U.S. virus cases 10 times reported rate: CDC
Researcher says spread accelerated by infected people without symptoms
The number of coronavirus infections in many parts of the United States is more than 10 times higher than the reported rate, according to data released Friday by the Centers for Disease Control and Prevention.
The analysis is part of a wideranging set of surveys started by the CDC to estimate how widely the virus has spread. Similar studies, sponsored by universities, national governments and the World Health Organization, are continuing all over the world.
The CDC study found, for instance, that in South Florida, just under two per cent of the population had been exposed to the virus as of April 10, but the proportion is likely to be higher now given the surge of infections in the state. The prevalence was highest in New York City at nearly seven per cent as of April 1.
“This study underscores that there are probably a lot of people infected without knowing it, likely because they have mild or asymptomatic infection,” said Dr. Fiona Havers, who led the CDC study.
“But those people could still spread it to others.”
The numbers indicate that even in areas hit hard by the virus, an overwhelming majority of people have not yet been infected, said Scott Hensley, a viral immunologist at the University of Pennsylvania who was not involved in the research.
“Many of us are sitting ducks who are still susceptible to second waves,” he said.
The difference between recorded infections and the actual prevalence in the data was highest in Missouri, where about 2.65 per cent of the population was infected with the virus as of April 26, although many people might not have felt sick. This number is about 24 times the reported rate: nearly 162,000 compared with the 6,800 thought to have been infected by then.
The results confirm what some scientists have warned about for months: that without wider testing, scores of infected people go undetected and circulate the virus.
“Our politicians can say our testing is awesome, but the fact is, our testing is inadequate,” Hensley said. “These are exactly the kind of studies we need right now.”
Dr. Robert Redfield, director of the CDC, hinted at this trend Thursday during a call with reporters.
“Our best estimate right now is for every case reported, there were actually 10 other infections,” Redfield said.
The source for his claim was unclear at that time. The CDC later posted the data on its website and on MedRxiv, a repository for scientific results that have not yet been vetted by peer review.
The CDC researchers tested samples from 11,933 people across six U.S. regions during discrete periods from March 23 through May 3: the Puget Sound region of Washington, where the first COVID-19 case in the country was diagnosed, as well as New York City, South Florida, Missouri, Utah and Connecticut.
The samples were collected at commercial laboratories from people who came in for routine screenings, such as cholesterol tests, and were evaluated for the presence of antibodies to the virus — which would indicate previous infection even in the absence of symptoms.
The researchers then estimated the number of infections in each area. New York City, for example, reported 53,803 cases by April 1, but the actual number of infections was 12 times higher, nearly 642,000.
The city’s prevalence of 6.93 per cent in the CDC study is well below the 21 per cent estimated by the state’s survey in April. That number was based on people recruited at supermarkets, so the results would have been biased toward people who would be out shopping during a pandemic — young people or those who had already had the virus and felt safe, experts said.