Herd Immunity Reached in Brazil's Amazon

According to a non-peer-reviewed study published on September 21, 2020, the transmission of the SARS-CoV-2 virus in Manaus, located in the Brazilian Amazon, increased quickly during March 2020, and then began to decline. The seroprevalence rate fell due to antibody waning. After correcting for this trend, these researchers estimate a final epidemic size of 66%.

Although non-pharmaceutical interventions and changes in population behavior may have helped to limit SARS-CoV-2 transmission in Manaus, the unusually high infection rate suggests that 'herd immunity' played a significant role in determining the size of the COVID-19 epidemic in Brazil.

As of September 28, 2020, Manaus (pop. of 1.8 million) had reported 4,031 fatalities related to COVID-19 during 2020.