Unfolding of a COVID-19 Transmission Wave in a Cluster of Highly Vulnerable Individuals in Rio De Janeiro – a Spatial Populational-Based Serosurvey

Background: COVID-19 serosurveys allow monitoring the level of SARS-CoV-2 transmission and support data-driven decisions, which are particularly useful in low-income settings with limited testing capacity and unequal COVID-19 burden. We estimated the seroprevalence of anti-SARS-CoV-2 antibodies in a large favela complexes in Rio de Janeiro, Brazil.Methods: A population-based panel study was conducted in Complexo de Manguinhos (16 favelas ) with a probabilistic sampling of participants aged ≥1 year. Participants answered a structured interview and provided blood samples for serology (anti-S IgG antibodies). Multilevel regression models (with random intercepts to account for participants spatial clustering) were used to assess factors associated with reactive serology. Secondary analyses estimated the seroprevalence using an additional anti-N IgG assay.Findings: 4,033 participants were included (from Sep/2020 toFeb/2021, 22 epidemic weeks), the median age was 39·8 years (IQR:21·8-57·7), 61% were female, 41% were mixed-race ( Pardo ) and 23% Black. Overall prevalence was 49·0% (95%CI:46·8%-51·2%) which varied across favelas (from 68·3% to 31·4%). Seroprevalence increased after epidemic week 15, concurrently with the upsurge of the Gamma (P1) variant in Rio de Janeiro. Lower prevalence estimates were found when using anti-N IgG assay across all age groups.Interpretation: We found a significantly higher prevalence of anti-S IgG antibodies than initially anticipated. Albeit high levels of seroprevalence levels, we documented the unfolding of a new transmission wave. Our results highlight the need for cautious interpretation of serosurveys estimates given the heterogeneity of exposure in communities, loss of immunological biomarkers, serological antigen target, and variant-specific test affinity.Funding: TST acknowledge funding from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ). GTG has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 765048 and CNPq funding 403679/2020-6. Declaration of Interest: All authors declare no competing interests.Ethical Approval: Local ethics committees approved the study (Instituto Nacional de Infectologia Evandro Chagas (INI)/Fiocruz, Escola Nacional de Saúde Pública (ENSP)/Fiocruz, and InstitutoOswaldo Cruz (IOC)/Fiocruz).