The new coronavirus was circulating in Brazil in early February, weeks earlier than initially detected, and just before millions of people were partying in the streets for carnival, according to a new study.
Brazil is the Latin American country hardest hit in the pandemic, with more than 11,500 deaths and 168,000 infections so far. Experts say under-testing means the real figures are probably far higher.
The study used statistical analysis to work backwards from the number of reported COVID-19 deaths and establish the probable time-frame of the virus' early spread in Brazil and other countries, said the Oswaldo Cruz Foundation (Fiocruz), the country's leading public health institute.
"The new coronavirus began spreading in Brazil around the first week of February. That is to say, more than 20 days before the first case was diagnosed in a traveler returning from Italy, on February 26... and more than 40 days before the first official confirmation of communal transmission," the institute said.
That means the local outbreak was already well under way when Brazil celebrated carnival from February 21 to 25, an event that draws millions of tourists and brings throngs of revelers into the streets in Rio de Janeiro, Sao Paulo, Salvador and other cities.
The study, which also analyzed data from Europe and the United States, found the virus was probably also spreading locally two to four weeks before the first cases were detected in Italy, the Netherlands and the United States.
"This lengthy phase of hidden communal transmission of the new coronavirus... indicates that containment measures should be taken at least as soon as the first imported cases are detected," said the lead researcher on the study, Gonzalo Bello.