The US has added to research from Italy and France that indicates the coronavirus might have been circulating among people in a number of countries before it was identified in China and erupted into a pandemic.
Scientists from the US Centres for Disease Control and Prevention (CDC) said on Monday that tests of blood samples taken in the United States from December 13 last year revealed evidence of antibodies for the Covid-19 virus, known as Sars-Cov-2.
The samples were taken more than two weeks before the December 31 official confirmation of the outbreak in the central Chinese city of Wuhan and as much as a month earlier than the first confirmed case of Covid-19 in the US on January 19, according to the CDC report.
Antibodies are generated by the human immune system to identify and attack pathogens in the body and are specific to each type of virus, bacteria or parasite.
“The presence of these serum antibodies indicate that isolated Sars-CoV-2 infections may have occurred in the western portion of the United States earlier than previously recognised,” CDC scientists wrote in the study, which was published in the peer-reviewed journal Clinical Infectious Diseases.
The finding follows an analysis published in Italy last month of blood samples taken since September 2019, which also indicated the presence of antibodies against the new coronavirus. In France, a swab taken from a hospital patient on December 27 later tested positive for the coronavirus, according to a report published in the International Journal of Antimicrobial Agents in June.
Both sets of samples were taken before the virus was identified in Wuhan and add to the politicised debate about the origins of the coronavirus.
The CDC report also comes as two international investigative teams, one organised by the World Health Organization and the other by The Lancet medical journal, try to get to the bottom of where the disease originated.
That search, which involves trying to find the animal species the virus is thought to have jumped from, has been described by Mike Ryan, an epidemiologist and executive director of the WHO health emergencies programme, as a hunt for a needle in a haystack.
At least two events have to occur for an emerging disease to become established, according to the Baylor College of Medicine in Texas.
First, the pathogen has to be introduced to a susceptible population. Second, it has to be able to spread readily from person to person and cause disease. The infection also has to be able to sustain itself through infecting more and more people.
The WHO has said its team will start its investigation in Wuhan. The goal is to try to determine how the new coronavirus crossed the animal-human barrier, an endeavour that could take years and never be achieved, if searches for other types of infectious diseases are precedents. The Lancet team will also focus on the early spread of Sars-CoV-2.
According to the US CDC report, researchers tested 7,389 blood samples collected between December 13, 2019, and January 17. The samples came from residents in nine states: California, Connecticut, Iowa, Massachusetts, Michigan, Oregon, Rhode Island, Washington and Wisconsin.
It found 106 reacted to tests for Sars-CoV-2 antibodies. Out of those, 39 were collected between December 13 and 16, while the rest were taken between December 20, 2019, and January 17, the report said.
Scientists did further tests on 90 of those samples to confirm the antibodies were not for other coronaviruses, such as those that cause the common cold. A total of 84, or about 93 per cent, were found to have Covid-19-specific antibodies, according to the report.
“These findings suggest that Sars-CoV-2 may have been introduced into the United States prior to January 19, 2020,” it said, adding that was not possible to determine whether the infections were linked to people who had recently travelled or a community spread.
Accurately tracing the introduction of a pathogen to new populations is important for predicting case burden, health care utilisation and fatalities of Covid-19, according to the report.
Chinese foreign ministry spokesman Zhao Lijian has said that tracing the origins of the virus is a complex scientific problem and could involve multiple countries and locations.
The WHO has said that if Wuhan is not the origin of Sars-CoV-2, the first step is to thoroughly investigate the city where the outbreak began to determine if that is so – to rule it out.
As with studies in other countries now finding earlier evidence of the coronavirus, Chinese government data seen by the South China Morning Post indicates the first Covid-19 cases in the country can be traced back to November 17, or much earlier than the confirmed clusters of infections in Wuhan at the end of December.
A number of the first infections in the city were linked to the Huanan Seafood Wholesale Market, which sold wild and farmed animals. The role, if any, of the market in the outbreak remains unclear because of “the absence of analytical epidemiological study among vendors and shoppers”, according to the WHO investigation team’s terms of reference.
Last month, Ryan, from the WHO, said the organisation would like its team of scientists to be “deployed as soon as possible” to Wuhan.
“We have reassurances from our Chinese government colleagues that … a field part of the mission will be facilitated as soon as possible, in order that the international community can be reassured of the quality of the science,” he said.