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Mounting evidence suggests that the novel coronavirus can be transmitted from presymptomatic or asymptomatic individuals, bolstering the argument for maintaining proper social distancing measures and possibly impacting the case-fatality rate for COVID-19, a new review from the Centers for Disease Control and Prevention (CDC) said.

The agency looked at reports pertaining to presymptomatic and asymptomatic transmission that was published in PubMed from Jan. 1 to April 2, which is when the CDC recommended that the public wear face coverings to further stop the spread of COVID-19. The instances of such transmission included reports of China, Germany and Singapore.

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“As the coronavirus disease pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) unfolds, an increasing number of reports have indicated that some infected persons may not exhibit signs or symptoms of illness, including persons who are presymptomatic (SARS-CoV-2 RNA is detectable before symptom onset) or asymptomatic (SARS-CoV-2 RNA is detectable but symptoms never develop),” researchers wrote. “The detection of SARS-CoV-2 RNA in presymptomatic or asymptomatic persons does not prove that they can transmit the virus to others. We describe evidence that supports the concept of transmission while presymptomatic and asymptomatic, which we found during a rapid literature review conducted at the Centers for Disease Control and Prevention (CDC) in early April 2020.”

One of the issues raised with the reports out of China was the inability to rule out alternative exposure to SARS-CoV-2, but the cases of possible presymptomatic or asymptomatic transmission concerning Germany occurred before widespread community transmission. Singapore’s instances further supported the possibility.

The CDC analyzed epidemiologic, virologic and modeling evidence and said that while each had limitations, “in the aggregate, these diverse studies suggest that SARS-CoV-2 can be transmitted by persons with presymptomatic or asymptomatic infection, which may meaningfully contribute to the propagation of the COVID-19 pandemic.”

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The agency further concluded that the evidence suggests the case-fatality rate for COVID-19 may be lower than currently estimated ratios if the asymptomatic infections are included, and that transmission while asymptomatic “reinforces the value of community interventions to slow the transmission of COVID-19.” The CDC also said the evidence supports the need for widespread testing and contact tracing.

The findings, the agency said, presents three questions pertaining to transmission that need to further be explored, including determining the incidence of asymptomatic compared with symptomatic infection. The agency also said “given that a large proportion of infections probably result from transmission from asymptomatic or presymptomatic persons, the effectiveness of public health interventions aimed at reducing their infectiousness needs to be quantified,” and lastly, determining the level of SARS-CoV-2 immunity among those who are asymptomatic.

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Doing so will help better determine whether the public will experience a resurgence of COVID-19.

“The answers to these questions will be crucial for guiding the gradual relaxing of community interventions, resuming the normal functions of society, and recovering from the COVID-19 pandemic,” the agency said.