Only 5 per cent of people who shared a home with coronavirus patients eventually contracted the disease, a new study by the National Centre for Infectious Diseases (NCID) has found.
And of this small group, half had just mild or no symptoms.
As a result, they were not picked up by the healthcare system until the two-week quarantine period was over.
Then, serology tests - which are done by taking blood samples - revealed that these people had developed antibodies against the virus, meaning they had been infected in the past.
"These were individuals under quarantine, and there is no evidence that these people passed the infection onwards to other people," said Dr Mark Chen, head of NCID's research office, when he presented the findings yesterday.
For the study, researchers took blood samples from 287 people who lived in the same household as confirmed Covid-19 cases. Participants were also asked to complete a questionnaire on their risk of exposure to the virus and past medical issues, among other things.
The eventual goal is to study a total of 2,000 such people, before looking at the transmission risk in settings such as transport or healthcare.
As a general rule, people quarantined after being in close contact with someone who had tested positive for the virus will be asked to monitor their health.
Those with "significant symptoms" like fever could be asked to take a swab test to pick up the virus.
"There will be some that remain untested to the end of their quarantine, and thus cannot be diagnosed positively," said Dr Ng Oon Tek, a senior consultant at NCID and one of the study's project leads.
"And we are attempting to catch all these people in our serology study."