Transmission electron microscopic image of an isolate from the first U.S. case of COVID-19, formerly known as 2019-nCoV. The spherical viral particles, colorized blue, contain cross-section through the viral genome, seen as black dots.

A new study from a University of Chicago lab group suggests that asymptomatic people make up more than 80% of COVID-19 cases and, along with presymptomatic cases, account for at least half of new infections. 

The paper, published last week in the Proceedings of the National Academy of Sciences, was written by Mercedes Pascual, a professor of ecology and evolution at the U. of C., along with Rahul Subramanian and Qixin He, a pair of researchers affiliated with her lab. 

The authors set out to determine approximately how many COVID-19 infections are asymptomatic. That’s important because the number of asymptomatic cases can provide information about how transmission of the disease works, helping to shape public health guidelines. (The researchers also looked at presymptomatic cases, in which a person does not yet show symptoms of COVID, since those can affect community transmission as well.) 

Asymptomatic cases also let us know whether we’re getting close to herd immunity, when spread of the disease slows significantly because a large enough percentage of the population has gotten it. 

But since the start of the pandemic, estimating the number of COVID cases that are asymptomatic has been a hurdle for scientists. One reason for that is the length of time it took to ramp up testing capacity early on.

“Without testing capacity data, it’s very difficult to estimate the difference between cases that were unreported due to a lack of testing and cases that were actually asymptomatic,” said Subramanian, a doctoral student in epidemiology at the U. of C. and lead author on the paper, in a press release. 

The U. of C. researchers used testing information from New York City between March and June of last year, when capacity had ramped up to capture most cases. 

They then used data from a serological study by Mount Sinai Hospital, which tested patients for coronavirus antibodies. By fitting their model to this study, as well as to the number of observed cases in New York, the researchers were able to estimate what proportion of cases were symptomatic and asymptomatic. 

Their model found that the proportion of symptomatic coronavirus cases likely lies between 12.9 and 17.4 percent. That means that the number of asymptomatic cases is over 80% — higher than usually assumed. 

Using these estimates, the researchers considered a couple of different scenarios that might explain how frequently the virus is transmitted depending on whether someone is symptomatic, presymptomatic, or asymptomatic. In each of those, they found that at least half of new infections come from asymptomatic and presymptomatic cases. 

“Since asymptomatic infections represent a large fraction of the infected population, they contribute substantially to community transmission in the aggregate together with presymptomatic cases, even when they individually transmit at a low per capita rate,” the researchers wrote. “They also contribute substantially to building herd immunity.”

Another study published last month came to a similar conclusion — it found that asymptomatic individuals on their own accounted for half of all new transmissions of COVID.  

As the new paper points out, guidance from the Centers for Disease Control and Prevention (CDC) has wavered on whether asymptomatic people should get tested. Last August, the CDC put out a new policy stating that people who had been exposed to someone with COVID did not necessarily need a test if they were asymptomatic. After an outcry from public health experts and the press, the agency reversed itself the following month — currently, the guidelines say that those who have been in close contact with a COVID-19 case should get tested, even if they’re asymptomatic. 

The U. of C. research corroborates this guidance, and suggests that public health authorities should continue to consider asymptomatic spread when formulating guidelines during the pandemic. 

“Even if asymptomatic people aren’t transmitting the virus at high rates, they constitute something like 80% of all infections,” said He, an assistant professor at Purdue University, in the release. “This proportion is quite surprising. It's crucial that everyone — including individuals who don't show symptoms — adhere to public health guidelines, such as mask wearing and social distancing, and that mass testing is made easily accessible to all.”


Christian Belanger graduated from the University of Chicago in 2017. He has previously written for South Side Weekly, Chicago magazine and the Chicago Reader.

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