James Hamblin writes:

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.

Yet now, the nightmare scenario is happening a second time. The situation defies expert expectations about how immunity would help protect the hardest-hit populations. By estimates of leading infectious-disease specialists, such as Anthony Fauci, when roughly 70 to 75 percent of the population is immune, there can still be clusters of cases, but sustaining a large-scale outbreak becomes mathematically impossible. Still somehow, according to The Washington Post, hospitals in Manaus that had thought they were well prepared are now overwhelmed.

Though many questions remain, one plausible explanation is that people who have already been infected by the virus are getting sick—and not mildly so. That possibility has been long feared throughout the pandemic, yet not previously seen on any significant scale. It comes at a time when variants of SARS-CoV-2 are being identified around the world, including a report in Minnesota of a case of the variant that has become dominant in Brazil. Although no known variants have been found to pose an immediate threat to vaccinated people, the capacity for reinfection to any significant degree would reshape the pandemic’s trajectory. [Continue reading…]