COVID-19 transmission can occur before symptom onset
based on clinical and epidemiologic data for all cases reported in the country by March 16.
As of that date, there had been 243 cases of COVID-19, of which 157 were locally acquired. Among those 157 were 10 cases (6.4%) that involved probable presymptomatic transmission, Wycliffe E. Wei, MPH, and associates said April 1 in the Morbidity and Mortality Weekly Report.
They defined presymptomatic transmission “as the transmission of SARS-CoV-2 from an infected person (source patient) to a secondary patient before the source patient developed symptoms, as ascertained by exposure and symptom onset dates, with no evidence that the secondary patient had been exposed to anyone else with COVID-19.”
Investigation of all 243 cases in Singapore identified seven clusters, each involving two to five patients, as sources of presymptomatic transmission. In four of the clusters, the “exposure occurred 1-3 days before the source patient developed symptoms,” said Mr. Wei of the Singapore Ministry of Health and associates.
These findings, along with evidence from Chinese studies – one of which reported presymptomatic transmission in 12.6% of cases – support “the likelihood that viral shedding can occur in the absence of symptoms and before symptom onset,” they said.
SOURCE: Wei WE et al. MMWR. 2020 Apr 1;69(ePub):1-5. doi: 10.15585/mmwr.mm6914e1.
