One to three days before symptoms appear, people with the novel coronavirus can spread the infection, a study by the United States Centers for Disease Control and Prevention (CDC) said.
The study looked at 243 cases of Covid-19 reported in Singapore between Jan. 23 and March 16, to underscore the importance of social distancing, Reuters reported.
The study identified seven "clusters" where pre-symptomatic transmission was likely.
Where the date of exposure could be determined in four such groups, pre-symptomatic transmission occurred one to three days before symptoms appeared in the source patient.
Of the cases in Singapore, 157 were locally acquired and 10 of these were likely transmitted before symptoms started to show.
The CDC's Morbidity and Mortality Weekly Report was published online on April 1.
The researchers expect asymptomatic illness to be under-detected with the focus currently on testing only people who show symptoms.
The CDC data adds to previous reports of cases in China that indicated the virus could spread before symptoms start to appear.
Not enough for people showing symptoms to limit contact
The findings suggest that it might not be enough for people showing symptoms to limit their contact to control the pandemic and this increases the challenges of containment measures, the researchers wrote.
But the magnitude of the impact depends on the extent and duration of transmissibility while a patient is pre-symptomatic.
This aspect has so far not been clearly established, the researchers said.
Public health officials conducting contact tracing should strongly consider including a period before symptom onset to account for the possibility of this type of transmission.
Transmissions might take place through vocal activities such as singing, with the rate of emission corresponding to voice loudness, the researchers noted.
Other ways of getting infected include via respiratory droplets or even speech.
While the cases were carefully investigated, the CDC said it was possible an unknown source might have caused the clusters.
The findings could also be affected by incorrect reporting of cases, especially if people had only mild symptoms.
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