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, according to Reuters.

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.

Local authorities on Friday enforced a 30-day ban on parties and other gatherings, and restricted restaurant and shopping hours, a setback for the city of 1.8 million after the worst of the pandemic seemed to be behind them.