Serial cross-sectional estimation of vaccine and infection-induced SARS-CoV-2 sero-prevalence in children and adults, British Columbia, Canada: March 2020 to August 2022

Abstract

ABSTRACTBackgroundWe chronicle SARS-CoV-2 sero-prevalence through eight cross-sectional sero-surveys (snapshots) in the Lower Mainland (Greater Vancouver and Fraser Valley), British Columbia, Canada from March 2020 to August 2022.MethodsAnonymized-residual sera were obtained from children and adults attending an outpatient laboratory network. Sera were tested with at least three immuno-assays per snapshot to detect spike (S1) and/or nucleocapsid protein (NP) antibodies. Sero-prevalence was defined by dual-assay positivity, including any or infection-induced, the latter requiring S1+NP antibody detection from January 2021 owing to vaccine availability. Infection-induced estimates were used to assess the extent to which surveillance case reports under-estimated infections.ResultsSero-prevalence was ≤1% by the 3rd snapshot in September 2020 and <5% by January 2021 (4th). Following vaccine roll-out, sero-prevalence increased to >55% by May/June 2021 (5th), ∼80% by September/October 2021 (6th), and >95% by March 2022 (7th). In all age groups, infection-induced sero-prevalence remained <15% through September/October 2021, increasing through subsequent Omicron waves to ∼40% by March 2022 (7th) and ∼60% by July/August 2022 (8th). By August 2022, at least 70-80% of children ≤19 years, 60-70% of adults 20-59 years, but ∼40% of adults ≥60 years had been infected. Surveillance case reports under-estimated infections by 12-fold between the 6th-7th and 92-fold between the 7th-8th snapshots.InterpretationBy August 2022, most children and adults had acquired SARS-CoV-2 vaccine and infection exposures, resulting in more robust hybrid immunity. Conversely the elderly, still at greatest risk of severe outcomes, remain largely-dependent on vaccine-induced protection alone, and should be prioritized for additional doses.