摘要Background This study aimed to estimate vaccine effectiveness(VE)against omicron variant infection and severe corona virus disease 2019(COVID-19)in children aged 5-11 years hospitalized with acute respiratory syndrome.Methods A test-negative,case-control analysis was conducted from February 2022 to June 2022.We enrolled 6950 eli-gible children,including 1102 cases and 5848 controls.VE was calculated after immunization with one and two doses of BNT162b2 or CoronaVac.The outcomes were hospitalization with acute respiratory symptoms and detection of severe acute respiratory syndrome coronavirus 2(SARS-CoV-2)infection and severe COVID-19.The adjusted odds ratio for the association of prior vaccination and outcomes was used to estimate VE.Results For fully vaccinated children,the overall estimated VE against hospitalization with SARS-CoV-2 infection was 42％[95％confidence interval(CI)26 to 54].VE peaked at 29-42 days(67％,95％CI 40％to 82％)and then declined to 19％(95％CI,-20％to 45％)at 57-120 days after the second dose.The BNT162b2 vaccine had a similar VE against hospitaliza-tion with SARS-CoV-2 infection(45％,95％CI,20 to 61)compared to the CoronaVac vaccine(40％,95％CI,17％to 56％).Among cases,56(5％)children died;53(94.6％)were not fully vaccinated.For cases,the two-dose schedule effectiveness against ICU admission,need for invasive ventilation,severe illness,and death were 10％(95％CI,-54％-45％),22％(95％CI-70％-68％),12％(95％CI,-62％-52％),and 16％(95％CI,-77％-75％),respectively.Conclusions For hospitalized children aged 5-11 years during the omicron-predominant period in Brazil,two doses of both vaccines had moderate effectiveness against hospitalization with acute respiratory symptoms and SARS-CoV-2 infection and offered limited protection against endpoints of COVID-19 severity.