Researchers in

say they have detected

in sewage sampled in November.

The conclusion comes days after scientists

found the virus in waste water samples collected in March, 2019, and adds to suggestions that the virus spread quietly before the alarm was first raised in China in late December.

“[The virus] was being shed within the community for several months prior to the first cases being reported by regional, national or Pan-American authorities,” Gislaine Fongaro, from the Federal University of Santa Catarina, said in a non-peer-review paper posted on the preprint server on Monday.

Fongaro’s study was based on samples collected from the waste water network in Florianopolis, a beach town in southern Brazil.

Previous studies indicated that the coronavirus could bind with cells in the intestines, and the researchers thought sewage could be a useful tool to monitor the rise and fall of the epidemic in the city, with up to 100 million copies of the virus per gram of faecal matter.

Fongaro and colleagues conducted tests for Sars-CoV-2, the formal name of the coronavirus, on samples dating from October last year to April this year. The tests targeted three different parts of the viral genome to reduce the risk of a false positive, and were carried out on samples that had been stored at a temperature of minus 80 degrees Celsius (-112 Fahrenheit).

The virus first appeared in two samples on November 27. Its presence was modest, and remained stable in the following months until a sudden peak in early March, according to Fongaro.

All tests were performed “in two independent experiments” and the positive samples were further confirmed in a separate laboratory, she added.

Doctors in the central Chinese city of Wuhan noticed some cases of atypical pneumonia in late December, and health authorities reported the disease was caused by an unknown strain of coronavirus on December 31, 2019.

Chinese scientists isolated a living strain of the virus a few days later and released the full genome sequence on January 11.

The first diagnosed case of Covid-19 in the Americas was reported in the United States on January 21. The virus that found its way into the drainage system of Florianopolis was “66 days in advance of the first Covid-19 confirmed case in the Americas, 91 days in advance of the first case in Brazil, and 97 days in advance of the first confirmed case in Santa Catalina Region”, the Brazilian researchers said.

In Barcelona, scientists found the virus in waste water samples dated back to March 12, 2019.

Professor Albert Bosch, president of the Spanish Society of Virology and lead scientist of the study, said local health authorities might have confused Covid-19 with influenza.

“Those infected with Covid-19 could have been diagnosed with flu in primary care by mistake, contributing to the community transmission before the public health took measures,” Bosch was quoted as saying in a statement posted on the website of the University of Barcelona on June 26.

An infectious disease researcher at the Chinese Academy of Sciences tracking worldwide reporting of the pandemic said a major concern about the drainage record was environmental contamination.

“With all respect to Bosch and his team, we need to know for sure no one handling the sample was infected,” said the researcher who asked not to be named because she was not authorised speaking to the media.

“Another question is, how could all these countries treat it as a flu?”

Some artificial intelligence analysis of chest scans from the US and Europe suggested that the novel coronavirus might have been causing lung infections with unknown cause long before local cases were reported. Some antibody surveys also suggested early, unnoticed spreads.

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