After more than a year since the novel coronavirus (SARS-CoV-2) disease 2019 or COVID-19 has reached the status of a global pandemic, the number of COVID-19 cases continues to rise in Brazil. As no effective treatment been approved yet, only mass vaccination can stop the spread of SARS-CoV-2 and end the COVID-19 pandemic. Multiple COVID-19 vaccine candidates are under development and some are currently in use. This study aims to describe the characteristics of individuals who have registered in an online platform to participate in clinical trials for COVID-19 vaccines. Additionally, participants' characteristics according to age and presence of comorbidities associated with severe COVID-19 and differences of SARS-CoV-2 testing across different geographical areas/neighborhoods are provided. This was a cross-sectional web-based study conducted between September and December/2020, aiming to reach individuals aged >/=18 years who live in Rio de Janeiro metropolitan area, Brazil. Among 21,210 individuals who completed the survey, 20,587(97.1%) were willing to participate in clinical trials for COVID-19 vaccines. Among those willing to participate, 57.8% individuals were aged 18-59 years and had no comorbidity, 33.7% were aged 18-59 years and had at least one comorbidity, and 8.6% were aged >/=60 years regardless the presence of any comorbidity. Almost half (42.6%) reported ever testing for COVID-19, and this proportion was lower among those aged >/=60 years (p < 0.001). Prevalence of positive PCR results was 16.0%, higher among those aged 18-59 years(p < 0.009). Prevalence of positive antibody result was 10.0%, with no difference across age and comorbidity groups. Participants from areas/neighborhoods with higher Human Development Index (HDI) reported ever testing for SARS-CoV-2 more frequently than those from lower HDI areas. Interest to participate in clinical trials for COVID-19 vaccines candidates in Rio de Janeiro was significantly high. The online registry successfully reached out a large number of individuals with diverse sociodemographic, economic and clinical backgrounds.