At the height of the pandemic, it was easy to worry that strangers would give you the virus. But a new study of what happened after people’s birthdays suggests that people we trust were also a common source of viral spread. Private gatherings have been harder for researchers to measure than big public events — they’re private, after all. And there has been a fierce debate for months among epidemiologists about just how big a factor they have been in how coronavirus moved from person to person. But researchers from Harvard, the RAND Corporation and Castlight Health used a creative method for finding them: Using health insurance claims data, they looked at the Covid rates of families in the two weeks after one of them had a birthday.
