Researchers investigate the number of asymptomatic COVID-19 cases

Concern over the number of individuals who have contracted COVID-19 without contact with a symptomatic individual have researchers rushing to get some kind of estimate of just how many individuals are infected with COVID-19 but show no symptoms.

The director of the University of Minnesota’s Center for Infectious Diseases Research and Policy Michael Osterholm stated, “Understanding the proportion of asymptomatic or mildly ill cases is going to be really important for us to understand what is driving this particular epidemic.”

To study asymptomatic infections, researchers in the U.S. and China have developed a model using thousands of laboratory-confirmed cases in Wuhan.  The group suggested that at one point in time as many as 59% of those infected with COVID-19 were unaware and many were contagious, despite having no symptoms, or just mild ones.

The methodology in the model is solid and the results are within estimates from other studies, said Adam Kucharski, a disease modeler at the London School of Hygiene and Tropical Medicine. 

“It’s the most recent analysis of the best data set we have,” he said.

There is a worry about that model though – it works with the assumption that entire communities have the same opportunity to be in contact with anyone else. Gerardo Chowell, a mathematical epidemiologist at Georgia State University says this may not be accurate.  In reality, “you have more chances of interacting with a small fraction of people: your family, your friends and your colleagues,” Chowell said. By assuming there is homogeneous mixing, he says, the model probably overestimates the transmission rate and exaggerates the number of infections with mild or no symptoms. But the result is in the right ballpark, he says.

In examining the cases on the Diamond Princess cruise ship, Chowell estimates that 17 percent of those who were infected with COVID-19 never showed symptoms, but he notes that this was a special population, with a significant number of elderly individuals. Older people tend to fare worse with this infection. Keeping that in mind, he believes that the 31 percent number from Japanese researchers may be closer to the ballpark than his own 17 percent when it comes to an estimate of the general population, at all ages.

In studying the power of viral shedding, other researchers in Germany found that some individuals who contracted COVID-19 had high levels of virus in the throat swabs early on, which meant they could easily pass on the virus through coughing or sneezing. A team in Germany found that a patient who was asymptomatic was still able to spread as much of the virus through a cough or sneeze as one who did in fact have symptoms.

Chowell says this is even more reason for strong social distancing.