Thousands of people who could unknowingly have been infected with Covid-19 - because they had been in close contact with patients - are being tested in a major exercise to look at the real rates of coronavirus infection in the community.

The National Centre for Infectious Diseases (NCID) is approaching people who were placed under quarantine from the start of contact tracing in January until April 4.

The aim is to determine the prevalence of infection and to assess exposure risks among close contacts of Covid-19 patients, whether they are at home or at work, or in social settings. The rates will also allow NCID to better estimate the number of undiagnosed cases among close contacts.

Serology tests will check blood samples for antibodies that would have developed if the person had already been infected. Some might have been asymptomatic, had mild symptoms or were never diagnosed to be Covid-19 positive.

Associate Professor Ng Oon Tek, a senior consultant at NCID, said that all participants in the study would have already completed their quarantine orders.

Participants will have to answer a questionnaire via phone or government e-form, which includes questions about exposure risks related to the patient tested positive. Thus, they would need to be able to identify the infected person they could have been in contact with, either by name or face.

Participants will also be required to provide a blood sample for antibody testing using a test developed by Duke-NUS Medical School.

The study includes household, work and social close contacts of Covid-19 patients.

NCID has approached about 2,500 household contacts and 1,800 social contacts so far. It aims to approach about 3,000 social contacts and 2,300 work ones.

In total, NCID has identified 1,100 contacts who are appropriate and agreeable to participate in the study. It hopes to enrol 2,000 contacts for the study, for which results should be available in two months.

The study is part of a set of three studies spearheaded by NCID. Another involves healthcare workers, and the third is on the rest of the population.

NCID said improved understanding of asymptomatic Covid-19 infections can lead to more effective transmission risk assessments and estimations of fatality rates.

"In addition, exposure risk factors for Covid-19 can offer insight into the nature of transmission, informing guidance and policies to stop its spread," it added.