Vaccination remains the most effective way to prevent SARS-CoV-2 infection or severe outcomes following infection. However, COVID-19 vaccine-induced humoral immune responses vary among individuals and wane over time. The variation and timing of this immune response is not fully understood, particularly in children. This research aims to 1) describe the SARS-CoV-2 anti-spike IgG antibody response to vaccination and 2) identify the health and demographic factors associated with this response among double-vaccinated children and adults in the Canadian CHILD Cohort.

This study included a subset of children (n= 153; mean age: 12 ±1.5 years, 46% female) and adults (n= 978; 44 ±6.0 years, 60% female) vaccinated with two doses. Blood samples were collected over two time-points- March 2021 to September 2021 (Phase A) and October 2021 to January 2022 (Phase B) using Dried Blood Spot (DBS) kits. SARS-CoV-2 anti-spike IgG antibody levels were quantified using automated chemiluminescent ELISAs and expressed in scaled luminescence. Demographic, vaccination, and health information were collected via online questionnaires. Associations were determined by linear regression.

In our cross-sectional data, we found a seropositivity rate of 95% following two COVID-19 vaccine doses for both children and adults. 14% (n= 157/1131) of participants had evidence of prior COVID-19. In both children and adults, the highest antibody levels were observed around three months post-vaccination and did not differ by biological sex. In a multivariable model, higher antibody levels were associated with: prior SARS-CoV-2 infection (β= 0.21 scaled luminescence units), age <18 years (β= 0.14) and receiving the Moderna mRNA (β= 0.20) or Moderna mRNA and Pfizer BioNTech vaccines (β= 0.20) vs. a combination of Moderna mRNA or Pfizer BioNTech vaccines and AstraZeneca Oxford vaccines. We did not observe any differences in antibody levels between a 3-8 compared to 9–16-week interval between first and second vaccine dose receipt.

In summary, we evaluated and characterized the anti-spike IgG antibody response following receipt of two COVID-19 vaccines and found antibody levels to be associated with age, previous SARS-CoV-2 infection, vaccine type, and time since vaccination. Understanding the determinants of vaccine responsiveness is an important research priority that could provide more insights into ways to improve vaccine immunogenicity and efficacy among different population subgroups.