In the survey, a plurality of epidemiologists said they would recommend limiting such gatherings to two households at a time. But a sizable number said larger gatherings among vaccinated people were OK... Those who said that vaccinated people should continue to limit the number of people they see indoors noted that vaccines aren’t 100 percent effective, and that the more people are in a space, the more chances the vaccine has to fail. Many said they would hesitate to gather indoors until children could also be vaccinated. (The F.D.A. approved use of the Pfizer vaccine for children 12 to 15 this week.) “It is not ‘one size fits all,’” said Alicia Riley, a sociologist and epidemiologist at the University of California, San Francisco, expressing a version of the profession’s unofficial motto: It depends. “How safe it is depends on the local levels of community transmission.”