Dressed in a hazmat suit, two masks and a face shield, Du Mingjun knocked on the mahogany door of a flat in a suburban district of Wuhan.

Key points: Doctors in Wuhan say there are growing numbers of cases in which people recover from the virus but continue to test positive

Doctors in Wuhan say there are growing numbers of cases in which people recover from the virus but continue to test positive In South Korea, about 1,000 people have been testing positive for four weeks or more

In South Korea, about 1,000 people have been testing positive for four weeks or more In Italy, health officials noticed that coronavirus patients could test positive for the virus for about a month

A man wearing a single mask opened the door a crack and, after Ms Du introduced herself as a psychological counsellor, burst into tears.

"I really can't take it anymore," he said.

Diagnosed with the novel coronavirus in early February, the man, who appeared to be in his 50s, had been treated at two hospitals before being transferred to a quarantine centre set up in a cluster of apartment blocks in an industrial part of Wuhan.

Why, he asked, did tests say he still had the virus more than two months after he first contracted it?

The answer to that question is a mystery baffling doctors on the frontline of China's battle against COVID-19, even as it has successfully slowed the spread of the coronavirus across the country.

Patients testing positive with no symptoms

Doctors in Wuhan still do not understand why some patients test positive so much later after recovering. ( AP: Chinatopix )

Chinese doctors in Wuhan say the growing number of cases in which people recover from the virus, but continue to test positive without showing symptoms, is one of their biggest challenges.

Those patients all tested negative for the virus at some point after recovering, but then tested positive again, some up to 70 days later, the doctors said. Many have done so over 50-60 days.

In South Korea, about 1,000 people have been testing positive for four weeks or more. In Italy, the first European country ravaged by the pandemic, health officials noticed that coronavirus patients could test positive for the virus for about a month.

The prospect of people remaining positive for the virus, and therefore potentially infectious, is of international concern, as many countries seek to end lockdowns and resume economic activity as the spread of the virus slows. Currently, the globally recommended isolation period after exposure is 14 days.

China has not published precise figures for how many patients fall into this category. But disclosures by Chinese hospitals to Reuters, as well as in other media reports, indicate there are at least dozens of such cases.

With less than four months of data, questions about immunity, especially in the long term, are difficult to answer.

But most experts agree these cases do not represent reinfections and are more likely to reflect the imperfect nature of testing.

As there is limited knowledge available on how infectious these patients are, doctors in Wuhan are keeping them isolated for longer.

Zhang Dingyu, president of Jinyintan Hospital, said health officials recognised the isolations may be excessive, especially if patients proved not to be infectious. But, for now, it was better to do so to protect the public, he said.

He described the issue as one of the most pressing facing the hospital and said counsellors like Ms Du are being brought in to help ease the emotional strain.

"When patients have this pressure, it also weighs on society," he said.

Positive retesting not seen in 2003 SARS outbreak

The plight of Wuhan's long-term patients underlines how much remains unknown about COVID-19 and why it appears to affect different people in numerous ways, Chinese doctors say.

Yuan Yufeng, a vice-president at Zhongnan Hospital in Wuhan, told Reuters he was aware of a case in which the patient had positive retests after first being diagnosed with the virus about 70 days earlier.

"We did not see anything like this during SARS," he said, referring to the 2003 Severe Acute Respiratory Syndrome outbreak that infected 8,098 people globally, mostly in China.

Wang Guiqiang, director of the infectious disease department of Peking University First Hospital, said the majority of such patients were not showing symptoms and very few had seen their conditions worsen.

Possible links to weak immune systems

Experts and doctors struggle to explain why the virus behaves so differently in these people.

Some suggest that patients retesting as positive after previously testing negative were somehow reinfected with the virus.

In Wuhan, doctors say patients who aren't showing symptoms are still testing positive to COVID-19. ( Supplied )

This would undermine hopes that people catching COVID-19 would produce antibodies that would prevent them from getting sick again from the virus.

Zhao Yan, a doctor of emergency medicine at Wuhan's Zhongnan Hospital, said he was sceptical about the possibility of reinfection based on cases at his facility, although he did not have hard evidence.

"They're closely monitored in the hospital and are aware of the risks, so they stay in quarantine. So I'm sure they were not reinfected," he said.

Jeong Eun-kyeong, director of the Korea Centres for Disease Control and Prevention, previously said the virus may have been "reactivated" in 91 South Korean patients who tested positive after having been thought to be cleared of it.

Other South Korean and Chinese experts have said that remnants of the virus could have stayed in patients' systems but not be infectious or dangerous to the host or others.

Few details have been disclosed about these patients, such as if they have underlying health conditions.

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Paul Hunter, a professor at the University of East Anglia's Norwich School of Medicine, said an unusually slow shedding of other viruses such as norovirus or influenza had been previously seen in patients with weakened immune systems.

In 2015, South Korean authorities disclosed they had a Middle East Respiratory Syndrome patient stricken with lymphoma who showed signs of the virus for 116 days. They said his impaired immune system kept his body from ridding itself of the virus. The lymphoma eventually caused his death.

The mental toll of COVID-19

For patients who keep testing positive, it can be a difficult time for their mental health. ( Reuters: China Daily )

As could be seen in Wuhan, the virus can also inflict a heavy mental toll on those caught in a seemingly endless cycle of positive tests.

Ms Du, who set up a therapy hotline when Wuhan's outbreak first began, allowed Reuters in early April to join her on a visit to the suburban quarantine centre on the condition that none of the patients be identified.

One man rattled off the names of three Wuhan hospitals he had stayed at before being moved to a flat in the centre. He had taken over 10 tests since the third week of February, he said, on occasions testing negative but mostly positive.

"I feel fine and have no symptoms, but they check and it's positive, check and it's positive," he said. "What is with this virus?"

Patients need to stay at the centre for at least 28 days and obtain two negative results before being allowed to leave. Patients are isolated in individual rooms they said were paid for by the government.

The most concerning case facing Ms Du during the visit was the man behind the mahogany door; he had told medical workers the night before that he wanted to kill himself.

"I wasn't thinking clearly," he told Ms Du, explaining how he had already taken numerous CT scans and nucleic acid tests, some of which tested negative, at different hospitals. He worried that he had been reinfected as he cycled through various hospitals.

His grandson missed him after being gone for so long, he said, and he worried his condition meant he would never be able to see him again.

He broke into another round of sobs. "Why is this happening to me?"

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Reuters