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

Zinszer K, Charland K, Pierce L, Saucier A, Hamelin MÈ, Da Torre MB, Carbonneau J, Nguyen CT, De Serres G, Papenburg J, Boivin G, Quach C. Infection-induced seroconversion and seroprevalence of SARS-CoV-2 among a cohort of children and youth in Montreal, Canada. Influenza Other Respir Viruses. 2023 Aug 25;17(8):e13186. doi: 10.1111/irv.13186.

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 Influenza Other and Respiratory Viruses, found previously seronegative children were 9 to 12 times more likely to seroconvert due to infection during the early Omicron-dominant period (May to October 2022) than in pre-Omicron periods. The study was led by Dr. Kate Zinszer (University of Montreal).

The EnCORE study is a prospective study of SARS-COV-2 in a cohort of children from Montreal, which has included five rounds of data collection to date. Based on data from the fourth round of data collection from May to October 2022 in a cohort of 726 children, aged 2- to 19- years-old, the study team estimated SARS-CoV-2 seroprevalence and seroconversion.

Key findings:

  • Seronegative children (children with no antibodies due to infection) in round 3 were approximately 9 to 12 times more likely to seroconvert (develop antibodies due to infection) during the early Omicron-dominant round 4 compared to pre-Omicron rounds.
  • Relative to the oldest age group, children aged 2- to 4-years-old had a 36% higher seroconversion rate while children aged 5- to 11-years-old had a 24% lower seroconversion rate. This could largely be explained by age group differences in vaccination uptake.
  • Vaccinated children had a 57% lower rate of infection-acquired seroconversion compared to unvaccinated children after adjusting for potential variables, with age group and vaccination remaining related.
  • During this period, adjusted rates of seroconversion were higher among females compared to males and among children of parents identifying as an ethnic or racial minority compared to children of white parents.
  • The unadjusted seroprevalence of the full sample of 726 children was 58.4%.

Vaccination was open to 12- to 19-year-olds in June 2021, and 98% were vaccinated prior to the study period (Round 4). 5–11-year-olds were eligible as of November 2021, and 91% were vaccinated by the study period. 2- to 4-year-olds could be vaccinated as of July 2022 and only 2% were vaccinated by the study period. All vaccinated 2- to 4-year-olds had only one dose, over 83% of 5–11-year-olds had two or more doses, and over 86% of the 12- to 19-year-olds had two or more doses. Moreover, the percentage of the age groups having a vaccination within 120 days of their dried blood spot (DBS) specimen for antibodies was 2% for 2- to 4-year-olds, 32% for 5- to 11-year-olds, and 27% for 12- to 19-year-olds, suggesting greater protection for the older participants. Despite Omicron being a highly immune-evasive variant, these data highlight the protective effects of vaccination in children.

The EnCORE study included five rounds of data collection to date: Round 1 (October 2020 to April 2021, N = 1632), Round 2 (May to September 2021, N = 936), Round 3 (November 2021 to February 2022, N = 723), Round 4 (May to October 2022, N = 726), and Round 5 (February to June 2023, results to be published soon).