For months, people have wondered how common asymptomatic cases of the coronavirus are. Understanding the pervasiveness of people with COVID-19 who have no symptoms is essential to mitigate the spread, as it is those people who are most likely to unknowingly infect others. The Centers for Disease Control and Prevention (CDC) have warned of the asymptomatic spread even before they had a clear understanding of the magnitude of the problem. But now, the CDC says that Almost half of those infected with coronaviruses are asymptomatic (40 percent, to be exact).
In May, CDC created five “COVID-19 pandemic planning scenarios” using data to estimate a range of possible trajectories for the coronavirus in the US At the time, CDC’s “best estimate” was that 35 percent of the cases were asymptomatic. Now, they have updated their predictions using new data until June 29. The CDC’s “best current estimate” is that about 40 percent of people infected with the coronavirus show no symptoms of the disease, a remarkable 5 percent increase.
And that’s not where the bad news ends. While in May, the CDC predicted that 40 percent of all coronavirus transmissions came from people with no symptoms, they now say 50 percent of all cases come from those who show no signs of the virus, either because they are asymptomatic or pre-symptomatic (meaning they eventually develop symptoms but do not have them at the time of transmission).
The issue of asymptomatic spread has been a hot topic recently, since an official with the World Health Organization (WHO) stated in June that “it seems rare for an asymptomatic person to transmit it to a secondary individual.” However, the WHO later returned to that comment. “Any proportion of the disease is transmitted from asymptomatic individuals … unknown,” another official said the following day. But, he added, “that is happening. I am absolutely convinced that this is happening.”
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There is also a bright spot in CDC’s updated assessments. The agency’s current best estimate for infectiousness of symptomless people, relative to symptomatic people, is now 75 percent, up from 100 percent in May.
While the CDC is using data to estimate these numbers, they emphasize that the true number of asymptomatic cases remains uncertain. “Asymptomatic cases are difficult to identify because people don’t know they are infected unless they do a test, which is usually only done consistently as part of a scientific study,” says the CDC. And for more information on the spread of the coronavirus, see This Is Who’s Transmitting Over 50 Percent of COVID Cases, one study finds.
