None of the coronavirus patients discharged from hospitals here has shown evidence of re-infection, said a National Centre for Infectious Diseases (NCID) senior consultant.

"Currently, our discharged patients remain well," Associate Professor David Lye told The New Paper.

High quality studies are needed to determine whether recovered patients can get re-infected, he added.

Prof Lye was responding to TNP's queries on recent reports of a woman in Japan testing positive again weeks after seemingly recovering from Covid-19.

The Osaka tour guide in her 40s tested positive in late January and was discharged from hospital on Feb 1. She later developed a sore throat and chest pains before testing positive again last Wednesday. Similar reports have surfaced in China, including a patient in Chengdu who was readmitted to hospital days after being discharged.

Prof Lye's view is shared by other public health experts.

Professor Tikki Elka Pangestu, a visiting fellow at the Lee Kuan Yew School of Public Policy, said more analysis and research are needed to determine if the Osaka woman had been re-infected by the coronavirus.

However, the former director of research policy and cooperation at the World Health Organisation said re-infection is unlikely if she had been infected the first time as she would have developed an immunity against the coronavirus.

Professor Wang Linfa of Duke-NUS School of Medicine told The Straits Times that he does not buy reports of re-infection as viral antibodies would have fought back.

HOW LONG?

Prof Wang, who helped to develop a test to detect antibodies in recovered patients to enable the authorities to link two church clusters, said he did not know how long Covid-19 patients would have the antibodies. But he noted severe acute respiratory syndrome (Sars) survivors here still have the Sars antibodies 17 years later.

Professor Paul Tambyah, an infectious disease expert from the National University of Singapore Yong Loo Lin School of Medicine, said it is possible the Osaka woman had a persistent infection and the test before discharge was a false negative.

"This has been reported in other settings. Persistence is quite different from re-infection and the genetic details from Japan will help clarify this," he added.

To confirm Covid-19 infections, a molecular test known as polymerase chain reaction (PCR) is used to detect the presence of genetic material, RNA in the case of viruses, in samples obtained from a patient, such as a throat or nasal swab.

Prof Tambyah said most modern PCR tests can pick up small amounts of RNA, but it is possible to have a false negative due to a limitation of the test or a problem with sampling.

"If the sample is not properly collected, there is a risk the test will miss (it)," he said, adding that false positives, on the other hand, are rare.

PCR tests also cannot tell if the virus detected in the sample is dead or alive. So even if a discharged patient tests positive, it is possible that he is simply shedding dead viruses.

"If the virus is dead, it cannot spread," Prof Tambyah added.

Over the past week, experts at the NCID and the Ministry of Health (MOH) have assured the public that discharged patients are no longer infectious.

During a media briefing last Thursday, MOH's director of medical services Kenneth Mak said discharged patients are given medical leave depending on their clinical circumstances, and they still have to go back to the hospital for follow- ups.

"The doctors at NCID and other hospitals have assured me that they have been quite disciplined and meticulous in checking, doing these tests and to make sure that all these tests on consecutive days are negative before they are assured that the patient is fit to go home," Associate Professor Mak said.

"(The patients) are not shedding any further virus and they are not a threat as far as transmitting any infection."

Tricky to tell when it's safe to stop isolation: NCID experts

Deciding when to take suspect coronavirus cases out of isolation has proven challenging, experts at the National Centre for Infectious Diseases (NCID) say, as there can be some delay between the onset of symptoms and a positive test result.

To address this, the NCID uses an algorithm to help decide when to continue isolating suspect cases, when to transfer them to other wards, and when to discharge them.

This allows for better management of isolation room occupancy at the 330-bed NCID, given the substantial number of suspect cases admitted for isolation and the need to hold them for repeated testing.

To reduce the risk of the virus spreading within the hospital - called nosocomial amplification - NCID staff continue to use surgical masks when caring for patients who have tested negative but are still within the first week of respiratory symptoms.

Writing about this ongoing challenge in an article published in the medical journal Clinical Infectious Diseases on Feb 26, NCID experts cited one case that tested positive only on the fifth repeated swab, on the seventh day of clinical illness.

"No single indicator may be effectively used to decide on de-isolation of a suspect case," the nine experts, including executive director Leo Yee Sin and clinical director Shawn Vasoo, wrote.

"Our suspect case was kept isolated because of a high index of clinical suspicion, with a clinically compatible illness and history of close contact with a laboratory-proven Covid-19 case."

DUAL INFECTIONS

Another issue when managing suspect Covid-19 cases is dual infections, which can occur in 10 per cent to 20 per cent of viral acute respiratory infections.

Detecting another virus in a patient may not definitively exclude Covid-19 infection, the experts said, highlighting the one patient here who had both Covid-19 and dengue fever.

Meanwhile, NCID senior consultant David Lye told The New Paper separately that many of the suspect cases who eventually tested negative were patients who would normally develop a sniffle or cough at this time of the year.

Among the confirmed cases, a majority had self-resolving illnesses, though as with any viral respiratory infection, some coughing may continue even after fever disappears.

Associate Professor Lye said runny nose and sore throat were so far quite mild and patients may have an occasional cough.

Prof Lye said some overseas reports showed more severe infections, which likely reflects more severe cases going to the hospital and milder cases going undetected.

He added: "Singapore has one of the best active detection systems for Covid-19 based on overseas assessment, so we may be seeing a wider spectrum of infections."

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This article was first published in The New Paper. Permission required for reproduction.