Figure 3. SARS-CoV-2 Antibody Boosting Between Rounds 3 and 4.

A, Round 4 antibody responses among the 232 participants who were seropositive at round 3 (65 participants aged <5 years, 58 participants aged 5-15 years, and 109 participants aged ≥16 years) are shown. The round 3 spike protein antibody response is shown on the x-axis, and the fold change between the round 4 and round 3 spike antibody responses is shown on the y-axis. We defined boosting as a 4-fold or greater increase (red dotted line). B, Participants were separated by vaccination status at round 4 and by tertiles of round 3 response (the second tertile is shown in the gray shaded rectangle). The colors of the points represent age groups, and the shapes of the points represent binned time since SARS-CoV-2 vaccination at the round 4 sample. Note that in panel B, boosting was observed in 41 of 47 participants vaccinated more than 3 weeks before their round 4 sample was collected. Among individuals who were not vaccinated, the proportion who demonstrated antibody boosting was 71% in the first tertile, 42% in the second tertile, and 11% in the third tertile. Among vaccinated individuals, the proportion who demonstrated antibody boosting was 97% in the first tertile, 91% in the second tertile, and 33% in the third tertile.