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

Piché-Renaud PP, Swayze S, Buchan SA, Wilson SE, Austin PC, Morris SK, Nasreen S, Schwartz KL, Tadrous M, Thampi N, Wilson K, Kwong JC; CANADIAN IMMUNIZATION RESEARCH NETOWRK (CIRN) Provincial Collaborative Investigators. COVID-19 Vaccine Effectiveness Against Omicron Infection and Hospitalization. Pediatrics. March 2023. DOI: 10.1542/peds.2022-059513

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

In this CITF-funded study published in Pediatrics, Drs. Jeffrey Kwong (University of Toronto) and Kumanan Wilson (University of Ottawa) highlighted that two doses of the monovalent Pfizer-BioNtech mRNA vaccine yielded moderate protection against symptomatic SARS-CoV-2 infection, while protection against severe COVID-19 outcomes was much higher among children between the ages of 5 and 11. Initially, vaccine effectiveness was higher when the interval between the two doses was longer, but protection waned quickly following each dose.

Key findings:

  • Vaccine effectiveness (VE) against symptomatic infection was estimated to be 24% up to a month after the first dose and increased to 66% up to a month after a second dose.
  • VE against symptomatic infection was higher (57%) for children who received their second dose at least 56 days after their first dose, compared to those who received it after an interval of 15-27 days (12%) or an interval of 28-41 days (38%).
  • Regardless of the dosing interval, the protection conferred by vaccination against symptomatic infection waned over time after each dose, becoming undetectable 60 days after the first dose and 120 days (4 months) after the second dose.
  • Cases of severe COVID-19 were rare among vaccinated children in the 5 to 11 age group. VE of the second dose against severe outcomes reached 94% within one month, but declined to 57% after 120 days (4 months) post-dose.

While these estimates suggest the administration of third doses for children due to waning immunity, further studies are necessary, particularly with the recent introduction of bivalent vaccines and potential newly emerging sub-variants of concern.

Children who showed COVID-related symptoms and underwent a RT-PCR test between January and August 2022 in Ontario, and who were not immunocompromised, were included in the study. Only those who received doses of the Pfizer/BioNtech mRNA vaccine were included. A total of 6,284 Omicron cases (Delta cases were not integrated in the analysis) and 8,389 controls who tested negative were analyzed. Severe outcomes included COVID-19-associated hospitalization or death.

If you want to learn about vaccine effectiveness in older adults, you can read our summary of another study conducted by Drs Kwong and Wilson published in Nature Communications in March 2023.