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Researchers have analysed the unhindered spread of Covid-19 in Manaus, Brazil, where over 70 per cent of the population was Covid-positive within seven months, to understand what would happen if the world had not taken any step to mitigate the spread of the pandemic.
According to the researchers, including Oxford University scientists, Brazil has witnessed one of the most rapidly growing Covid-19 outbreaks. Amazon was the worst-hit region in Brazil.
The researchers said that in Manaus, the largest metropolis in the Amazon, the first SARS-CoV-2 case was reported in mid-March, after which non-pharmaceutical interventions (NPIs), such as social distancing, were introduced, as per the findings published in the journal Science .
A city in Brazil may have become immune to Covid-19: Study
The researchers noted that the explosive spread of coronavirus was followed by high mortality rates and then the epidemic witnessed a sustained drop despite relaxed NPIs.
Study design
To understand whether the population achieved herd immunity after the wild spread of Covid-19, scientists collected data from blood donors in Manaus. They inferred the virus attack rate from the collected blood samples and compared this data with that of Sao Paulo, which was less impacted.
The researchers estimated a 76 per cent attack rate in Manus by October, including adjustments for waning antibody immunity. By comparison, the attack rate in Sao Paulo by October was 29 per cent, partly explained by the larger population size, they added.
The researchers maintained that the attack rates remained lower than predicted in a mixed population with no mitigation strategies despite the widespread Covid-19 in these two cities. “It is likely that NPIs worked in tandem with growing population immunity to contain the epidemic,” they noted. They also acknowledged voluntary behavioural changes as helping.
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However, according to the authors of the study, further research is needed to determine the longevity of population immunity.
“Monitoring of new cases...will also be vital to understand the extent to which population immunity might prevent future transmission, and the potential need for booster vaccinations to bolster protective immunity,” they wrote in the study.
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