The number of coronavirus patients aged between 20 and 29 has overtaken that of patients aged 60 and above to become the largest infected group, with most young adults having caught the bug overseas, primarily in Britain.

Of the 558 Covid-19 patients, 141 are between 20 and 29 years old, compared with 111 patients who are 60 and above. Out of these 141 cases of young adults, 78 per cent, or 111 cases, were imported.

Around three in five, or 68 of the 111 imported cases of young adults, had a travel history to the United Kingdom, The Straits Times (ST) has found, based on government figures and profiles of the patients.

Professor Teo Yik Ying, dean of National University of Singapore's (NUS) Saw Swee Hock School of Public Health, told ST: "The number of young people being infected reflects the demographics of people returning to Singapore as a result of the global situation, where a number of countries in Europe and North America are seeing widespread community transmission.

"A large segment of the people coming into Singapore in the past week are locals who have been overseas for studies, work placements or internships. They are predominantly people in their 20s and 30s."

Though past reports have suggested the elderly are more likely to be infected, young people are not immune to the virus, said Prof Teo.

He said: "I must emphasise that Covid-19 is not a disease of the elderly. Children, as well as healthy adults, are also susceptible to it."

While the elderly and those with pre-existing health issues are more likely to progress to more severe infections and experience higher mortality rates, he said, "in terms of infection, both groups are equally at risk".

Visiting professor Annelies Wilder-Smith of Nanyang Technological University's Lee Kong Chian School of Medicine, said: "Older people, even if they spend less time in the community, will be infected by the younger people who interact with the older people at home.

"For this reason, some countries are trying different strategies to protect this most vulnerable group."

On the perception that younger adults might be less risk averse, Associate Professor Hsu Li Yang, infectious diseases programme leader at the Saw Swee Hock School of Public Health, said: "It is likely that a small fraction of young people here will defy stay-home notices... and continue to go out. This is possibly because they do not appreciate the risk or feel that they are not at risk, or else are unable to tolerate 'cabin fever' being cooped up in their rooms for two weeks.

"This is why the penalties are set relatively high as a deterrence."

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Those who breach the stay-home notice will face fines of up to $10,000 or jail of up to six months, or both, the Ministry of Health said yesterday.

National Development Minister Lawrence Wong said on Tuesday: "We have always been doing enforcement through the use of remote monitoring, and spot checks, but we are putting in place enhanced enforcement.

"Whoever flouts stay-home requirements and are caught - we will not hesitate to prosecute, and to apply these penalties."

Around 20 per cent of cases here involve patients aged 60 and above.

Prof Teo said: "It is a reflection of the way the disease is spreading in Singapore - we have had a few large clusters like the Safra Jurong cluster which involved people in that age group."

Those younger than 20 make up just a small fraction - 17 out of 558 confirmed cases.

Infectious disease expert Leong Hoe Nam said: "It begs the question of the manner in which the disease was acquired - if it was at work or in the community and less so in schools. There is also a possibility of many asymptomatic individuals."

He added: "More research into this will be required."

Correction note: This story has been corrected to say that those who breach the stay-home notice will face fines of up to $10,000 or jail of up to six months, or both.