ABSTRACT
Background: Measurement of SARS-CoV-2 antibody seropositivity is important to accurately understand exposure to infection and/or vaccination in specific populations. Methods: Children with or without prior SARS-CoV-2 infections, was enrolled in Calgary, Canada in 2020. Venous blood was sampled 4 times from July 2020 to April 2022 for SARS-CoV-2 nucleocapsid and spike antibodies. Demographic and clinical information was obtained including SARS-CoV-2 testing results and vaccination records. Results: 1035 children were enrolled and 88.9% completed all 4 visits; median age 9 years (IQR: 5,13); 519 (50.1%) female; and 815 (78.7%) Caucasian. Before enrollment, 118 (11.4%) had confirmed or probable SARS-CoV-2. By April 2022, 39.5% of previously uninfected participants had a SARS-CoV-2 infection. Nucleocapsid antibody seropositivity declined to 16.4% after more than 200 days after diagnosis. Spike antibodies remained elevated in 93.6% of unvaccinated children after more than 200 days after diagnosis. By April 2022, 408 (95.6%) children 12 years and older had received 2 or more vaccine doses, and 241 (61.6%) 5 to 11 year-old children had received 2 vaccine doses. At that time, all 685 vaccinated children had spike antibodies, compared with 94/176 (53.4%) of unvaccinated children. Conclusions: In our population, after the first peak of Omicron variant infections and introduction of COVID-19 vaccines for children, all vaccinated children had SARS-CoV-2 spike antibodies, in contrast to 53.4% of unvaccinated children. It is not yet known whether a high level of seropositivity at a point in time indicates sustained population-level protection against SARS-CoV-2 transmission or severe COVID-19 outcomes in children.