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A new report from the Centers for Disease Control and Prevention (CDC) found that those infected with the novel coronavirus can spread it for one to three days before showing signs of the illness — such as a fever, cough or shortness of breath — hence making them “presymptomatic” spreaders. The findings further emphasize how infectious COVID-19 is and why containment measures may be difficult.

The CDC in its Morbidity and Mortality Weekly Report (MMWR) looked at some 243 cases of COVID-19 in Singapore from Jan. 23 through March 16. Specifically, researchers “identified seven clusters of cases in which presymptomatic transmission is the most likely explanation for the occurrence of secondary cases.”

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Presymptomatic is defined in the report 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.” SARS-CoV-2 is the name of the virus that causes COVID-19.

Of the 243 cases, 157 contracted the disease locally. Presymptomatic transmission likely occurred in at least 10 of the 157 cases, according to the report.

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“In the four clusters for which the date of exposure could be determined, presymptomatic transmission occurred 1–3 days before symptom onset in the presymptomatic source patient,” it reads.

“To account for the possibility of presymptomatic transmission, officials developing contact tracing protocols should strongly consider including a period before symptom onset,” researchers suggested in the report, noting that presymptomatic transmission “underscores the critical role social distancing, including avoidance of congregate settings, plays in controlling the COVID-19 pandemic.”

Though there are various ways researchers think the virus can spread, including direct contact with symptomatic patients or touching surfaces contaminated with the virus, the Singapore study and others “support the likelihood that viral shedding can occur in the absence of symptoms and before symptom onset.”

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Essentially, the study suggests that anyone may be a carrier of the virus, whether they show symptoms or not.

“These findings also suggest that to control the pandemic it might not be enough for only persons with symptoms to limit their contact with others because persons without symptoms might transmit infection,” they concluded, in part.