To estimate the vaccine effectiveness after first and second dose of ChAdOx1 nCoV-19 against symptomatic COVID-19 and infection in a socially vulnerable community in Brazil when Gamma and Delta were the predominant variants circulating.
We conducted a test-negative study at the community “Complexo da Maré”, the largest group of slums (n=16) in Rio de Janeiro, Brazil, from Jan 17, 2021 to Nov 27, 2021. We selected RT-qPCR positive and negative tests from a broad community testing program. The primary outcome was symptomatic COVID-19 (positive RT-qPCR with at least 1 symptom) and the secondary outcome was infection (any positive RT-qPCR). Vaccine effectiveness was estimated as 1 – OR, which was obtained from adjusted logistic regression models.
We included 10,077 RT-qPCR tests (6,394, 64% from symptomatic and 3,683, 36% from asymptomatic individuals). The mean age was 40 (SD: 14) years and the median time between vaccination and RT-qPCR testing among vaccinated was 41 [p25-p75: 21-62] days for the first dose and 36 [p25-p75: 17-59] days for the second dose. Adjusted vaccine effectiveness against symptomatic COVID-19 was 31.6% (95% CI, 12.0-46.8) after 21 days of first dose and 65.1% (95% CI, 40.9-79.4) after 14 days of second dose. Adjusted vaccine effectiveness against COVID-19 infection was 31.0% (95% CI, 12.7-45.5) after 21 days of first dose and 59.0% (95% CI, 33.1-74.8) after 14 days of second dose.
ChAdOx1 nCoV-19 was effective on reducing symptomatic COVID-19 in a socially vulnerable community in Brazil when Gamma and Delta were the predominant variants circulating.

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