Babies younger than six-months-old whose mothers received an mRNA COVID-19 vaccine had a lower risk of hospitalisation 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 hospitalised for COVID-19 (cases) were born to mothers who received two doses of an mRNA COVID-19 vaccine versus 29% of hospitalised infants who tested negative for SARS-CoV-2 (controls), reported Dr Manish Patel, MSc, of the US Centres for Disease Control and Infection (CDC) Division of Viral Diseases in Atlanta, and colleagues.
Overall, maternal vaccination was 52% (95% CI 33-65) effective against infant COVID-related hospitalisation, 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 hospitalisation, and in particular, hospitalisation in an ICU, was reduced among babies 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 hospitalisation 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 hospitalisation 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 Immunisation Practices recently recommended mRNA COVID vaccines in children six months and older, but no vaccine is authorised for children under six months of age.)
Overall, vaccine effectiveness against infant hospitalisation 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, Dr Sonja Rasmussen of the University of Florida College of Medicine in Gainesville, and Dr Denise Jamieson, MPH, of Emory University School of Medicine in Atlanta, said this study provides “compelling evidence” that maternal vaccination may reduce risk of COVID hospitalisation among infants under six months, “a finding that further supports recommendations for COVID-19 vaccination during pregnancy”.
But they also pointed out that maternal vaccine coverage is not optimal: Only 71% of pregnant women in the US 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 woman’s decision-making process. “This evidence that COVID-19 vaccines help to protect infants as well as mothers is highly relevant for patient counselling: 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 hospitalisation.
However, previous estimates had been limited to the Delta strain. COVID-19 had serious adverse outcomes for infants under six months, particularly during the Omicron wave. Hospitalisation rates among babies in this age group were six times as high during the Omicron surge compared with the Delta peak.
Additionally, COVID was the primary reason for hospitalisation 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 hospitalisation. 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 two 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 was vaccinated, the researchers found.
Study limitations included 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 generalisable to less severe cases of COVID, and the researchers were unable to assess the effects of a maternal booster, they said.
Maternal Vaccination and Risk of Hospitalization for Covid-19 among Infants
Natasha Halasa, Samantha Olson, Mary Staat, Margaret Newhams, Ashley Price, Pia Pannaraj, Julie Boom, Leila Sahni, Kathleen Chiotos, Melissa Cameron, Katherine Bline, Charlotte Hobbs.
Published in the New England Journal of Medicine on 22 June 2022
Infants younger than 6 months of age are at high risk for complications of coronavirus disease 2019 (COVID-19) and are not eligible for vaccination. Transplacental transfer of antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) after maternal COVID-19 vaccination may confer protection against COVID-19 in infants.
We used a case–control test-negative design to assess the effectiveness of maternal vaccination during pregnancy against hospitalisation for COVID-19 among infants younger than 6 months of age. Between July 1, 2021, and March 8, 2022, we enrolled infants hospitalised for COVID-19 (case infants) and infants hospitalised without COVID-19 (control infants) at 30 hospitals in 22 states. We estimated vaccine effectiveness by comparing the odds of full maternal vaccination (two doses of mRNA vaccine) among case infants and control infants during circulation of the B.1.617.2 (delta) variant (July 1, 2021, to December 18, 2021) and the B.1.1.259 (omicron) variant (December 19, 2021, to March 8, 2022).
A total of 537 case infants (181 of whom had been admitted to a hospital during the delta period and 356 during the omicron period; median age, 2 months) and 512 control infants were enrolled and included in the analyses; 16% of the case infants and 29% of the control infants had been born to mothers who had been fully vaccinated against COVID-19 during pregnancy. Among the case infants, 113 (21%) received intensive care (64 [12%] received mechanical ventilation or vasoactive infusions). Two case infants died from COVID-19; neither infant’s mother had been vaccinated during pregnancy. The effectiveness of maternal vaccination against hospitalisation for COVID-19 among infants was 52% (95% confidence interval [CI], 33 to 65) overall, 80% (95% CI, 60 to 90) during the delta period, and 38% (95% CI, 8 to 58) during the omicron period. Effectiveness was 69% (95% CI, 50 to 80) when maternal vaccination occurred after 20 weeks of pregnancy and 38% (95% CI, 3 to 60) during the first 20 weeks of pregnancy.
Maternal vaccination with two doses of mRNA vaccine was associated with a reduced risk of hospitalisation for COVID-19, including for critical illness, among infants younger than 6 months of age. (Funded by the Centers for Disease Control and Prevention.)
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