CDC estimates 35 percent with coronavirus may be asymptomatic — which highlights need for social distancing, say experts

Experts continue to theorize that the coronavirus may be spreading through asymptomatic cases. Here, New Yorkers practice social distancing in a Brooklyn park. (Photo: Getty Images).
Experts continue to theorize that the coronavirus may be spreading through asymptomatic cases. Here, New Yorkers practice social distancing in a Brooklyn park. (Photo: Getty Images).

With more than 1.5 million cases of the coronavirus in the U.S. alone, experts are still searching for answers about the way in which the virus may be spreading silently. On Friday, many news organizations began citing a number listed in the Centers for Disease Control and Prevention guidance on reopening, which noted that the organization’s “best estimate about viral transmission” is that 35 percent of individuals infected with SARS-CoV-2 may be asymptomatic.

The number was listed in the CDC’s “COVID-19 Pandemic Planning Scenarios,” which relied on mathematical models to “help evaluate the potential effects of different community mitigation strategies.” Dr. Kavita Patel, a nonresident fellow at the Brookings Institution and Yahoo Life medical contributor, says the number is meant to be used for state and local modeling. “This highlights the importance of social distancing, nonmedical masks and hand hygiene even more since some proportion, potentially as many as 35 percent, are asymptomatic,” says Patel.

Dr. Dara Kass, an emergency medicine doctor and Yahoo Life medical contributor, also cautions that the number is an estimate and should not be taken as definitive. “It’s a complicated model that really means we need stronger surveillance and testing,” says Kass. “The more testing we have available, the easier it will be track back to asymptomatic carriers and find if they have any common attributes.”

The CDC’s estimate, although only a model, raises important questions about asymptomatic carriers. To help clarify, here are some of the points you need to know.

Other coronaviruses can spread through asymptomatic individuals

Dr. Amesh Adalja, an infectious disease expert and senior scholar at the Johns Hopkins Center for Health Security, says that coronaviruses have a history of spreading through asymptomatic carriers. “This is the seventh human coronavirus ...and this novel coronavirus does in many ways behave like many common cold-causing coronaviruses in the way that they spread,” he tells Yahoo Life. “And we know from those coronaviruses that there are a very high number of asymptomatic individuals.” Influenza has also been shown to spread through asymptomatic carriers, with the CDC writing that just 50 percent of those infected with influenza will develop “classic” symptoms.

Estimates of asymptomatic COVID-19 individuals vary widely

The CDC does not cite data for its new model, but there is preliminary evidence to suggest that there may be many individuals carrying the virus without symptoms. One study from the CDC, published on April 3, found that 56 percent of the residents at one Seattle nursing home who tested positive for SARS-CoV-2 were either asymptomatic or pre-symptomatic at the time of testing. Another study out of Wuhan looked at Japanese citizens evacuated out of Wuhan and found that over 30 percent who tested positive were asymptomatic. A third study, of individuals at a homeless shelter in Boston, found that of the 36 percent that tested positive for the virus, not a single one was showing symptoms.

Lack of symptoms does not mean you’re not contagious

Dr. Willam Schaffner, an infectious disease expert and professor at the Vanderbilt School of Medicine, says he believes 35 percent seems like a “reasonable estimate” for the number of asymptomatic individuals, given what we know about other viruses. But he adds that it’s important to understand what asymptomatic means. “With influenza, you can be without symptoms or have only mild symptoms and still put out an awful lot of virus,” says Schaffner. “So just because you have less disease or no symptoms, doesn't mean that you're not contagious.”

It’s for this reason that large gatherings are still considered dangerous

“With asymptomatic people, they could spread it to everyone who gathers in a group if they do it casually and not carefully,” says Schaffner. “That's why the continuing recommendations are to be cautious.” He notes that individuals, especially those who are at high risk, should remain far apart and wearing masks. The CDC continues to recommend that individuals think twice about coming together in large groups, particularly for religious services, concerts, sporting events or weddings. Although it’s unclear exactly how easily the virus spreads from person to person, a recent study found that 2,600 respiratory droplets were released in a single second of people talking.

Asymptomatic spread may be one reason why COVID-19 has been difficult to control

Adalja notes that more testing is required in order to determine how many people are shedding the virus unknowingly, but that in the past, a disease that can spread silently proves extremely tricky to contain. “When infectious diseases have a high proportion of asymptomatic individuals who are contagious, that becomes very hard to control,” says Adalja. “And that's probably part of the reason why this has been so hard to control. But what force does it have in the general public is an important question that I think needs to be fully more fully answered.”

For the latest coronavirus news and updates, follow along at According to experts, people over 60 and those who are immunocompromised continue to be the most at risk. If you have questions, please reference the CDC’s and WHO’s resource guides.

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