On April 3, an assistant manager at the Lido branch of McDonald's saw a doctor for a sore throat and fever. She was given five days' medical leave.

At the end of her medical leave on April 8, the 44-year-old mother of a teenage boy went back to the doctor.

This time, she was sent to Seng-kang General Hospital for a Covid-19 swab test.

It was positive.

Over the next nine days, six other employees of the fast-food chain, who had worked across nine outlets, were diagnosed with the coronavirus.

Over at the Ministry of Health, doctors and officials watched with increasing concern.

Steps had already been taken by McDonald's to contain this cluster, but did they go far enough?

The company had told all employees from the affected outlets to isolate themselves for 14 days. These restaurants were also closed and underwent deep cleaning.

On April 18, McDonald's stopped takeaways - Singapore had already banned dining-in - and switched to delivery and drive-through service only.

But the ministry - by then battling huge numbers of foreign workers who got infected at worksites and living quarters - felt more had to be done.

A decision was made: From 11am the following day - April 19 - all 135 McDonald's outlets would close down till May 4.

The decision wasn't taken lightly.

The chain employs more than 10,000 people and serves more than six million hungry customers every month.

But, as McDonald's noted on its website: "These are unprecedented times for all of us. With the safety of all our customers and employees as the top priority, we will do all we can to help Singapore flatten the curve."

The company promised to pay salaries to its employees while operations were suspended.

Prof Tan, who was director of medical services during Sars, says the most important takeaway from that period was experience.