Scientists tracking the spread of the coronavirus reported on Monday that, for every confirmed case, there are most likely another five to 10 people in the community with undetected infections. These often-milder cases are, on average, about half as infectious as confirmed ones, but are responsible for nearly 80 percent of new cases, according to the report, which was based on data from China.

The researchers modeled the virus’s natural spread in China before the government instituted a travel ban and an aggressive testing policy. During that time, from December of last year through late January, about 6 in 7 cases went undetected. That situation is analogous to the current state of affairs in the United States and other Western countries, where tests are not widely available, the researchers said.

“If we have 3,500 confirmed cases in the U.S., you might be looking at 35,000 in reality,” said Jeffrey Shaman, an epidemiologist at Columbia University and the senior author of the new report, which was posted by the journal Science.

The report is among the first to address two of the most pressing questions about the pandemic: How many people are walking around with unrecognized infections, and how infectious are they? As American policymakers have begun taking more aggressive measures to slow transmission, such as canceling events and closing restaurants, access to tests for the virus has been difficult or nonexistent in much of the country.