Coronavirus: COVID-19 can lurk undetected in places 'free' of the virus - study

COVID-19 might be able to lurk in the community undetected for months, a worrying new study has found, with thousands of Wuhan residents infected after China declared the city free of the virus.  

Wuhan, where the SARS-CoV-2 virus behind the disease was first detected late 2019, saw its last patient discharged from hospital at the end of April after a strict lockdown appeared to wipe out the disease. Testing of the city's entire population in early June found no confirmed infections, according to Chinese media reports. 

Between March and May, only a handful of new cases were reported in China each day - sometimes down to single-digits - and only a fraction of them were due to local transmission.

But testing of blood samples collected at the time tells a different story. Researchers in China say the prevalence of antibodies is far greater in blood collected in Wuhan than elsewhere in the country, which isn't surprising since that's where the outbreak began; but nearly half a percent of them contained IgM antibodies for the virus, indicative of a recent infection.

With a population of 10 million, that suggests potentially 50,000 people were infected in Wuhan between March and May - during which time the lockdown ended and China said the outbreak was under control. 

"We estimate thousands of people had been infected asymptomatically in a period of two months from March 6 to May 3 of 2020," the researchers said in their new study, published Friday in journal PLOS Neglected Tropical Diseases.

A massive 1.68 percent of people had IgG antibodies, indicative of a historic infection - that's 168,000 Wuhan residents, or double the official confirmed infection tally for all of China. 

"We conclude that… a large amount of asymptomatic carriers of SARS-CoV-2 existed after elimination of clinical cases of COVID-19 in Wuhan," the researchers said.

"Avirulent SARS-CoV-2 strains may still cause symptoms in extremely susceptible individuals and it may also revert to a highly virulent strain to reignite the epidemic of COVID-19 in China."

A separate study last week came up with an even greater figure - 4.4 percent or more than 400,000 infections. 

Wuhan isn't alone. A study this week published in JAMA Network Open found 47 million Americans had likely had the virus by November 15, with 28 million of them symptomatic. The official infected count is about 21 million - at November 15 it was 11 million. 

Deaths have also likely been underestimated, the researchers said - at November 15 the official US death toll was 250,000, but it was likely more in the range of 305,000. 

The origin of Auckland's outbreak in August was never found, with some suggesting at the time it could have been a flare-up of undetected community transmission.