BEIJING - When China's top respiratory disease expert, Professor Zhong Nanshan, first shocked the country on Jan 20 by confirming its worst fear that the new coronavirus was being transmitted through human contact, face masks sold out overnight from big cities to small villages.

Suddenly, everyone was wearing one out on the streets, even though they were not told to do so at that point in time.

Two days later, Wuhan, the epicentre of the outbreak, mandated that everyone had to don a mask in public. This was a day before the city was locked down.

Anticipating a travel crunch as millions of people headed home for the Chinese New Year holiday that weekend, Beijing pushed out a nationwide hygiene campaign urging people to wear masks, wash their hands regularly and avoid crowded places.

Since then, many cities and provinces made mask-wearing compulsory. People who did not wear one were arrested or shamed. This, even as medical workers on the front lines were battling an acute shortage of masks themselves.

But as cities like Beijing have started to relax its rules on public mask-donning as the epidemic comes under control, there is still the expectation that anyone out in public should simply put one on.

Taxi and private car hire Didi drivers can refuse to take passengers who do not have their faces covered. In public parks, such as Fragrant Hills in Beijing, minders politely remind visitors to put their masks on, even if they take them off for a minute to get some fresh air.

Unlike other countries such as the United States, there is little to no debate here over whether masks are effective in keeping the coronavirus at bay.

"There is good evidence that maximal shedding of virus in Covid-19 is before onset of symptoms, therefore mask wearing by the public, not as a self-protection measure, but to limit transmission from minimally symptomatic individuals to others will be reduced," said Professor Babak Javid, an infectious disease expert at Beijing's Tsinghua University.

The emergence of cases of transmission by asymptomatic carriers in the past week has reinforced the Chinese belief in mass mask-wearing.

On Tuesday, the National Health Commission, under public pressure, said it would step up the screening of these "silent carriers" and include them in its daily report.

"The ordinary people in China have always used masks to prevent dust and disease. It has become ingrained in our culture. And since mask-wearing is already common on smog-filled days, wearing one during this outbreak is neither inconvenient nor uncomfortable," said Mr Yu Han, 48, a school teacher in Guangzhou.

Governments around the world have been split on whether to promote the wearing of masks during this coronavirus outbreak.

The World Health Organisation advises healthy people to wear a mask only if they are taking care of someone suspected of being infected, or if they are coughing or sneezing.

China's National Health Commission on March 18 published guidelines on who and where one should wear a mask. For instance, it recommended wearing masks in crowded places, and while taking the lift or public transport. And disposable masks should not be used for more than eight hours.

"Wearing a mask is for self-protection and also gives us a sense of security. Foreigners may not be used to wearing masks and so are not comfortable doing so," said Beijing-based traditional Chinese medicine physician Liu Yijun, 28.

"Plus, we have had the experience of Sars. The government has been actively pushing its public health message and people now understand that wearing masks equals saving lives."

Professor Javid says it is good to encourage civil participation instead of just relying on laws.

"In this context, the response of the citizenry of China to the pandemic has been highly commendable," he said.