Can COVID-19 patients keep testing positive but no longer be infectious? Here's what you need to know

Can COVID-19 patients keep testing positive but no longer be infectious? Here's what you need to know

Singapore swab test for COVID-19
A healthcare worker dressed in personal protective equipment collects a nasal swab sample from a migrant worker for COVID-19 testing at a foreign workers' dormitory in Singapore on Apr 27, 2020. (Photo: AFP/Roslan Rahman)

SINGAPORE: On Saturday (May 16), it was revealed that 18 COVID-19 positive patients who had stayed at community care facility D'Resort for between 38 and 51 days were discharged after it was determined that they were no longer infectious to others. 

These patients were "clinically very well" but persistently tested positive for COVID-19. They were referred to a medical review committee, said the Ministry of Health (MOH) in response to queries from CNA.

"The committee deliberated on each of these cases individually and determined that the patients are shedding dead viral components, detectable through the PCR (Polymerase Chain Reaction) test, but which are no longer transmissible and infective to others," said MOH. "After careful consideration, the Ministry of Health decided that these individuals can be cleared for discharge."

READ: 18 COVID-19 positive patients discharged from D'Resort after it was determined they were no longer infective to others: MOH

As an added precautionary measure, these patients are required to be home quarantined for a further seven days, said MOH.

Following the publication of CNA's article, questions were raised on social media, including as to whether there were risks involved.

CNA spoke to experts to find out how COVID-19 patients are tested, why they could continue to test positive after prolonged periods of time and why it's possible to test positive for COVID-19 but not be infectious.

1. How is it possible for a patient to be COVID-19 positive but not infectious?

The PCR test, which is used to identify the presence of the Sars-CoV-2 virus, is a “very sensitive” method of detection, said Professor Leo Yee Sin, who is the executive director of the National Centre for Infectious Diseases (NCID).

The PCR test detects genetic fragments of the virus - a positive result does not represent the "full presence" of the virus, nor does it represent that the virus is still viable, she said in a previous interview with CNA.

Patients are most infectious prior to the onset of their illness and in the first week, said Dr Leong Hoe Nam, an infectious disease specialist from the Mount Elizabeth Novena Hospital.

Experts that CNA spoke to said research has shown that samples of the virus cannot be cultured starting from as early as day 10 of the illness. This points to the fact that patients are shedding non-infectious or dead virus particles.

This means that from or after day 10, the likelihood of a person successfully infecting another person is extremely low.

In contrast, the presence of the virus in the PCR test results could last for a range of between two to six weeks, added Dr Wong Sin Yew, an infectious disease physician at Gleneagles Hospital. 

“So if you bring this message forward, we are very sure that individuals by the time they reach 38, 51 days of the clinical illness, they are no longer infectious,” added Prof Leo.

"Infectious diseases experts have advised the Ministry of Health that patients are unlikely to be infectious beyond 14 days from their onset of illness, as the virus can no longer be cultured from biological samples taken from these patients, even if they continue to test PCR positive," MOH had said previously.

2. How common is it for somebody to test positive for COVID-19 but no longer be infectious? 

A "significant proportion" of patients would fall under this category, said Associate Professor Hsu Li Yang from NUS’ Saw Swee Hock School of Public Health.

“Presumably, this would apply to all patients in the third week following the onset of their illness and who still test positive for the virus, which is a significant proportion of all patients,” he pointed out.

3. Is there any risk to others when patients are discharged even if they test positive for COVID-19? 

Experts CNA spoke to believe that the risk is very low or otherwise negligible.

“The risk of onward transmission is extremely low and possibly zero. That is why the strategy for “discharge and de-isolation” is evolving”, said Dr Wong, who gave the example of the United States' Centres for Disease Control and Prevention.

Should the patient be “shedding dead viral components” as said by MOH in the case of the 18 patients, there is “no risk” of infecting someone else, added Dr Wong.

The move to discharge these patients was a “wise” one, added Dr Leong.

“It frees up resources, it frees up our patients, it frees us from the prison of fear,” he said. “We are beginning to understand the virus. And it is a defeatable virus, with weakness. And let’s not stigmatise the patients. 

“They didn’t choose to have the virus, they didn’t choose to fall sick. But we can choose to give them a breathing space.”

Understanding about the virus also continues to evolves on a regular basis. For instance, scientists from the Korean Centers for Disease Control and Prevention recently found that patients who test positive for COVID-19 after testing negative in previous tests were not capable of transmitting the virus. 

4. If these discharged patients are not a risk, why then is there a need for them to serve a home quarantine order?

Such a move would be in order to err on the side of caution and to “truly minimize” the risk that there might be any spread the virus to others in the community, said Prof Hsu.

Infectious diseases expert Professor Dale Fisher concurred that this would act as an “extra bit of insurance”.

“It is important for us to understand that they are no longer infectious. They are now back in the community and it is safe to interact with them,” added Prof Leo.

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Source: CNA/mt

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