Key message:
3-dose COVID-19-vaccine has excellent safety profile in adolescents with rheumatic diseases; minimal risk of exacerbation. Adequate long-term immunogenicity in healthy controls and most rheumatic adolescents; the third dose restored titres. Breakthrough COVID-19 infection rates and time from last vaccination to infection were similar between groups. BNT162b2 mRNA COVID-19 booster vaccine is recommended for adolescents with rheumatic diseases