A new CDC report found that 23 residents at a nursing home in King County, Washington, tested positive for COVID-19.
Half of those infected had not yet shown symptoms, but the report said they were highly contagious.
Evidence from Wuhan, China, suggests people can spread the disease without showing symptoms.
High-risk populations, particularly in close quarters, may need greater precautions to prevent the virus, including stricter distancing efforts and personal protective equipment for residents and staff at care facilities, the CDC report said.
Two weeks after the first COVID-19 diagnosis at a nursing facility in King County, Washington, 23 more residents tested positive for the disease caused by the novel coronavirus — but half of them had not yet shown any symptoms, according to a new report.
The paper, featured in the CDC's Morbidity and Mortality Weekly Report on Friday, adds to previous research that people can have the virus for days before developing the tell-tale fever, cough, or shortness-of-breath, and in that time they may well be contagious.
It is further evidence, the authors say, that people working with high-risk populations such as the elderly as a care home restrict person-to-person contact, practice social distancing, and provide protective equipment for all residents and staff.
"Symptom-based screening in skilled nursing facilities could fail to identify approximately half of residents with COVID-19," the authors wrote.
The spread of coronavirus in King County was the first major outbreak of the virus seen in the US.
The first case of COVID-19 at a King County nursing home was identified February 28. Contact tracers believe it started with a healthcare professional working at the home, who had symptoms of the coronavirus.
Within a week, seven residents had testing positive after showing symptoms. Others later developed symptoms, and also tested positive.
As the epidemic escalated, the home started conducting blanket tests, including on residents who did not show any symptoms, eventually testing 76 of the 82 people in the facility. Three refused to be tested, two were already in hospital being treated for the disease by the time testing began, and one was not available for testing.
According to the report, a third (23) of them tested positive, but 13 of them had no symptoms at all. Of those who did exhibit symptoms, eight had typical symptoms, but two had more unusual symptoms: melancholy and nausea.
A week later, 10 of the 13 with asymptomatic cases had developed symptoms.
The report does have limitations. It only addresses a single long-term care facility, and its authors acknowledge it can be difficulty to accurately assess symptoms among patients with dementia and other forms of cognitive decline.
But the novel coronavirus is already known to spread rapidly, particularly under close confines such as a care facility or nursing home.
Currently, attempts to slow the spread of the virus primarily rely on common symptoms like fever, coughing, and shortness of breath to identify and isolate new cases.