This is a summary, written by members of the CITF Secretariat, of:

Jorgensen SCJ, Hernandez A, Fell DB, Austin PC, D’Souza R, Guttmann A, Kwong JC. Estimated effectiveness of postpartum maternal messenger RNA COVID-19 vaccination against Delta and Omicron SARS-CoV-2 infection and hospitalization in infants younger than 6 months. JAMA Pediatr. 2023 Feb 27. doi:

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

A CITF-funded study published in JAMA Pediatrics found that the infants of mothers who were vaccinated postpartum had moderately effective protection against Delta infection (73%), but little protection against the Omicron variant (13%). The study applied to infants below the age of six months. It identified postpartum vaccination as part of cocooning, a strategy for conferring protection against many respiratory infections to infants by vaccinating people who are in close contact with them. Although this study did not look at the vaccination status of others in the immediate environment, postpartum vaccination of mothers did provide important levels of protection against the Delta variant of SARS-CoV-2. This study was led by Drs. Jeffrey Kwong (University of Toronto) and Deshayne Fell (University of Ottawa).

Key findings:

  • Vaccine effectiveness (VE) against Omicron-related infant hospitalizations was 36%.
  • There were no Delta-related hospitalizations in infants of vaccinated mothers.
  • Two doses of vaccine increased the VE against infection against Delta to 73%, from 39% after one vaccine dose. However, a second dose of vaccine did not increase VE against Omicron infection.

The authors highlighted that cocooning is an important strategy when it comes to protecting young infants against SARS-CoV-2 infection. Mothers vaccinated postpartum protected their infants against SARS-CoV-2 infection and hospitalizations to a greater extent against Delta than Omicron. Further studies are needed to assess the waning of VE for third doses.

The study included infants under six months born between May 7, 2021 and March 31, 2022, who received a SARS-CoV-2 polymerase chain reaction (PCR) test between June 18, 2021 and September 5, 2022. Postpartum maternal vaccination was defined as at least two mRNA vaccine doses between birth and 14 days before the infant’s test. Vaccination rates were 39.1% for the Delta analyses, and 33.6% for the Omicron analyses. Fewer than five mothers received a third vaccine dose.