How an Amazonian city that shunned lockdown may have hit herd immunity

The authorities in Manaus made little attempt to halt spread of Covid-19 - yet the virus seems to have run out of steam there

Mass graves dug for Covid victims in Manaus
A coronavirus mass grave in Manaus, Brazil, earlier this year
(Image credit: Michael Dantas/AFP/Getty)

Located deep in the Amazon rainforest, Manaus has always been a coronavirus outlier.

Having been cited as a warning lesson at the beginning of the pandemic, the Brazilian city now provides scientists with a puzzling case study in how Covid-19 might behave if left unchecked.

“Manaus was once a symbol of the threat that the virus might pose to the developing world,” says The Times. “Drone images of mass graves caused alarm around the world four months ago as Covid-19 ravaged the city and burials were running at five times their normal rate.”

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Hospitals ran out of beds, oxygen and stretchers, and gravediggers couldn’t keep up with the backlog of bodies.

“But then, unexpectedly, it started to let up ­- without the interventions seen elsewhere,” The Washington Post reports.

The city has never imposed a lockdown, encouraged social distancing or required face masks, yet excess deaths have dropped from about 120 per day in May to “practically zero”, says the newspaper.

Precisely why the virus has fizzled out remains unclear, but a hypothesis has been offered by Jarbas Barbosa da Silva, assistant director of the Pan American Health Organization. “The peak we had in Manaus was very strong, and there was such widespread community transmission that it may have produced some kind of collective immunity,” he suggests.

The concept of herd immunity has “proved highly contentious”, says The Times. Since most epidemiologists assumed that 70% of people would have to be infected with the Covid-19 coronavirus before the disease could be controlled, the “policy was deemed unacceptable by almost all governments”.

Yet in Manaus, only 20% of the 1.8 million-strong population are thought to have had the virus - about the same proportion as in London and New York.

Even if the city has reached herd immunity, da Silva says it “paid a very large price”.

More than 3,000 Covid-related deaths were reported in Manaus, “the equivalent of one in 500 residents being killed by the virus”, says the Daily Mail. And in the rest of the country, the “outbreak is still rattling on, with Brazil registering 1,271 Covid-19 deaths and 47,134 new cases on Monday”.

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