Unchecked spread of the coronavirus disease (Covid-19) in the Brazilian city of Manaus offers some clues on what achieving herd immunity against the virus may take, a new study has suggested.
The pandemic ripped through Manaus, a city of 2.1 million that lies at the centre of the Amazon rainforest region, in March and April with tens of thousands of infections, but the spread slowed down.
Now, a study -- published on the pre-print server MedRxiv before peer review – said the results of antibody analysis of banked blood and mathematical modeling led them to estimate that between 44% and 66% of the population of the city was infected.
This, the study said, was sufficiently high to have reached the threshold of herd immunity, in which enough members of a population develop immunity to a disease so as to break the chain of infection.
“The unusually high infection rate suggests that herd immunity played a significant role in determining the size of the epidemic,” the researchers wrote in the study, which was first reported by MIT Technology Review.
“All signs indicate that it was the very fact of being so exposed to the virus that brought about the reduction in the number of new cases and deaths in Manaus,” the study’s coordinator, University of Sao Paulo professor Ester Sabino, told the Sao Paulo State Research Support Foundation (FAPESP), which helped fund the study.
Manaus recorded its first infection in March and over the next two months was ravaged by the disease in the absence of quick state response or social distancing. Media reported overwhelmed health care workers struggling to control the disease, overrun hospitals and mass graves dug to accommodate rising fatalities.
But deaths started slowing down in recent weeks – from a daily high of 79 to just four in the past 14 days. The city is currently reopening after a months-long lockdown in Brazil, the country with the second-highest death toll worldwide.
“The elevated mortality and the rapid and sustained drop in cases suggest population immunity played a significant role in determining the size of the epidemic in Manaus,” the study said.
The authors cautioned that their results could not be extrapolated directly to other contexts due to differences in population demographics, behaviour, vulnerability to infection, as well as implementation and adherence to non-pharmaceutical measures.
The concept of herd immunity has triggered vigorous debate across the world with health experts warning that any attempt to reach the threshold was a risky policy that endangered the health of thousands of people.
“Community immunity via natural infection is not a strategy, it’s a sign that a government failed to control an outbreak and is paying for that in lives lost,” tweeted Florian Krammer, professor of microbiology at the Icahn School of Medicine at Mount Sinai Hospital in New York.