The largest city in Brazil's Amazon has closed bars and river beaches to contain a fresh surge of coronavirus cases, a trend that may dash theories that Manaus was one of the world's first places to reach collective, or herd, immunity.

When a large portion of a community becomes immune to a disease, its spread becomes unlikely.

University of Sao Paulo researchers suggested that a drastic fall in COVID-19 deaths in Manaus pointed to collective immunity at work, but they also believe that antibodies to the disease after infection may not last more than a few months.