In Germany, authorities say they don’t know where 75% of people who currently test positive for the coronavirus got it. In Austria, the figure stands at 77%. In Spain, the health ministry said that it was able to identify the origin of only 7% of infections registered in the last week of October. In France and Italy, only some 20% of new cases have been linked to people who previously tested positive.

Jay Varma, senior adviser for public health in the New York City mayor’s office, said 10% of the city’s infections are due to travel, 5% from gatherings, and another 5% from institutional settings such as nursing homes.

“The vast majority of the remainder—somewhere probably around 50% or more—we don’t have a way to directly attribute their source of infection,” Mr. Varma said. “And that’s a concern.”…

Marc Lipsitch, an epidemiologist from Harvard T.H. Chan School of Public Health, points to government data in several countries showing most people are becoming infected at home. The home, while undoubtedly a driver of infections, probably tops the list, he said, because of how hard it is to trace infections that originate elsewhere.