Results:

A total of 262 participants (150 [57%] participants aged 12-17 years and 112 [43%] aged 6-11 years) were randomly assigned to receive vaccination with two doses of either ChAdOx1 nCoV-19 (n = 211 [n = 105 at day 28 and n = 106 at day 84]) or control (n = 51 [n = 26 at day 28 and n = 25 at day 84]). While solicited adverse effects were common (80% and 76% after the first and second dose, respectively), no serious adverse events related to ChAdOx1 nCoV-19 administration were recorded by the data cutoff date on October 28, 2021. Pain and tenderness were the most common local solicited adverse events. Among seronegative participants who received ChAdOx1 nCoV-19, anti-SARS-CoV-2 IgG and pseudoneutralizing antibody titers at day 28 after the second dose were higher in participants aged 12–17 years with a longer interval between doses (geometric means of 73,371 arbitrary units [AU]/mL [95% CI, 58,685–91,733] and 299 half-maximal inhibitory concentration [IC50; 95% CI, 230–390]) compared with those who received their vaccines 28 days apart (43,280 AU/mL [95% CI, 35,852–52,246] and 150 IC50 [95% CI 116–194]). Humoral responses were higher in those aged 6–11 years than in those aged 12–17 years receiving their second dose at the same 112-day interval (geometric mean ratios 1.48 [95% CI, 1.07–2.07] for anti-SARS-CoV-2 IgG and 2.96 [1.89–4.62] for pseudoneutralizing antibody titers). Cellular responses peaked after a first dose of ChAdOx1 nCoV-19 across all age and interval groups and remained above baseline after a second vaccination.