Two months ago, the Government urged people to wear masks only if they were feeling unwell.

Now, everyone is told to don a mask if they go out, unless they are exercising, or face a $300 fine.

Professor Teo Yik Ying, dean of the National University of Singapore's Saw Swee Hock School of Public Health, said the change in advice comes as more becomes known of the coronavirus that has caused a global pandemic.

"As countries start to learn more, policies need to change accordingly," he said.

Furthermore, Singapore is now in a different phase of the outbreak and the response requires different tools. There were just a handful of Covid-19 cases until early February. Now, there are more than 1,000 new cases a day.

Even so, Prof Teo does not think masks are that important in preventing the spread of the virus, though he also said mask wearing plays an important role in Singapore's defence against Covid-19.

He explained why he thinks masks confer little protection, saying that with only a few people wearing masks in February, the number of infections then would have been far higher "if masks were really so important in preventing transmission and infection".

Professor Dale Fisher, a senior infectious diseases consultant at the National University Hospital, said wearing a mask serves as "a constant reminder to be careful and to do all those things that we feel strongly about, like social distancing, not going out unless you have to, and hand washing".

However, masks do little to stop the transmission of the virus, especially if people practise social distancing.

He said: "This virus doesn't just float along the street. You really do need to be close to somebody or touch a contaminated surface."

"It's not so much about filtering the air that you're inhaling," he said. "It's about modifying behaviour."

Professor Leo Yee Sin, executive director of the National Centre for Infectious Diseases, said there has been a lot of debate over the usefulness of masks, not just during the current pandemic but also for other respiratory outbreaks, including influenza.

People should not see masks as the only form of protection, she said, but as part of precautionary measures.

Prof Leo also said: "Having a mask on carries very different behaviour, connotations and meaning. It constantly reminds us that we are in a very unusual period of time and I hope that that kind of notion will also help us fully understand the importance of social responsibility."

Prof Teo added that even though they provide limited protection, given the large number of cases today, "it is prudent, it is necessary, to protect ourselves with masks".