Three quarters of the population of Manaus in Brazil's Amazon have been infected with coronavirus since the city's first outbreak in March.

Researchers from Imperial's COVID-19 Response Team, and a team of international collaborators found that:

• 76% population in Manaus became infected with SARS-CoV-2 between March and October 2020
• In contrast, they find that 29% became infected in São Paulo, the first city detecting SARS-CoV-2 circulation in Latin America

The international team, including scientists from the University of São Paulo, the Fundação Hospitalar de Hematologia e Hemoterapia do Amazonas, Fundação Pró-Sangue/Hemocentro de São Paulo, and the University of Oxford, have collected and analysed data on the prevalence of SARS-CoV-2 antibodies, a marker of prior infection, among blood donors in the Brazilian cities of Manaus and São Paulo.

By testing approximately 1,000 blood donation samples each month in both São Paulo and Manaus, the researchers show how the proportion of blood donors with antibodies against SARS-CoV-2 changed between February, before the first Brazilian COVID-19 case, and October, eight months into the on-going epidemic there.

The Amazon region is hardest hit

"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" Professor Nuno Faria Imperial College London

The results, published in the journal Science , show that, while both cities have experienced large epidemics with high mortality, as much as three-quarters of the population in Manaus was infected between March and October, and a third of the population in São Paulo.

The researchers used mathematical models to take into account waning of antibody titers measured by the researchers, as the proportion of blood donors testing positive halved between June and August.

The authors say that these results are a data-based warning of what may be the extent of SARS-CoV-2 transmission in the absence of effective mitigation.

Brazil has experienced an unprecedented epidemic caused by SARS-CoV-2, with more than 6.5 million cases reported to date and more than 175,000 deaths.

The Amazon region, in the north of the country, has been hardest hit. In Manaus, mortality increased rapidly in April.

The researchers say that the large transmission rate in Manaus may be explained by socioeconomic conditions, household crowding, limited access to clean water, and reliance on high transmission risk boat travel, in which over-crowding results in accelerated contagion, similar to that seen on cruise ships.

'Warning for other cities'

Professor Nuno Faria , Reader at Imperial College London and Associate Professor at the University of Oxford, principal investigator of CADDE project, 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 São Paulo, said: “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.”

The research was supported by Itaú Unibanco Todos pela Saúde programme; CADDE Medical Research Council - Fundação de Amparo à Pesquisa do Estado de São Paulo; Wellcome Trust and Royal Society, and National Heart, Lung, and Blood Institute of the US National Institutes of Health Recipient Epidemiology and Donor Evaluation Study (REDS).