent mRNA vaccine (vs. the primary monovalent vaccine in the current study), and the potential benefits of additional boosters, they added.

“Although Jorgenson and colleagues’ study reinforces the value of maternal vaccination against COVID-19 during pregnancy, more studies are needed to better inform vaccination recommendations in an evolving landscape of new SARS-CoV-2 strains and novel vaccines,” the editorialists concluded.

The study was supported by ICES, which is funded by an annual grant from the Ontario Ministry of Health and the Ministry of Long-term Care; the study also received funding from the Canadian Immunization Research Network and the Public Health Agency of Canada. Dr. Jorgensen and the editorialists had no financial conflicts to disclose.

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