The Brazilian city of Manaus has witnessed so many COVID-19 infections that the local population may now be benefitting from "herd immunity", according to a preliminary study published on medRxiv.
The novel coronavirus pandemic has devastated Manaus city, situated in the Amazon rainforest.
Brazil reported 45.9 lakh COVID-19 cases as of September 24. This is the third-highest caseload in the world after the United States and India. Brazil has also reported 1.38 lakh fatalities related to COVID-19 – the second-highest death toll in the world.
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The study suggests that the transmission of SARS-CoV-2, which causes COVID-19, spiked during March and April in Manaus before falling slowly between May and September.
In June, one month after the outbreak peaked locally, 44 percent of the population was sero-positive for the virus. This number was adjusted to 52 percent after taking into account the false-negatives. This sero-prevalence rate fell in July and August due to waning of antibodies, the study suggests.
After data adjustment, the study estimates that the final epidemic size was 66 percent in terms of sero- prevalence.
While interventions by authorities and change in population behaviour may have helped limit the spread of COVID-19 in the city, the “unusually high infection rate suggests that herd immunity played a significant role in determining the size of the epidemic”, according to the study.
The fall in number of cases being reported on a daily bases has meant that businesses are reopening in Manaus faster than the rest of Brazil. Health experts, however, have cautioned that attempting to deliberately reach herd immunity is dangerous.
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