SINGAPORE - The 62 Covid-19 patients who have been discharged here no longer have the virus in them and cannot pass the infection on to others, experts at the National Centre for Infectious Diseases (NCID) said.

However, it is not known if patients can get infected more than once, as not enough is known yet about a person’s immune response to the disease.

To be doubly sure Dr Shawn Vasoo, clinical director of the NCID, said the centre has started swabbing patients who are well when they return for a follow-up check two weeks after discharge.

He thinks it is rare for a patient to become infectious again, but said “we will be evaluating this”.

The Japanese authorities said that a woman there tested positive for the disease for a second time three weeks after recovering and being sent home. But not enough information is available on the case, and the Singapore authorities are reaching out to their Japanese counterparts to determine the exact circumstances.

Reports have also surfaced in China that one in seven patients who has recovered is still infectious.

The Straits Times spoke to experts about how Singapore makes sure patients who are discharged are not infectious, and about the possibility of getting Covid-19 more than once.

Professor Leo Yee Sin, executive director of NCID, said doctors monitor virus-shedding in patients' respiratory tract - in other words, they check if the patient is still releasing live virus and thus remains contagious.

This is done by taking nasal or throat swabs.

Virus-shedding stops if the patient no longer has the virus in him.

"Patients are only discharged when they have clinically recovered and molecular testing indicates they have stopped shedding the virus," she said.

This means that should they sneeze or cough, there is no virus from them that could contaminate surfaces or infect others.

Many of the hospitalised patients here are feeling well, especially during the last few days of their hospital stay.

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But keeping track of virus-shedding is a precaution Singapore has taken.

Prof Leo said when tests show that patients are completely cured and no longer have the virus in them, they are still kept in the hospital for at least another day waiting for the results of a second test done 24 hours after they have received the first all-clear results.

Beyond that, she added: "Cases who are discharged are reviewed at our clinic."

Dr Li Yueping, director of the intensive care unit at China's Guangzhou No.8 People's Hospital, had said that test results of 14 per cent of its patients who had been discharged returned to positive when they went for follow-up checks.

Dr Vasoo said this is not surprising - nor is it worrying.

The virus was found in anal swabs.

He said data from the Sars (severe acute respiratory syndrome) outbreak in 2003 found virus in patients' stool for about four weeks after they had recovered.

But this does not mean the virus is still viable, he said, adding that the coronavirus is spread through droplets.

"As infection is spread by respiratory droplets, if (viable) virus-shedding is absent from respiratory secretions, then these people should not go on to infect others," Dr Vasoo said.

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Furthermore, he said, even if the virus in the stool is still active, it "should not be a source of transmission with modern sanitation and hygiene being observed".

On whether these discharged patients can be infected again by the coronavirus - as Osaka's prefectural government said had happened to one of its earlier patients, a female tour bus guide - Dr Vasoo said doctors currently do not know how long patients remain immune to the disease.

"We need to study the immune response of patients infected with the Covid-19 virus. At the moment it is unknown if patients infected by the Covid-19 will have long-lasting immunity to the virus," he said.