Mr. Biden came into office vowing to make testing for the virus cheap and easily accessible, and there have been some improvements since he was sworn in. Laboratory tests are more plentiful now, and more than a dozen at-home tests are available, up from zero in January. The Food and Drug Administration has sped up its approval process, and the supply of at-home tests has increased steadily since August; last month, it was expected to double by March.

But the United States remains a far cry from Europe, where more than three dozen types of at-home tests are available for as little as $1 to $2 per test. Americans can pay as much as $25 for a box of two, and Mr. Biden’s plan to have insurers reimburse for those purchases will not take effect until mid-January at the earliest.

In Miami, cars lined up bumper to bumper this week at a drive-through test site. In Providence, R.I., there were no testing appointments available at a local CVS; those looking to be tested were instructed to buy over-the-counter at-home tests.