We used a test-negative design to estimate the vaccine effectiveness of Ad26.COV2.S (Janssen) against symptomatic COVID-19 and clinical outcomes in Mato-Grosso do Sul, Brazil. We analyzed 11,817 RT-PCR tests. The mean age was 37 (SD=17) years, 2,308 (20%) of individuals more or equal than 50 years and almost two-thirds of the population was Brown/Pardo. Adjusted effectiveness against symptomatic COVID-19 after 28 days of the single dose was 50.9% (95% CI, 35.5-63.0). Adjusted effectiveness against clinical outcomes was 72.9% (95% CI, 35.1-91.1) for hospitalization, 92.5% (95% CI, 54.9-99.6) for ICU admission, 88.7% (95% CI, 17.9-99.5) for mechanical ventilation and 90.5% (95% CI, 31.5-99.6) for death. Despite lacking precision on some estimates, a single dose of Ad26.COV2.S vaccine continues to protect specially for severe forms of COVID-19 in the context of new variants.
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