Associate Professor University of British Columbia
Background: Prior to fall 2023, coronavirus disease 2019 (COVID-19) vaccine series consisting of two primary mRNA vaccine doses plus one booster dose was recommended for healthy children aged 5-11 years in Canada. Studies are needed to understand longitudinal immunity to COVID-19 vaccination in the pediatric population. Objective: The Severe acute respiratory syndrome-related coronavirus 2 PRevalence In children and youNG adults in British Columbia (SPRING) sub-study and Enfants et COVID-19: Étude de seroprevalence (EnCORE) cohorts aimed to investigate antibody responses to COVID-19 vaccination in healthy children aged 5-11 years. Design/Methods: Ninety-five children without immunocompromising conditions and not taking immunosuppressive medications were enrolled in either study (BC or Quebec) prior to dose one or dose two of their COVID-19 vaccine series. Responses were quantified via anti-index virus spike protein specific IgG (S-IgG) avidity, reported as total relative avidity index (TRAI) (avidity units, AU) and total absolute avidity index (TAAI) (absolute avidity units, AAU/mL). TRAI is a weighted sum of the proportions of all S-IgG avidity levels while TAAI is a weighted sum of the concentrations of S-IgG avidity levels. Responses at one- and six-months post-dose three were compared with corresponding post-dose two timepoints. A Welch’s t-test compared responses between timepoints. Results: Together, cohorts consisted of 56% males and 44% females with a median age of 8 years. TRAI increased one-month post-dose three compared with post-dose two (79 vs. 64, p < 0.0001) but did not significantly change by six-months post-dose three compared with post-dose two (72 vs. 67, p = 0.295). TAAI increased both at one- (1246 vs. 853, p = 0.047) and six-months (640 vs. 233, p = 0.004) post-dose three in comparison with the corresponding post-dose two timepoints. This increase in avidity was due to higher concentrations of high and very high avidity S-IgG at one- and six-months post-dose three compared with post-dose two timepoints.
Conclusion(s): Anti-index virus S-IgG avidity increased post-dose three in comparison with corresponding post-dose two timepoints. This led to the hypothesis that repeated exposure to the S-protein resulted in affinity maturation and ultimately in the production of higher avidity antibodies post-dose three. This finding suggests that S-IgG avidity should be considered when determining protection against COVID-19 and needs to be further explored to support future booster recommendations. Future data will include participants from three additional cohorts located in Alberta, Manitoba and Ontario.
