SINGAPORE — Some Covid-19 patients continue to test positive for the infection even though they are recovering and are clinically well and this may be because tests are picking up fragments of the coronavirus that are no longer viable, Health Minister Gan Kim Yong said on Monday (May 4).

He said this in Parliament in response to a question by Member of Parliament for Fengshan Single Member Constituency Cheryl Chan who raised the issue of Covid-19 patients who are recovering well but continue to test positive for the coronavirus despite being housed in community care facilities.

In some cases, patients are in these facilities for over a month.

Mr Gan explained that in some cases the fragments may return a positive result on the test but these patients are no longer infectious.

The Government, he said, is in talks with infectious disease experts on what more can be done for such patients.

TODAY spoke to infectious diseases experts to understand the peculiarity of these test results.  

WHAT ARE THESE FRAGMENTS? 

The coronavirus requires many components to be fully infectious, said Associate Professor Richard Sugrue, who is a virologist from Nanyang Technological University’s School of Biological Sciences.

First, the particle is surrounded by a membrane and on this, there is a spike protein, which forms the knob-like structures that appear on many visual representations of the virus. 

These structures are used by the virus to gain entry into cells. 

The virus genome is buried inside the particle and is associated with a protein that stabilises and protects it from other cell enzymes that could degrade it.

When the virus particle is disrupted into its separate parts by the patient’s immune system, the virus becomes non-infectious. 

Currently many diagnostic tests seem to be detecting these residual fragments, said Assoc Prof Sugrue.

Elaborating, Associate Professor Hsu Li Yang, co-director of National University Singapore’s Saw Swee Hock School of Public Health’s global health department, said the coronavirus PCR (polymerase chain reaction) tests, which is what Singapore is using, do not distinguish between “live” virus fragments or fragments of virus genetic material — the “dead” virus.

“The PCR tests identify small segments of genetic codes unique to the virus and amplify them. These amplified portions of the genetic code can then be detected by various means depending on the testing systems used.”

WHY DO PATIENTS WHO FEEL WELL SHED THE VIRUS? 

There are many reasons why some patients continue to shed the virus — both “live or “dead” — after they have recovered, said Assoc Prof Hsu.

But a key reason is that recovering from an infection does not always go in tandem with how and when viruses multiply and spread. 

This means an individual’s virus load is not necessarily representative of the severity of their symptoms or how poorly they are feeling. 

For instance, there is evidence that other viruses, such as those that cause influenza or chickenpox, can shed and spread to others before the patient develops symptoms.

The virus for these infections can also be detected in a patient days after he or she has recovered. 

“But the virus is ‘dead’ most of the time or shed in very low quantity and will usually not infect anyone,” said Assoc Prof Hsu.

Dr Leong Hoe Nam, an infectious diseases specialist from Mount Elizabeth Hospital, noted that the same has been uncovered for the coronavirus.

Studies have now shown that the virus that causes the Covid-19 disease can be found in a patient one to two days before the onset of fever, said Dr Leong. He added that coronaviruses are known to shed for up to six weeks.

So far, the longest known period that these fragments can stay in the body is over 70 days from the start of the infection, said Assoc Prof Hsu.

WHY DO SOME PEOPLE TAKE LONGER THAN OTHERS TO RID THEMSELVES OF THE VIRUS? 

Not much is known as to how and why this differs from person to person.

Said Assoc Prof Hsu: “We do not yet understand the mechanism, but people who have been more ill, or who have had their immune systems suppressed, tend to shed virus for a longer period of time.”

Professor Wang Linfa of Duke-NUS Medical School said there is still no conclusive evidence to show the exact mechanism of why this is happening in some patients. 

“More research is needed on the topic before (infectious diseases experts) can make any conclusive remarks.”

IS IT SAFE TO HAVE CLINICALLY WELL BUT POSITIVE PATIENTS DISCHARGED? 

While the risk of this group passing the infection on to others is slim, Dr Leong said it would still be “a big leap of faith” to let them back into the community while they still test positive for Covid-19.

“All we need is to make one mistake and we could undo our (progress fighting the virus),” he said.

However, one scenario where he sees the authorities considering discharging these patients back to the community is if the healthcare system no longer has the capacity to take in new patients. 

These individuals would likely still have to be kept away from the greater community and be put under more stringent rules than those under current stay-home notices and instead follow quarantine order rules even while at home. 

“The person would have to stay in his own room, have food placed outside his room and someone would have to help launder his clothes, for example, without him having to come out,” said Dr Leong.

But he added that at the moment, the Government has “no stress” to make such a move as it is still planning to open more community care facilities for Covid-19 patients.