In infants younger than 6 months of age, maternal immunization with 2 doses of an mRNA SARS-CoV-2 vaccine is associated with a reduced risk for hospitalization for COVID-19, including critical illness, although the reductions were less evident when the omicron variant was predominant. These findings were published in The New England Journal of Medicine.

A case-control, test-negative design was used to evaluate the effectiveness of maternal vaccination for COVID-19 among infants younger than 6 months who had been hospitalized for suspected SARS-CoV-2 infection between July 1, 2021, and March 8, 2022. Infants who were hospitalized with COVID-19 (case infants) and infants with Covid-19-associated symptoms who had a negative SARS-CoV-2 reverse-transcriptase–polymerase-chain-reaction or antigen test (control infants) were included in the study. All of the enrollees had been hospitalized at 30 pediatric hospitals in 22 states in the United States.

Researchers sought to assess the extent to which transplacental transfer of antibodies against SARS-CoV-2 infection following maternal vaccination may confer protection against COVID-19 in infants. They estimated vaccine effectiveness by comparing the odds of full maternal vaccination (2 doses of an mRNA vaccine) among case infants and control infants during circulation of the B.1.617.2 (delta) variant between July 1, 2021 and December 18, 2021, and the B.1.1.259 (omicron) variant between December 19, 2002, and March 8, 2022.


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The researchers enrolled a total of 537 case infants (181 of whom had been admitted during the period of delta variant circulation) and 512 control infants (median age of 2 months in both cohorts). Overall, 16% of the case infants and 29% of the control infants had been born to mothers who were fully vaccinated against SARS-CoV-2 during their pregnancy.

In all, 21% of the case infants needed to receive intensive care, with 12% requiring mechanical ventilation or vasoactive infusions. Among the case infants, 2 died from COVID-19, with neither of these infant’s mothers having been vaccinated during pregnancy.

The effectiveness of maternal vaccination against COVID-19 hospitalization was 52% (95% CI, 33%-65%) overall, 80% (95% CI, 60%-90%) in the period during which the delta variant was circulating, and 38% (95% CI, 8%-58%) during the period of omicron circulation.

The effectiveness against COVID-19-associated hospitalization for either variant among infants was 69% (95% CI, 50%-80%) when maternal vaccination took place after 20 weeks of pregnancy, compared with 38% (95% CI, 3%-60%) when maternal vaccination occurred during the initial 20 weeks of pregnancy. In a similar fashion, during both the delta-predominant period and the omicron-predominant period, the point estimates were higher when maternal vaccination took place after 20 weeks of pregnancy vs during the initial 20 weeks of pregnancy.

Limitations of the study include that the researchers are unable to evaluate potential biases associated with natural SARS-CoV-2 infection before or during pregnancy. Further, the possibility of residual confounding resulting from differences between case infants and control infants cannot be excluded from the analysis. Additionally, the findings from the study might not apply to less severe cases of COVID-19 or to other variants.

“These findings provide additional support for the current recommendations regarding Covid-19 vaccination during pregnancy,” the researcher wrote.

Disclosure: Some of the study authors have declared affiliations with biotech, pharmaceutical, and/or device companies. Please see the original reference for a full list of authors’ disclosures. 

Reference

Halasa NB, Olson SM, Staat MA, et al; Overcoming Covid-19 Investigators. Maternal vaccination and risk of hospitalization for Covid-19 among infants. N Engl J Med. Published online June 22, 2022. doi:10.1056/NEJMoa2204399