Shoppers line up for their turn to buy food at the Trader Joe’s grocery store on 14th Street NW, near the intersection of U street, in Washington on April 10. (John McDonnell/The Washington Post)

Researchers tracking smartphone data say they recently made a disturbing discovery: For the first time since states began implementing stay-at-home orders in mid-March to limit the spread of the novel coronavirus, Americans are staying home less.

The nationwide shift during the week of April 13 was relatively slight. However, any loss of momentum, particularly when stay-in-place orders remain in effect across most of the country, has some public health experts worried about “quarantine fatigue.” Any increase in travel, they say, is premature when staying home remains the most effective way to limit the spread of the virus until widespread testing and contact tracing become available.