Newborns’ Protection With Moms’ COVID Vax Dips During Omicron

Infants under 6 months of age whose mothers received an mRNA COVID-19 vaccine had a lower risk of hospitalization with the virus, but protection dropped considerably during the Omicron era, according to a case-control study.

In the Overcoming COVID-19 study, 16% of infants hospitalized for COVID-19 (cases) were born to mothers who received two doses of an mRNA COVID-19 vaccine versus 29% of hospitalized infants who tested negative for SARS-CoV-2 (controls), reported Manish Patel, MD, MSc, of the CDC Division of Viral Diseases in Atlanta, and colleagues.

Overall, maternal vaccination was 52% (95% CI 33-65) effective against infant COVID-related hospitalization, and 70% (95% CI 42-85) effective against ICU admission, they wrote in the New England Journal of Medicine.

“The finding that the risk of hospitalization, and in particular, hospitalization in an ICU, was reduced among infants whose mothers had been fully vaccinated during pregnancy provides evidence of additional benefits of maternal vaccination beyond those previously reported for the mother,” Patel and colleagues stated.

But maternal vaccination was associated with lower protection against COVID-related infant hospitalization during the Omicron period (38%, 95% CI 8-58%) compared with when Delta was the predominant strain (80%, 95% CI 60-90).

This “moderate” reduced risk of infant hospitalization during the Omicron period is still meaningful for an age group not likely to get access to a licensed vaccine in the foreseeable future, Patel’s group noted. (CDC’s Advisory Committee on Immunization Practices recently recommended mRNA COVID vaccines in children 6 months and older, but no vaccine is authorized for children under 6 months of age.)

Overall, vaccine effectiveness against infant hospitalization was higher when administered after 20 weeks’ pregnancy compared to when it was given earlier in pregnancy (69% vs 38%, respectively).

In an accompanying editorial, Sonja Rasmussen, MD, of the University of Florida College of Medicine in Gainesville, and Denise Jamieson, MD, MPH, of Emory University School of Medicine in Atlanta, said that this study provides “compelling evidence” that maternal vaccination may reduce risk of COVID hospitalization among infants under age 6 months, “a finding that further supports recommendations for Covid-19 vaccination during pregnancy.”

But Rasmussen and Jamieson also pointed out that maternal vaccine coverage is not optimal: Only 71% of pregnant women in the U.S. were fully vaccinated as of mid May, with a significantly lower rate (58%) observed in non-Hispanic Black women.

They stressed that healthcare provider recommendations and data on safety and efficacy are key to a pregnant person’s decision-making process. “This evidence that Covid-19 vaccines help to protect infants as well as mothers is highly relevant for patient counseling: a ‘two-for-one’ deal may encourage more mothers to receive Covid-19 vaccination,” Rasmussen and Jamieson wrote.

Patel’s group previously reported that maternal vaccination with two doses of an mRNA vaccine was associated with a 61% reduced risk of infant hospitalization. However, previous estimates had been limited to the Delta strain.

COVID-19 had serious adverse outcomes for infants under 6 months, particularly during the Omicron wave. Hospitalization rates among babies in this age group were six times as high during the Omicron surge compared to the Delta peak. Additionally, COVID was the primary reason for hospitalization among 85% of babies in this age group.

Patel’s group used a case-control, test-negative design to assess efficacy of maternal vaccination, defined as completion of a two-dose series of the Pfizer/BioNTech or Moderna vaccine, on infant hospitalization. The researchers enrolled infants between July 2021 and March 2022 from 30 hospitals in 22 states.

All case infants included in the study had a clinical presentation consistent with acute COVID-19, as well as a positive SARS-CoV-2 PCR or antigen test. The researchers excluded infants who were tested for SARS-CoV-2 more than 10 days after symptom onset or more than 72 hours after hospital admission.

There were 537 case infants and 512 control infants included in the analysis. Of the case infants, 181 were admitted to the hospital during Delta predominance, and 356 during Omicron. The median age of infants in both groups was 2 months. Around 19% of case infants and 24% of control infants had at least one underlying health condition (including respiratory and cardiovascular), and there was a lower proportion of non-Hispanic white babies in the case cohort than the control group (34% vs 44%, respectively).

Overall, 21% of case infants received intensive care, and 12% received mechanical ventilation or vasoactive infusions. Compared to case infants whose mothers had not been fully vaccinated, those whose mothers were vaccinated had a lower incidence of ICU admission, critical COVID-19, mechanical ventilation, and vasoactive infusions. Two case infants died from COVID-19, and neither of their mothers were vaccinated, the researchers found.

Study limitations included the fact that the authors were unable to assess potential biases related to natural infection with SARS-CoV-2 before or during pregnancy. Additionally, the study may be subject to residual confounding from differences between the case and control groups. These findings may not be generalizable to less severe cases of COVID, and the researchers were unable to assess the effects of a maternal booster, they said.