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Manaus, a city with a population of two million, never introduced a full lockdown during the pandemic.
While non-essential businesses were closed, many people ignored social distancing guidelines.
Scientists from Imperial College London, University of Sao Paulo and Oxford University warned that the city is an example of the extent that COVID-19 can spread “without effective mitigation”.
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In April and May, so many Manaus residents were dying that its hospitals collapsed and mass graves had to be dug.
The city is the largest in the Amazon region – the worst hit area of Brazil, which has itself been one of the worst hit countries in the world.
The nation has seen around 6.5 million reported cases and more than 175,000 deaths.
The team of researchers behind the report found that 76% of Manaus’ population had been infected with COVID-19 between March and October.
In contrast, they found that only 29% became infected in Sao Paulo – another city that has been hit hard by the pandemic but did introduce full lockdown measures.
The report said the large transmission rate in Manaus may also be explained by socioeconomic conditions, household crowding and limited access to clean water.
Reliance on high transmission risk boat travel, in which over-crowding results in accelerated contagion – similar to that seen on cruise ships – also likely contributed, the scientists added.
When cases suddenly plummeted in June after Manaus’ huge infection and death rates during the first wave, some scientists theorised that the city had reached a form of herd immunity.
Many thought that Manaus may vindicate the argument for countries to adopt a herd immunity approach to the pandemic.
However, this was quickly thrown into doubt when a resurgence in cases swept through the city in September.
Professor Nuno Faria, from Imperial College London and the University of Oxford, said: “The large burden of illness and death caused by COVID-19 in Manaus emphasises the importance of face coverings, social distancing and hand washing to stop the spread of infection throughout Brazil – measures that will be reinforced by vaccination to immunise individuals at risk and, ultimately, to protect whole populations.”
Professor Ester Sabino, immunologist at the University of Sao Paulo, added: “Infection rates are currently high in Latin America and we find particularly high infection rates in Manaus, the largest urban metropolis in the Amazon region.
“Manaus is a warning for other cities, for example Sao Paulo could more than double the number of deaths if it reached a similar level of infection.”
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