A Longitudinal Seroepidemiology Study to Evaluate Antibody Response to SARS-CoV-2 Virus and Vaccination in Children in Calgary, Canada from July 2020 to September 2022.

Abstract

Introduction: Measurement of SARS-CoV-2 antibody seropositivity is important to accurately understand exposure to infection and/or vaccination in populations. Methods: A cohort of 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, with an additional 5th visit in the fall of 2022. Demographic and clinical information was obtained including SARS-CoV-2 test results and vaccination records. Results and analysis: 1035 children were enrolled and 88.9% completed 4 visits; median age 9 years (IQR: 5,13); 519 (50.1%) female; and 815 (78.7%) Caucasian. 477 (46.1%) participants attended Visit 5. Before enrollment, 118 (11.4%) had confirmed or probable SARS-CoV-2 infection. By September 2022, the total cumulative percentage of previously uninfected participants diagnosed with COVID-19 was 53.9% by Visit 5. (0/917 (0%), 15/873 (1.7%), 31/837 (3.7%), 280/820 (34.1%), and 63/439 (14.4%) at Visits 1-5, respectively). Nucleocapsid antibody seropositivity declined to 18.0% after more than 200 days after diagnosis. In contrast, spike antibodies remained elevated in 97.6% of unvaccinated children after more than 400 days after diagnosis. By September 2022, 97.1% (232/239) of children 12 years and older, 85.9% (171/199) of children 5-11, and 15.4% (6/39) of children under 5 received at least 1 dose of vaccine. At that time, all 409 vaccinated children had spike antibodies, compared with 38/64 (59.4%) of unvaccinated children (P<0.001 for comparison of proportions). Conclusions: By September 2022, most children in the study had a serologic response against the SARS-CoV-2 virus from infection and/or vaccination, with unvaccinated children much less likely to have a serologic response. Ongoing studies of serologic status are needed to estimate population levels of virus exposure and durability of antibody response after infection and/or vaccination.