As of Friday, May 15, testing tracked by state officials in North Dakota indicated 35% of those who test positive for the coronavirus are asymptomatic at the time of testing, said Tammy Miller, Gov. Doug Burgum’s chief operating officer and a leader in guiding the state’s response.
Public health officials and health experts have said asymptomatic carriers of the virus pose a significant challenge, since they can unknowingly transmit the virus — and some studies suggest they may play a major role in the coronavirus’s spread.
On the other hand, a high asymptomatic rate means the death rate from coronavirus likely is lower than reported. People without symptoms often are never tested, skewing the numbers, said Molly Howell, an epidemiologist and assistant director of disease control for the North Dakota Department of Health.
North Dakota’s high asymptomatic rate might be the result of the state’s aggressive testing efforts, she said, such as surveillance testing events like those held at the Fargodome.
As of Friday, North Dakota reported 51,715 test results, or a rate of 68 per 1,000 residents.
“Because COVID-19’s so new, everything kind of surprises us,” Howell said, referring to the disease caused by the coronavirus and its many mysteries.
In Cass County, North Dakota’s hotspot, testing indicates that between 10% and 30% of those who test positive show no symptoms, depending upon the testing event, said Desi Fleming, director of Fargo Cass Public Health and leader of the Red River Valley COVID-19 Task Force.
“We learn more about this virus every day,” she said. “Early on, we were focusing on those who were symptomatic, and now we know we have individuals that don’t show any symptoms.
“This is why we use contact tracing efforts, to identify close contacts as no one may have realized someone was infected, as well as identifying clusters of positives with our epidemiology data which helps drive some of our targeted testing efforts.”
The asymptomatic rate is one of many factors the state uses in its complex computer modeling to predict the spread of the virus and what will be required from the public health system and health providers in response, Miller said.
Those who test positive are quarantined for 14 days or longer, and public health officials work to identify their close contacts, so they also can be isolated to prevent transmitting the infection.
So far, most testing has focused on those with symptoms, but the task force is also targeting testing on high-risk groups, including residents and employees of nursing homes and other congregate living centers.
Studies and estimates of asymptomatic coronavirus carriers range widely, with some ranging as high as 25% to 50%.
Dr. Robert Redfield, the director of the Centers for Disease Control and Prevention, told NPR that a significant percentage of those infected — as many as 25% — remain asymptomatic.
Researchers in Iceland reported 50% of new coronavirus cases were people who showed no symptoms in testing by deCODE, a subsidiary of U.S. biotechnology company Amgen.
People with no symptoms were the source of 44% of diagnosed COVID-19 cases, according to a study published in the journal Nature Medicine.
Seven clusters of the coronavirus in China were traced to presymptomatic virus carriers by researchers in Singapore, the CDC reported.
In yet another study of asymptomatic carriers, researchers at New York Presbyterian/Columbia University Irving Medical Center found about 15% of women who came in for child delivery tested positive, but 88% of birthing mothers showed no symptoms and 13.5% of all patients during a two-week period were infected but showed no symptoms.
Little is understood about the role of asymptomatic carriers in transmission, Howell said. It’s likely those who aren’t coughing or sneezing aren’t shedding as much of the virus.
Still, she said, the high rate of asymptomatic coronavirus carriers is a reminder of the urgent need to maintain precautions to slow the spread, including maintaining a distance of six or more feet from others, wearing a mask when that’s not possible, avoiding large gatherings, working from home if possible and washing hands thoroughly and frequently.
Employers are asked to separate workers as much as possible and to stagger shifts to prevent spreading the virus.
The Centers for Disease Control and Prevention, which has sent three teams to North Dakota, is aware of the state’s high asymptomatic testing rate, Howell said.
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