Mustafa Centre is believed to have been the starting point for hundreds of coronavirus infections at foreign worker dormitories, said the Health Ministry's director of medical services Kenneth Mak yesterday.

The workers are likely to have been infected after visiting the shopping mall, where some employees had fallen ill. They transmitted the infection to their co-workers, who then infected others at their dormitories.

Preliminary investigations have linked the Mustafa Centre cluster to clusters at the construction site at Project Glory and five dormitories, Associate Professor Mak said.

These are S11 Dormitory @ Punggol, Tampines Dormitory, Toh Guan Dormitory, Sungei Tengah Lodge and Cochrane Lodge II.

The majority of yesterday's 287 new cases were linked to foreign worker dormitories.

Thousands of healthy workers are being moved to locations such as army camps and floating hotels, and the Government has started to actively screen and test workers for the disease.

Four dormitories have also been gazetted as isolation areas.

At a press conference yesterday, Manpower Minister Josephine Teo was asked if anything could have been done to avoid having to quarantine thousands of foreign workers.

In response, she said the Government had taken steps since the start of the outbreak to engage dormitory operators and break up large gatherings of foreign workers.

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"It is not as if we have not done anything to try and manage the situation at the dorms," Mrs Teo added, noting that there have been no clusters in dormitories until only recently, even though a number of foreign workers had tested positive for the virus.

"Within dorms, workers interact with each other very regularly, very closely - they're like family, so the risk of transmission was always there," she said.

Preventing clusters at dormitories would have required drastic measures involving the closure of work sites and malls, and were not easy decisions to make when the evidence at that time did not suggest they were necessary, Mrs Teo said.

"We couldn't have so easily said: 'Well, let's do it', when there were no similar occurrences of the clusters that we see today," she added. "It's not a button that you press easily, and it is a non-trivial decision to make."

National Development Minister Lawrence Wong said: "If I had known, of course, I would have done things differently - but no one can tell the next step.

"The reality is no one will know... if one case can very quickly spread into a cluster."

At present, the Government's priority is to contain the spread in dormitories for foreign workers while ensuring that their welfare is looked after, he said.

At yesterday's press conference, Mr Wong also stressed that the disease is not a "foreign worker issue" and that any individual can be a weak link if he does not comply with safe distancing measures.

"One lapse can result in a cluster forming, so all of us have to do our part to stabilise the situation and keep it under control," he said.

"We have a responsibility for these foreign workers who have come all the way here, at considerable expense, to make a living in Singapore. So we will do our part, and we will do our very best to take care of them, and ensure their safety and their well-being."