New CDC report finds COVID-19 can be spread 1-3 days before onset of symptoms

A new report from the Centers for Disease Control and Prevention on Wednesday provides new evidence to bolster early reports that the transmission of the coronavirus — which has now infected over 887,000 people worldwide — can happen prior to symptoms. Published in the CDC’s Morbidity and Mortality Weekly Report (MMWR), the study underscores the potential difficulty of containing a virus that may be spreading silently.

The report focused on the transmission of COVID-19 from Jan. 23 to March 16 in Singapore — specifically, seven “clusters” in which presymptomatic transmission occurred. Presymptomatic transmission is defined by the researchers as “the transmission of SARS-CoV-2 from an infected person (source patient) to a secondary patient before the source patient developed symptoms.”

Of the 243 cases recorded in Singapore by the end of the study, 157 were transmitted locally (meaning they were not brought in from travelers), and of those, at least 10 of the cases were connected to presymptomatic transmission. “The evidence of presymptomatic transmission in Singapore, in combination with evidence from other studies supports the likelihood that viral shedding can occur in the absence of symptoms and before symptom onset,” the authors write.

Although it remains unclear exactly how long individuals may be carrying the virus before symptoms appear, the researchers found that exposure “occurred 1–3 days before the source patient developed symptoms.” At least two of the cases occurred from individuals who acquired the virus during a singing practice, another occurred between two housemates.

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The researchers say the virus may have been spread in multiple ways. “Presymptomatic transmission might occur through generation of respiratory droplets or possibly through indirect transmission,” they write. “Speech and other vocal activities such as singing have been shown to generate air particles, with the rate of emission corresponding to voice loudness.”

The study is the latest to explore whether or not people who appear healthy may be contagious.

In a March 16 report, epidemiologists at the University of Texas at Austin join with a team of scientists France, China and Hong Kong to study over 450 cases of COVID-19 spread across 53 cities in China. Of those, they found that as many as 10 percent were caused by presymptomatic individuals. Lauren Ancel Meyers, PhD, an integrative biology professor at UT Austin and one of the study’s authors suggested the findings were troubling, revealing a potentially “elusive” virus.

“The data suggest that this coronavirus may spread like the flu,” Ancel said in a press release from UT Austin. “That means we need to move quickly and aggressively to curb the emerging threat.“ Ancel said the study highlighted the need for government officials to maintain quarantines and shelter-in-place orders.

“This provides evidence that extensive control measures including isolation, quarantine, school closures, travel restrictions and cancellation of mass gatherings may be warranted,” Meyers said. “Asymptomatic transmission definitely makes containment more difficult.”

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While the MMWR report has limitations, including self-reported symptoms (which may have been too mild to be considered symptoms), it provides crucial information to Americans trying to stay safe. As researchers work to determine how likely it is that individuals can spread the illness without symptoms, the new report can serve as a warning to be increasingly cautious.

“These findings … 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,” the authors write. “These findings underscore the importance of social distancing in the public health response to the COVID-19 pandemic, including the avoidance of congregate settings.”

Cover Photo: A woman wearing a protective mask is seen in New York. (Photo by Jeenah Moon/Getty Images)

With Yahoo news report

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