The largest population level study of the prevalence of antibodies against the virus that causes Covid-19 was recently completed in Brazil. Population based information on Covid-19 is crucial to understand the impact of the virus on various segments of the population and to guide evidence-based policy decisions on control measures.
The study estimated that in some cities only one in ten infections with the virus that causes Covid-19 were reported as Covid-19 cases. The low proportion of reported cases demonstrates the need for expanded testing in Brazil. The study also found disparities in individuals most affected by infection with the virus that causes Covid-19. These disparities highlight the importance of making policy decisions based on scientific evidence to protect all groups against the negative impacts of Covid-19.
To determine the percentage of people with antibodies against the virus that causes Covid-19, researchers randomly tested residents of 132 cities in Brazil for these antibodies. If a person has antibodies against the virus that causes Covid-19, it suggests that the person was likely to have been infected with this virus previously. In each city, the goal was to test 250 individuals selected at random. However, the number of people who were in fact tested varied by each city. The study’s findings on antibody prevalence are likely to underestimate the true extent of previous infection because of how quickly antibody levels decrease and become undetectable by an antibody test, especially among the majority of cases who are asymptomatic.
Researchers conducted two surveys two weeks apart, so they could measure the change in antibody prevalence over time. The first survey occurred at the end of May and included 25,025 participants. The second survey occurred at the beginning of June and included 31,165 participants. Randomly selected participants provided two drops of blood that was tested for the presence of antibodies using the WONDFO SARS-CoV-2 point of care antibody test. Participants also completed a survey that asked questions about their demographics, socioeconomic status, and uptake of Covid-19 prevention behaviors. The demographic information included sex, ethnicity, age, and education level. The five groups recognized in the official Brazilian classification of ethnicity are: Branco (white), Pardo (Brown), Preto (Black), Amarelo (East Asian), and Indígena (Indigenous).
There were large disparities in infection rates between the ethnic groups and income levels in Brazil. The prevalence of antibodies against Covid-19 in the second survey was 6.4 percent in Indigenous groups compared to 1.4 percent in the white population. Among the poorest twenty percent of participants, prevalence in the second survey was 3.7 percent compared to 1.7 percent among the wealthiest twenty percent of participants. Members of the Indigenous population were over four times more likely to have antibodies against Covid-19 in their blood compared to white people. This large difference is partially explained by region, number of household members, and wealth. Even after controlling for all these factors, Indigenous people were still over fifty percent more likely to have antibodies against Covid-19 in their blood than their white counterparts.