Background Vaccination helps prevent SARS-CoV-2 infection and severe COVID-19. However, vaccine-induced humoral immune responses vary among individuals and wane over time. We aimed to describe the SARS-CoV-2 anti-spike IgG antibody response to vaccination and identify health and demographic factors associated with this response among children and adults.

Methods We studied a subset of double-vaccinated children (n= 151; mean age: 12 ±1.5 years, 46% female) and adults (n= 995; 44 ±6.0 years, 60% female) from the Canadian CHILD Cohort. Dried blood spots were collected over two time periods (March 2021 to September 2021; October 2021 to January 2022). Antibody levels were quantified using automated chemiluminescent ELISAs. Demographic, vaccination, and health data were collected via online questionnaires. Associations were determined using multivariable regression.

Results Our cohort had SARS-CoV-2 anti-spike seropositivity rate of 97% following two COVID-19 vaccine doses. In both children and adults, the highest antibody levels were observed around three months post-vaccination and did not differ by biological sex. Higher antibody levels were associated with: prior SARS-CoV-2 infection (β=0.15 scaled luminescence units, 95%CI, 0.06-0.24), age <18 years (β=0.15, 95%CI 0.05-0.26) and receiving the Moderna mRNA (β=0.23, 95%CI 0.11-0.34) or Pfizer-BioNTech mRNA vaccines (β= 0.10, 95%CI, 0.02-0.18) vs. a combination of mRNA and Oxford-AstraZeneca viral vector vaccines. There were no differences in antibody levels when comparing a 3-8 vs. 9-16-week interval between vaccine doses.

Interpretation We identified key factors associated with post-vaccination antibody responses in children and adults, which could help improve future vaccine development and deployment among different population subgroups.

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