To keep the virus off a campus, test beyond its borders, a new study suggests.

A new analysis suggests that schools and colleges, large companies and other organizations that want to keep themselves safe with frequent mass testing should think beyond their own personnel.

By dedicating a substantial portion of their tests to people in the surrounding community, they can reduce the number of Covid-19 cases among their members by as much as 25 percent, researchers report in a new paper, which has not yet been published in a scientific journal.

“It’s natural in an outbreak for people to become self-serving, self-focused,” said Dr. Pardis Sabeti, a computational biologist at Harvard University and the Broad Institute who led the analysis. But she added, “If you’ve been in enough outbreaks, you just understand that testing in a box doesn’t makes sense. These things are communicable, and they’re coming in from the community.”

The study has “really profound implications, especially if others can replicate it,” said David O’Connor, a virologist at the University of Wisconsin, Madison, who was not involved in the analysis but reviewed a draft of the paper.

Early in the pandemic, when testing resources were in short supply, many colleges proposed intensive, expensive testing regimens focused entirely on their own campuses. When they sought Dr. Sabeti’s advice, she said, she told them they ought to test friends, relatives and neighbors of their students and employees as well.

It was not an easy idea to sell without data on its effectiveness, so Dr. Sabeti and her colleagues developed an epidemiological model to simulate how a virus might spread through a midsize institution, Colorado Mesa University, and what would happen under different testing policies. They found that allocating some tests to community contacts would significantly reduce the expected number of Covid cases on campus.