People who have been infected with coronavirus can spread it to numerous others during a crucial one-to-three day period before they start showing symptoms, according to a study by the US Centers for Disease Control and Prevention.
The study focused on 243 cases of COVID-19, the illness caused by the coronavirus, reported in Singapore between January 23 and March 16.
It identified seven “clusters” of the disease that were likely triggered by people with coronavirus who appeared healthy.
“Such clusters had clear contact between a source patient and a patient infected by the source (a secondary patient), had no other likely explanations for infection, and had the source patient’s date of symptom onset occurring after the date of exposure to the secondary patient who was subsequently infected,” researchers wrote in the CDC’s Morbidity and Mortality Weekly Report, published online on Wednesday.
Ten pre-symptomatic cases within the clusters accounted for 157 patients who had not recently travelled and acquired coronavirus locally, researchers said.
In four “clusters”, where the date of exposure could be determined, pre-symptomatic transmission occurred one-to-three days before symptoms such as a runny nose, cough and diarrhoea appeared in the source patient.
One of the patients studied, a 52-year-old woman, was infected after sitting in a seat at a church that had been occupied earlier in the day by two tourists from Wuhan, China, who showed no symptoms but later fell ill.
“Pre-symptomatic transmission might occur through generation of respiratory droplets or possibly through indirect transmission,” researchers said.
“Speech and other vocal activities such as singing have been shown to generate air particles, with the rate of emission corresponding to voice loudness.”
Call for contact tracing to be expanded
The data highlights the importance of social distancing in an effort to combat the highly infectious deadly disease, but also reveals the difficulty the existence of pre-symptomatic transmission could have in regards to contact tracing.
The study authors urged public health officials conducting contact tracing to strongly consider including a period before symptom onset to account for the possibility of this type of transmission.
The findings increase the challenges of containment measures, the researchers wrote, but said the magnitude of the impact depends on the extent and duration of transmissibility while a patient is pre-symptomatic and that has so far not been clearly established.
While the cases were carefully investigated, the CDC said it was possible an unknown source might have caused the clusters.
An earlier study that focused on China, where the virus was first identified, suggested that more than 10% of transmissions were from people who were infected but did not yet feel sick.
- with Reuters and AP
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