What we know and don't about asymptomatic transmission and coronavirus

With new stories emerging about the rapid spread of COVID-19, public health experts are now warning the public to stay inside even if they feel healthy. The reason? You may be an asymptomatic carrier.

"Asymptomatic transmission means you can be infected with the virus, have no symptoms and still be contagious," Dr. William Schaffner, professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center, told ABC News.

Most people who pass along the virus do so while they’re sick, usually because they’re coughing or sneezing, which sheds a lot of infectious viruses. But, there is still a significant number of people who transmit that are asymptomatic.

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For example, Dr. John Williams, chief of the division of pediatric infectious diseases at University of Pittsburgh Medical Center Children’s Hospital, pointed out that "asymptomatic infection is common in children, occurring in 10-30%" of cases.

Experts don’t know what portion of adults with COVID-19 are asymptomatic.

"At a time when there is a lot of community transmission, figuring out how much is silent transmission is tricky at best," Dr. Todd Ellerin, chief of infectious disease at South Shore Health, told ABC News.

But this week, the Centers for Disease Control and Prevention Director Dr. Robert Redfield estimated asymptomatic transmission could be as high as 25%. And early data from pandemic hotspots seems to indicate that many people don’t have symptoms on the day they were tested.

For example, among the over 3,700 passengers on the Diamond Princess cruise liner who tested positive for COVID-19, more than 46% were not showing symptoms at the time they were tested. In Iceland, about half of the people who tested positive for COVID-19 were asymptomatic, according to one study.

And here in the United States, during the outbreak at a long-term care skilled nursing facility in King County, Washington, 23 residents tested positive, despite 13 being asymptomatic that day, according to the CDC.

This issue gets to the heart of why the COVID-19 pandemic has been hard to contain. Even though many people feel fine, they are still capable of transmitting the virus to others.

"The virus has a long incubation period so symptoms might not appear until five to 14 days," Dr. Simone Wildes, an infectious disease specialist at South Shore Health, said. "Therefore, people can be spreading the disease without actually knowing they are sick."

What to know about the novel coronavirus:

• How it started and how to protect yourself: coronavirus explained
• What to do if you have symptoms: coronavirus symptoms
• Tracking the spread in the US and worldwide: coronavirus map

The European Center for Disease Prevention and Control estimates that the novel coronavirus that causes COVID-19 can be picked up by a conventional test about one to two days before symptoms appear.

This means that -- for a few days at least -- people have enough virus in their bodies to be detected by lab tests, but they might not feel sick yet. Some of them will never get sick, which means they are totally "asymptomatic," while others will eventually come down with symptoms, which is called "presymptomatic."

With testing in the United States is still reserved mostly for those who are already sick, there's no way to know if you are one of these asymptomatic carriers. Right now, the country does not have enough tests to warrant mass testing among people who feel healthy.

"There may be a role for testing certain high-risk groups to rule out asymptomatic infection, but at this stage, that would be an exception and not the rule," Ellerin said. "That is not ready for prime time."

But our new understanding about asymptomatic transmission has public health experts weighing the merit of asking everyone to wear masks -- not just healthcare workers and people who are already feeling symptoms.

The federal government’s recommendations around mask-wearing could soon evolve, with Dr. Anthony Fauci, Director of the National Institute of Allergy and Infectious Diseases telling CNN that the idea that all Americans should wear masks is "under very active consideration."

"There isn’t a recommendation on that yet, but if it were to occur one of the driving forces would be to decrease transmission from a symptomatic or asymptomatic infected individual to uninfected individuals," Ellerin said.

For the time being, public health experts are asking everyone to assume they could be spreading the virus.

"This is why social distancing isn’t based on who "looks sick,’" Williams said.

"You can catch [the virus] from someone who is perfectly normal and has no symptoms," said Schaffner. On the flip side, he said, "you can be perfectly healthy and be a risk to other people because you may have the infection and be contagious and not know it."

"Stay at home. No one wants to be the dreaded spreader," Schaffner said.

Angela N. Baldwin, M.D., M.P.H., is a pathology resident at Montefiore Health System in the Bronx and is a contributor to the ABC News Medical Unit. Sony Salzman is the unit's coordinating producer.