It was thought that when a large portion of a community had been exposed to Covid, its spread became far less likely.
Related articles University of Sao Paulo researchers suggested that a drastic fall in COVID-19 deaths in Manaus pointed to collective immunity at work, but they now 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.
Non-essential businesses were closed but many simply ignored social distancing guidelines.
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The study by the University of Sao Paulo's Institute of Tropical Medicine tested newly donated banked blood for antibodies to the virus and used a mathematical model to estimate contagion levels.
The high infection rate suggested that herd immunity led to the dramatic drop in cases and deaths, the study said.
Daily burials and cremations fell from a peak of 277 on May 1 to just 45 in mid-September, according to the mayor's office.
But now the numbers are on the rise again.
Brazil's right-wing President Jair Bolsonaro (Image: Getty) The study's lead researcher, Ester Sabino, declined to be interviewed for this article because the Manaus herd immunity study awaits peer review for publication.
Authorities warned Manaus residents they were ignoring the virus and risked a second wave of contagion by not wearing masks, packing into bars and attending parties.
Epidemiologist Dr André Patricio Almeida, of the Adventist Hospital of Manaus, said cases are rising again mostly among younger, wealthier people who go to bars who show milder symptoms but often infect older relatives who need to be treated in hospital.
Almeida said too little is known about COVID-19 and whether re-infection is possible to verify if herd immunity was reached in Manaus, but some short-lived immunity probably had been attained.
"There could be immunity that does not last for long," Mayor Virgilio agreed.