By Rich Haridy

Up to 25 percent of people infected with COVID-19 may present no symptoms, yet still transmit the disease, says Robert Redfield, director of the US Centers for Disease Control and Prevention (CDC). The comment comes alongside a new report published by the CDC tracking transmission clusters that suggests infected individuals may be contagious between one and three days before any symptoms appear.

General COVID-19 health advice is pretty clear – at the first sign of any symptoms, such as dry cough or fever, self-isolate and get tested. But one of the bigger unanswered questions is, how many people are contracting the virus without presenting any symptoms?

In a recent interview with NPR, Robert Redfield suggested the CDC is confident nearly one in four people infected with the novel coronavirus may not display any symptoms. And more concerning, these “stealth carriers” would most likely be contagious for a period of time, transmitting the virus unknowingly.

“One of the [pieces of] information that we have pretty much confirmed now is that a significant number of individuals that are infected actually remain asymptomatic,” Redfield said in the NPR interview. “That may be as many as 25 percent. That’s important, because now you have individuals that may not have any symptoms that can contribute to transmission, and we have learned that in fact they do contribute to transmission.”

Without massive non-discretionary testing it is nearly impossible to clearly know how many asymptomatic cases of COVID-19 may be spreading through a community. And this particular concern is fueling calls for universal mask-wearing recommendations, something Redfield claims is being closely considered by the CDC.

“… you can look at masks in two ways … Is the mask something that protects me or … if I wear a mask, is it something that protects others, from me?” Redfield says. “I can tell you that the data and this issue of whether it’s going to contribute [to prevention] is being aggressively reviewed as we speak.”

While asymptomatic transmission is proving challenging to quantify, CDC researchers are increasingly understanding issues surrounding presymptomatic transmission. Presymptomatic transmission essentially covers the period an individual may be contagious before they themselves begin displaying symptoms.

A new report from Singapore’s Ministry of Health, and published by the CDC, tracks seven clear clusters of cases likely attributed to presymptomatic transmission. The data suggests presymptomatic transmission occurred between one and three days before symptom onset in the presymptomatic source patients.

“The possibility of presymptomatic transmission of SARS-CoV-2 increases the challenges of COVID-19 containment measures, which are predicated on early detection and isolation of symptomatic persons,” the report concludes. “The magnitude of this impact is dependent upon the extent and duration of transmissibility while a patient is presymptomatic, which, to date, have not been clearly established.”

Between the unknown number of asymptomatic carriers, and the clear potential of presymptomatic transmission, Redfield stresses the importance of social distancing as our most powerful weapon against the spread of COVID-19. Until we can explicitly identify who is infected with COVID-19, and when they may be most contagious, social distancing is the best strategy for containment.

“So this social distancing that we’re pushing … is a powerful weapon, and that will shut this outbreak down sooner than it otherwise would have been shut down,” Redfield said to NPR. “And as next season comes up, it’s going to be important that we re-embrace that social distancing.”