"Usually, healthy people whose blood oxygen reading falls below 88 percent find it hard to breathe and immediately need to go to hospital to have a nasal oxygen tube inserted, which helps alleviate symptoms while their condition is being monitored.

"It's not uncommon for somebody with COPD to have a much lower oximeter reading, even if they don't have COVID-19. These people may have got used to compromised breathing, but they are at grave risk."

With COVID-19, those who are obese or have high blood pressure or diabetes are also regarded as highly vulnerable. Such conditions can impair the microvascular system, leaving the person prone to clotting once COVID-19 infection triggers inflammation.

Oximeters that can help sound the alarm had largely disappeared from pharmacy shelves by mid-March.

At the height of the pandemic in April, a video on the internet showed a tearful New York doctor lamenting patients being delivered in ambulances to hospitals when it was too late, because they were dead.

Jiang Shasha, an intensive care doctor with Northwell Health in Queens, New York, said: "I saw a young man whose blood oxygen reading had plummeted from 90 to 60 percent within half an hour, before he suffered a cardiac arrest and died.

"We call it Code Blue," says Zhang, referring to an emergency situation when a patient's blood oxygen level plunges, leaving him or her close to death.

Apart from self-monitoring, Zhang sees "early supportive care" as essential in improving a patient's chance of beating COVID-19.

"The whole purpose of supportive care is to make patients feel more comfortable-no high fever, no severe coughing, no dehydration-so they are better placed for the gradual release of their own immune power," she says.

"There's no medication to fight the virus directly, but some drugs can slow down viral replication and give the immunity system the chance to begin restoring itself."

One such drug Zhang has been giving to her patients is hydroxychloroquine, whose effectiveness and possible side effects are a matter of strong contention. The drug, routinely used to prevent or treat malaria, has been found to make cells less hospitable for viruses, and there have been hopes that it could do the same to coronavirus-2(SARSCoV-2) that causes the disease now called COVID-19.