Speculation has been rife for five years about the surprisingly large number of Americans who skipped the second dose of an mRNA COVID-19 vaccine. Months into the jabs' broad availability in 2021, the Centers for Disease Control and Prevention estimated five million people, or 8% of COVID vaccine recipients, had stopped with a single jab. Some of them told The New York Times they feared side effects, felt protected by one dose or their vaccine provider had run out of second doses.