Even in a year of horrendous suffering, what is unfolding in Brazil stands out. In the rainforest city of Manaus, home to 2 million people, bodies are reportedly being dropped into mass graves as quickly as they can be dug. Hospitals have run out of oxygen, and people with potentially treatable cases of COVID-19 are dying of asphyxia. This nature and scale of mortality have not been seen since the first months of the pandemic.
This is happening in a very unlikely place. Manaus saw a devastating outbreak last April that similarly overwhelmed systems, infecting the majority of the city. Because the morbidity was so ubiquitous, many scientists believed the population had since developed a high level of immunity that would preclude another devastating wave of infection. On the whole, Brazil has already reported the second-highest death toll in the world (though half that of the United States). As the country headed into summer, the worst was thought to be behind it.
Data seemed to support the idea that herd immunity in Manaus was near. In Science this month, researchers mapped the virus’s takeover last year: In April, blood tests found that 4.8 percent of the city’s population had antibodies to SARS-CoV-2. By June, the number was up to 52.5 percent. Since people who get infected do not always test positive for antibodies, the researchers estimated that by June about two-thirds of the city had been infected. By November, the estimate was about 76 percent. In The Lancet this week, a team of Brazilian researchers noted that even if these estimates were off by a large margin, infection on this scale “should confer important population immunity to avoid a larger outbreak.” Indeed, it seemed to. The city was able to largely reopen and remain open throughout its winter with low levels of COVID-19 cases.