The F.D.A. is expected to approve a new round of shots by Pfizer and Moderna as early as Monday to prepare Americans for the fall and winter season when infections usually tick upward.
Infections remain very low, despite signs of a slight increase. Now, experts are looking for clues to what living with the coronavirus will be like this winter and beyond.
Since 2011, there have been more human swine flu cases reported in the United States than anywhere else in the world. Most have occurred at farm-animal showcases.