FULLERTON, Calif. — The post-Thanksgiving rush for coronavirus testing is on: Pharmacies in the Southern California suburbs are advising customers lucky enough to score appointments that it could be four days before they receive results. In Chelsea, Mass., a line of people who hoped for testing, pelted by rain and wind, strung along an entire block early this week. In Atlanta, people have idled in cars, sometimes for hours, to get swabbed at drive-throughs.

Testing has long been one of the keys to controlling the spread of a virus that with the onset of winter is entering its most dangerous phase. Yet even as cases per capita have rocketed, securing a test has become enough of an ordeal that many people have been dissuaded from even trying.