The city of Manaus, in Amazonas, the largest in the Brazilian Amazon, closed bars and beaches on rivers to contain a new wave of coronavirus cases.
The trend threatens to contradict theories that the region would be one of the first places in the world to have achieved so-called herd immunity, when a large part of a community is immune to a disease and its spread is less likely.
Researchers at the University of São Paulo (USP) suggested that the dramatic drop in deaths from covid-19 in Manaus indicated 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 decided to ban parties and other gatherings of people for 30 days, also restricting restaurant and shopping hours, in a new setback in the city of 1.8 million people after the worst of the pandemic seemed to be behind them. .
Between April and May, so many residents of Manaus were dying of covid-19 that hospitals collapsed and cemeteries were unable to dig pits quickly enough.
The city never imposed a complete lockdown. Non-essential deals were closed, but many simply ignored the social detachment guidelines.
In June, deaths dropped unexpectedly. Public health experts then began to question whether so many residents had caught the virus that there was no one else to be infected.
A survey published last week on medRxiv, a website that distributes non-public health articles, estimated that between 44% and 66% of the population of Manaus had been infected between the height of the pandemic, in mid-May, and August.
The study, from the Institute of Tropical Medicine at the University of São Paulo, tested a new blood bank recently donated for antibodies against the virus and used a mathematical model to estimate levels of contagion.
The high rate of infection suggested that herd immunity had led to a dramatic drop in the number of deaths and cases, according to the study.
Daily burials and cremations dropped from a peak of 277 on May 1 to just 45 in mid-September, according to the city. The daily count of Covid-19 deaths in the city, which peaked at 60 on April 30, according to official data, dropped to just two or three a day in late August.
Now, the numbers have grown again.
The researcher who led the study, Ester Sabino, declined to be interviewed because her work on Manaus herd immunity awaits peer review before being published.
Authorities warned residents of Manaus that they were ignoring the virus and that there was a risk of a second wave of contagion due to the absence of masks, crowded bars and parties. Beaches on Manaus rivers where rave parties were being held were closed.
The mayor of Manaus, Arthur Virgilio, blamed President Jair Bolsonaro, who he said minimized the severity of the pandemic by encouraging people to return to life and work normally instead of waiting for the development of a vaccine.
“The government needs to take the situation seriously and speak the truth. If it says it is okay, it encourages people to ignore our decrees,” the mayor said in an interview with Reuters news agency.
Epidemiologist André Patricio Almeida, from the Adventist Hospital of Manaus, said that cases are growing again mainly among young people, healthier people who go to bars and show mild symptoms, but often pass the disease on to older relatives who need to be treated in hospitals.
Almeida said that little is known about covid-19 and whether reinfection is possible to see if Manaus has achieved herd immunity, but he believes that some short-term immunity has probably been achieved.
The study by the University of São Paulo stated that antibodies against coronavirus appear to decrease after just a few months, which may explain the resurgence of cases in Manaus.
“Something that was made clear in our study – and that is also being shown by other groups – is that antibodies against SARS-CoV-2 decay quickly months after infection,” said one of the study’s authors, Lewis Buss, in a statement from the São Paulo State Research Support Foundation (Fapesp), which accompanied the article.
“This is clearly happening in Manaus,” said Buss.