But there is no standard or scientific definition of what a mass gathering of people is, said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. And scientists who have studied the 1918 Spanish flu pandemic and more recent outbreaks have not found strong evidence that restricting mass gatherings alone is effective in controlling an outbreak, Dr. Osterholm said.

In addition to the size of a gathering, public health experts may consider factors such as the density of people in one place when making recommendations on social distancing, Dr. Osterholm said.

Events where people congregate in a small space may increase the opportunity for person-to-person transmission of the virus. Gatherings that last for longer periods of time, such as rallies, concerts or conferences, may also increase the opportunities for transmission compared with brief encounters that may occur at a pharmacy or a grocery store. That is why the context of the gathering may be more important than the actual number of people in some situations.

“What is proper social distancing for a major metropolitan city with sustained community acquired transmission is going to be different from a sleepy farming community,” Ms. Hills said. “You wouldn’t want to just one-size things.”

The prevalence of a pathogen in the community is another important factor to consider. If a pathogen is extremely widespread, then it makes sense to limit gatherings to much smaller numbers, Dr. Osterholm said.

“If 20 percent of the population is infected, you can have one meeting with 10 people in it and pretty well assume that there will be someone there who can transmit the virus,” Dr. Osterholm said. Because of a lack of testing, it is not yet clear what portion of the population is infected with the coronavirus.