Tracking COVID-19’s explosive spread in the Brazilian Amazon

Brazil has had one of the world’s worst COVID-19 epidemics, with a particularly rapid and deadly outbreak in the Amazon region. A new study co-authored by researchers from Harvard T.H. Chan School of Public Health, and colleagues in Brazil and the U.K., tracked what happened when the coronavirus spread largely uncontrolled through the densely populated city of Manaus, capital of the state of Amazonas.

The study was published December 8, 2020 in Science. Harvard Chan School co-authors included Susie Gurzenda, a researcher in the Department of Global Health and Population (GHP) and Marcia Castro, Andelot Professor of Demography and GHP chair.

By testing for SARS-CoV-2 antibodies in the blood of a sample group of donors in Manaus the researchers estimated that 44% of the population had been infected by June, one month after the epidemic peaked in the city. By October, the infection rate rose to 76%, followed by a sustained drop in new cases. According to the researchers, these results are a data-based warning of what may be the extent of SARS-CoV-2 transmission in the absence of effective mitigation.

The researchers explored whether the epidemic may have reached a threshold in Manaus, or if the drop in cases was a result of non-pharmaceutical public health interventions or behavioral changes. Some mitigation strategies such as social distancing policies were introduced in the city in March but later relaxed. Castro has previously noted that strategies that may work in the U.S. or Europe can be less effective among low-income populations in Brazil, who may live in dense households with poor ventilation and lack of clean water.

Although the outbreak was devastating in Manaus—excess deaths had increased by 4.5-fold at its peak in early May—it would likely have been even worse with no mitigation strategies, according to the researchers’ prediction models. This suggests that public health interventions and behavioral changes worked in tandem with population immunity to contain the epidemic in Manaus, they wrote.

More research is needed to understand why the virus spread so rapidly in the city, the researchers wrote. They suggested that possible explanations may include socioeconomic conditions, overcrowding in housing and on transportation, and a young, mobile population. Monitoring of new cases is also needed to further understand population immunity.

Read coverage in Hindustan Times: Scientists decode outcome of unmitigated spread of Covid-19