SINGAPORE - More than 85 per cent of patients with Covid-19 since January have yet to be discharged, remaining in hospital or isolation facilities.

As of Monday, only 13.5 per cent of patients or 3,225 of the 23,787 people infected have recovered and gone home.

Twenty-one have died.

Not all those who have yet to be discharged are still sick though. Some have recovered, but are not allowed to go home because they may still transmit the coronavirus, which causes Covid-19, to others.

The National Centre for Infectious Diseases (NCID) said four in five patients become only mildly ill, displaying upper respiratory tract symptoms but no lung infection.

They usually remain sick for about two weeks, but might need almost another fortnight before they stop shedding the virus.

So the majority of patients will be discharged after about four weeks.

Professor Leo Yee Sin, executive director of the NCID said the viral load is higher at the start of illness and declines over the first week.

By the 26th day of their illness, 90 per cent of patients will stop shedding the virus. The remainder take longer to rid themselves of the live virus, or dead virus parts.

Singapore is taking the extra precaution of keeping patients still shedding the virus in isolation because the polymerase chain reaction (PCR) test is designed to only detect a certain segment of the virus gene. It is not able to differentiate whether the virus is intact and alive, or dead and in fragments.

"Prolonged PCR positive is a commonly found phenomenon," said Prof Leo.

About 20,000 of those infected were diagnosed only within the past month - the reason why so few have been discharged so far.

Aside from that, about one in five patients becomes very sick. They generally take three to six weeks to recover.

Prof Leo said that about 15 per cent of patients become severely ill with infected lungs, difficulty in breathing and reduced oxygen levels in their blood.

One in 20 patients becomes critically ill. There are about 20 patients in intensive care right now. Their heart and lung functions are often severely compromised, and other organs may also be affected.

The organs of the 58-year-old woman who died about a week after she was cleared of Covid-19 suffered irreparable damage. While she was no longer shedding the coronavirus, she had to remain in hospital because of other complications arising from the infection.

There are other patients who have found themselves in similar circumstances.

Said Prof Leo: "We know that Covid-19 tends to cause more severe illness in the older age groups (above 60 years old) and those with underlying medical conditions.

"In patients with pre-existing lung conditions, the infection can place additional stress on the respiratory systems, causing breathlessness or respiratory failure."

In the meantime, the number of people being discharged is edging upwards

On Monday (May 11), more than 500 patients were discharged - the highest number so far.

That helps to free up hospital beds and isolation facilities.