Margot Sanger-Katz writes:

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. Overall, their paper, published in Jama Internal Medicine, found that a recent family birthday increased Covid risk by nearly a third in counties where the virus was widespread.

Their theory is that the increased risk is almost certainly explained by birthday parties. Though the insurance claims don’t show whether any individual Covid patient had actually held a birthday party, several aspects of the data strongly suggest a connection. [Continue reading…]