Vaccine effectiveness of ChAdOx1 nCoV-19 against COVID-19 in a socially vulnerable community in Rio de Janeiro, Brazil: a test-negative design study

Abstract

Objectives 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. Methods 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. Results 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. Conclusions 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.

Vaccine effectiveness of ChAdOx1 nCoV-19 against COVID-19 in a socially vulnerable community in Rio de Janeiro, Brazil: author's response

“…We would like to highlight that our vaccine estimates were in accordance to previous literature and robust to several sensitivity analyses, as shown in the supplementary material [ 2 ].…”

“…We appreciate the interest in our publication [ 1 , 2 ]. First, we would like to highlight that our study followed a pre-specified protocol, which was based on other protocols for evaluation of COVID-19 vaccine effectiveness applying case-control, test-negative study design conducted in Brazil and in accordance with the World Health Organization (WHO) recommendations [ 3 , 4 ].…”

“…Regarding the decrease in effectiveness following the first dose, the change overtime for the first dose is shown in Table 1 of the original article [ 2 ]. We did not evaluate change overtime for the second dose because we had not sample size/power to conduct this analysis.…”

“…Regarding potential confounding factors, we excluded previous infected individuals (eFigure 3, supplementary material of the original article [ 2 ]). We agree that personal behaviour could affect the risk of infection, and if this behaviour is different between vaccinated individuals compared to unvaccinated individuals, we could have residual confounding, as acknowledged in the article [ 2 ]. However, as also discussed in the manuscript, our indicator bias did not indicate any remarkable bias, under the assumption of a case-control, test-negative design [ 5 ].…”

Re: Vaccine effectiveness of ChAdOx1 nCoV-19 against COVID-19 in a socially vulnerable community in Rio de Janeiro, Brazil by Ranzani et al.

“…Ranzani and colleagues evaluated the effectiveness of ChAdOx1 NCOV-19 against COVID-19 in a socially vulnerable community, and observed vaccine effectiveness increased up to 53.2% during 42-55 days of the first dose, and decreased afterwards in those that did not take the second dose 1 . They hypothesized that this decrease might occur in part because of an increase on the Delta dominance and wanin.…”