LAUSR

Abstract Background We conducted a prospective study of severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) messenger RNA (mRNA) vaccination of children and adolescents who were taking immunosuppressive agents. Methods Two doses of SARS‐CoV‐2 mRNA vaccine were administered to patients taking immunosuppressive agents. Titers of SARS‐CoV‐2 spike protein receptor‐binding domain antibodies were measured before and after vaccination. Vaccine failure was defined as a postvaccination antibody titer of <0.8 U/mL. Seroconversion rates, factors associated with antibody titers after vaccination, clinical effectiveness against breakthrough infection, and adverse events were evaluated. Results A total of 42 patients (median age, 18.1 years) were enrolled. Immunogenicity was measured in 34. The median SARS‐CoV‐2 spike antibody titer was 329 U/mL (interquartile range [IQR] 50–812 U/mL). Seroconversion (≥0.8 U/mL) was achieved in 29 patients (85%), whereas vaccine failure was diagnosed in 5 (15%). All patients with vaccine failure were recipients of solid organ transplants (SOTs) and were taking two immunosuppressants. The median antibody titer in SOT recipients (57 U/mL) was significantly lower than that in nonrecipients (653 U/mL, p = 0.0002), and that of patients taking two immunosuppressive agents (93 U/mL) was lower than that of patients taking one (506 U/mL, p = 0.003). Breakthrough infection occurred in three patients (7%). Adverse events were nonspecific, and no flares of primary disease or acute rejection in SOT recipients occurred. Conclusions SARS‐CoV‐2 mRNA vaccine was immunogenic in children and adolescents taking immunosuppressive agents, although SOT recipients and patients taking two immunosuppressive agents tended to show lower postvaccination antibody titers.