Federal health officials said on Tuesday that they were directing nearly all drugstores and grocery-store pharmacies to offer second doses of Covid-19 vaccines to people who received their first shot from a different provider.

Growing numbers of Americans who received a first shot of the two-dose Pfizer-BioNtech or Moderna vaccine are not getting their second shots, in part because of challenges with access. The Centers for Disease Control and Prevention estimates that more than 5 million people, or nearly 8 percent of those who were partially vaccinated, have missed getting their second dose.