No More Than 10 People in One Place, Trump Said. But Why?

As the new coronavirus continues to spread, state and local governments are shutting down schools and businesses and setting limits on the sizes of public gatherings. The latest recommendation announced Monday by the federal government to promote social distancing and limit the transmission of the coronavirus: no more than 10 people in one place.

“We’d much rather be ahead of the curve than behind it,” President Trump said at a White House news conference. “Therefore, my administration is recommending that all Americans, including the young and healthy, work to engage in schooling from home when possible, avoid gathering in groups of more than 10 people, avoid discretionary travel and avoid eating and drinking at bars, restaurants and public food courts.”

But where did that number come from? The variations in crowd size requirements and recommendations from assorted government authorities may add to confusion about what people should do to protect themselves and prevent transmission of disease.

“It feels like somebody is using a Magic 8-Ball to make these decisions,” said Kelly Hills, a bioethicist and co-founder of the consulting firm Rogue Bioethics. “There is no consistency. There doesn’t even seem to be consistency in who’s making these decisions.”

Size limits for group gatherings in public and private settings have varied widely over the past week.

• St. Louis banned gatherings as large as 1,000 on Thursday.

• As recently as Sunday, the federal Centers for Disease Control and Prevention recommended that events consisting of 50 people or more be canceled or postponed for the next eight weeks.

• Gov. Charlie Baker of Massachusetts prohibited gatherings of over 25 people on Sunday.

• On the same day, officials in San Mateo County, Calif., reduced their recommendation to 10 people while officially banning crowds of 50 or more.

• That change came at a time when California state officials had declared 250 as the ceiling for gatherings.

Social distancing is a broad category of precautions that includes standing approximately six feet from others when possible, not shaking hands, applying self-quarantine or isolation for individuals who feel sick and avoiding mass gatherings. These precautions can help slow the spread of infections and buy health officials more time to stock up on supplies needed to treat the critically ill.

Dr. Deborah Birx, the White House coronavirus response director, said during Monday’s news conference that the federal recommendation of no more than 10 was reached because officials had been working with researchers modeling the spread of the virus.

“The most important thing was if one person in the household became infected, the whole household self-quarantined for 14 days because that stops 100 percent of the transmission outside of the household,” Dr. Birx said, without offering more detail about how they reached the number 10.

One study from researchers at Imperial College London predicted that without action to stem the spread, the virus could cause more than two million deaths in the United States. It suggested that the most effective way to avert that outcome was to limit interactions between people for a period of time.

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. But that is a piece of information that researchers don’t have yet for the new coronavirus.

Does that mean it’s safe to have up to 10 people in your home for a dinner party, a book club meeting or some other small gathering?

The safest thing to do for the moment would be to cancel such plans, or perhaps shift them to videoconferencing.

And for those who go forward with guests, it is important to consider whether anyone in the household or among the invited guests may be elderly or have underlying health conditions. Research suggests that individuals in these categories are more likely to catch an infection, and they may develop more severe disease if they get sick.

Family-style meals and open buffets should be avoided in any case.

“You might want to have one person in charge of dishing out all of the plates, rather than each person going in and interacting with the food,” Ms. Hills said.

And if you or one of your guests develops a fever or cough or simply feels fatigued, it is best to err on the side of caution and cancel plans, Ms. Hills said. “Everything can be rescheduled.”