Hospitalizations of coronavirus patients plummeted in the state from a peak of more than 1,300 in May to fewer than 300 in August. Excess deaths in Manaus fell from around 120 per day to practically zero. The city closed its field hospital.

In a country devastated by the novel coronavirus, where more than 3.6 million people have been infected and over 114,000 killed, the reversal has stunned front-line doctors. Manaus never imposed a lockdown or other strict containment measures employed successfully in Asia and Europe. And what policies did exist, many people ignored…

The factors that are helping to keep the virus at bay in Manaus and other cities remain unclear. Changed behaviors and individual community characteristics surely play a role. Manaus is testing far more than it once did. But whatever the dynamic, scientists and health officials are starting to wonder whether early prognostications about herd immunity overshot the mark.

It was initially believed that between 60 and 70 percent of the population needed to develop antibodies to reach collective immunity. But Guayaquil never broke 33 percent. Manaus, the capital of Amazonas state, never got past 20.