Extra Covid-19 Shots Pose Quandary for Vaccination Sites

David MacMillan lucked into a dose of Moderna Inc.’s Covid vaccine late on New Year’s Day. While grocery shopping at Giant Food in Washington, D.C., the 31-year-old paralegal passed by the in-store pharmacist who was scrambling to find takers for two doses after a pair of health-care workers missed their appointments. The store was scheduled to close in 10 minutes and the thawed doses would have to be thrown out.

Two of the store’s associates and another customer had already declined the shots. Mr. MacMillan and his 25-year-old friend happily...