The uncontrolled spread of the coronavirus disease (Covid-19) in the Brazilian city of Manaus offers some clues about what it may take to achieve herd immunity to the virus, a new study suggests.
The pandemic ripped through Manaus, a city of 2.1 million in the middle of the Amazon rainforest, in March and April with tens of thousands of infections, but the spread is slowing.
Now, a study – published on MedRxiv’s pre-print server before peer review – says results from blood bank antibody analysis and mathematical modeling led them to estimate that between 44% and 66% of the city’s population is infected.
This, the study says, is high enough to reach the herd immunity threshold, where enough members of the population develop immunity to a disease to break the chain of infection.
“The extremely high infection rates suggest that herd immunity plays a critical role in determining the size of the epidemic,” write the investigators for the study, first reported by the MIT Technology 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 professor Ester Sabino, told the Sao Paulo State Research Support Foundation (FAPESP), which helped fund the research.
Manaus recorded his first infection in March and over the next two months was ravaged by the disease due to lack of rapid state response or social distancing. Media reported overwhelmed health care workers struggling to control the disease, flooding hospitals and mass graves to accommodate the rising death toll.
But deaths have started to slow in recent weeks – from a daily high of 79 to just four in the last 14 days. The city is currently reopening after months of lockdown in Brazil, the country with the second highest death toll in the world.
“The rising mortality rate and the rapid and sustained decline in cases suggest population immunity played an important role in determining the size of the epidemic in Manaus,” said the study.
The authors caution that their results cannot be extrapolated directly to other contexts because of differences in population demographics, behavior, susceptibility to infection, and application and adherence to non-pharmaceutical measures.
The concept of herd immunity has sparked heated debate around the world with health experts warning that any attempt to reach a threshold is a risky policy that puts the health of thousands of people at risk.
“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.
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