The Brazilian city of Manaus, devastated by the coronavirus pandemic, may have suffered so many infections that its population now benefits from “herd immunity,” according to preliminary studies.
Published on the medRxiv website, the study analyzed infection data with mathematical modeling to estimate that 66 percent of the population had antibodies to the new corona virus in Manaus, where the spread of a pandemic so fast is brutal.
That may be high enough to reach the threshold of herd immunity, where enough members of the population immune to the disease can no longer spread effectively, said the study’s authors, a group of 34 Brazilian and international researchers.
“The very high infection rates suggest that herd immunity plays an important role in determining the size of the epidemic,” they wrote in the study, which has not undergone peer review.
“All the signs point to the fact that exposure to the virus is what led to the reduction in the number of new cases and deaths in Manaus,” the study coordinator, University of Sao Paulo medical professor Ester Sabino, told the Sao Paulo State Research Support Foundation (Fapesp), which helped fund the research.
Located in Amazon rainforest, Manaus is the scene of gruesome images of a flooded hospital, mass graves and bodies piled on refrigerator trucks when the pandemic reached its peak in May.
But deaths in the city of 2.2 million people have fallen dramatically in recent weeks, to an average of just 3.6 per day over the past 14 days.
Manaus is now one of the fastest reopening cities from lockdown in Brazil, the country with the second highest death toll in the world, after the United States, with nearly 139,000 people killed.
That includes the famous school, business, nightlife and opera house.
However, health experts caution that efforts to achieve herd immunity are a dangerous path for policymakers.
“Community immunity through natural infection is not a strategy, it’s a sign that the government is failing to control the outbreak and is paying for it in the lives lost,” tweeted Florian Krammer, professor of microbiology at the Icahn School of Medicine at Mount Sinai Hospital in New York.
Other experts warn that immunity to the virus may be short-lived.
Manaus has recorded 2,462 deaths from Covid-19.
If it were a country, it would have the second highest mortality rate in the world, with 100.7 deaths per 100,000 population.