But many epidemiologists are far less certain, saying there is little evidence to suggest that household gatherings were the source of the majority of infections since the summer. Indeed, it has become much harder to pinpoint any source of any outbreak, now that the virus is so widespread and Americans may be exposed in so many ways.

“Somebody says something, and somebody else says it, and then it just becomes truth,” said Julia Marcus, an infectious disease epidemiologist at Harvard University. “I worry about this narrative that doesn’t yet seem to be data-based.”

Most states don’t collect or report detailed information about the exposure that led to a new infection. But in states where a breakdown is available, long-term care facilities, food processing plants, prisons, health care settings, and restaurants and bars are still the leading sources of spread, the data suggest.