As COVID-19 testing becomes more widespread, scientists are learning that far more people infected with the coronavirus show little or no symptoms than what was originally thought.
That means the disease could be less deadly than some feared but also harder to control as infected people unknowingly spread the disease to others.
"We learned that coronavirus could cause the entire spectrum of symptoms in people — from none at all to putting you in the grave," said Dr. Bill Schaffner, an infectious disease specialist at Vanderbilt University Medical Center. "What we still have not determined is the ratio of asymptomatic to symptomatic to seriously symptomatic."
In the beginning, health officials said the virus usually causes mild or moderate flu-like illness — but that information was based on known cases, and evidence is growing that a substantial number of people may never appear sick.
The head of the U.S. Centers for Disease Control and Prevention said 25% of infected people might not have symptoms. The vice chairman of the Joint Chiefs of Staff, Gen. John Hyten, thinks it may be as high as 60% to 70% among military personnel, the Associated Press reported.
Schaffner said testing in group settings, such as cruise ships, nursing homes, prisons and meat packing plants, brought the idea of asymptomatic transmission to the forefront.
At the Bledsoe County Correctional Complex west of Pikeville, Tennessee, 586 prisoners recently tested positive for the virus, but of those, 580 were showing no symptoms.
Tennessee Health Commissioner Dr. Lisa Piercey called the high number of asymptomatic or mildly symptomatic inmates a "head scratcher" when it first came to light.
"These are things we wish we knew, and we learn more and more every day. That's why they call it the novel coronavirus. It's brand new, and so we're gathering more science with each passing week," Piercey said in April.
Schaffner said researchers suspect that younger adults have a greater proportion of asymptomatic carriers, but that there are likely other factors that determine how many, if any, symptoms someone shows. Either way, it's why everyone should wear a face mask, he said.
"Asymptomatic transmission can be so common that we now recommend that everyone wear a mask," he said. "The reason I wear a mask is not so much to protect me, but mostly because here I am without symptoms, and I could be infected. I'm wearing the mask to protect you, and you're wearing the mask to protect me."
Schaffner said there are many other viruses, including the flu, that can be transmitted by people who never show symptoms or before they do. Some studies of influenza show that the virus can be spread to others 24 to 48 hours before symptoms show up, but similar studies of coronavirus haven't yet been completed.
"It's a clever way that the virus gets through a population," he said. "It's sneaky that way, because when people get sick they begin to sequester themselves — withdraw from contact — but if the virus can get around before you get sick, that's an easy way for it to keep spreading."
Testing is the best way to combat a disease such as coronavirus. Without a cure or a vaccine, controlling the spread of the pandemic requires finding out who's infected and isolating those individuals.
Efforts to reopen the economy amplify the need for robust testing in order to spot resurgences and flare ups that could overwhelm health systems and lead to more deaths, experts say.
Until recently, COVID-19 testing in the community was primarily based on testing people with symptoms. That testing strategy makes statistics such as infection rate impossible to calculate.
Gov. Bill Lee has made a series of major pushes to get Tennesseans tested in response to the coronavirus pandemic, not just among the general population but for specific vulnerable populations. The list has focused on facilities with large populations including prisons, nursing homes and now public housing where there can be wide community spread.
Dr. Janara Huff, a pediatric infectious disease specialist at Erlanger Children's Hospital, said the percentage of people who are infected but asymptomatic has become more obvious as more tests are done for the purposes of screening, and not because someone is ill.
"If you just tested random asymptomatic persons in a park, very few of them would be positive. If you test a closed group of persons in a factory or prison where some people are known to be ill with COVID, you are much more likely to find many infected people and a significant number will be asymptomatic. This could be anywhere from 20-30% or more of those infected," Huff said by email.
While asymptomatic people are unlikely to suddenly suffer severe illness, the problem is that they are unaware of the infection and shedding virus, which could infect other people, Huff said.
"This is why masks are recommended — to reduce spread of the virus by someone who is contagious but not sick," she said.
It is difficult to contain COVID-19, but testing the contacts of sick people will help find infected but asymptomatic people and their contacts, and allow them to self-quarantine and avoid spreading the virus further.
Melissa McPheeters, a Vanderbilt health policy researcher, said although the virus can spread quickly in a group setting, contact tracing is more straightforward.
"You have people in one place for the most part, and so you have less contact tracing that you need to do outside of that environment," McPheeters said.
"Certainly people who are employed there leave and go home and you have to follow up with them, but you have the ability to very quickly test a lot of people and get a very clear idea of what's going on in that environment," she said. "That's different than if it's spreading throughout a city, and you have to call every person and they need to remember every person they've been in contact with. It can be tricky."
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