Here’s the rosy scenario: By Labor Day, if all goes well, nearly every adult in the United States who wants a COVID-19 vaccine will have received one. Deaths and hospitalizations from the coronavirus will have fallen sharply; transmission will have plummeted as the population reaches herd immunity. Everything from schools to restaurants will have fully reopened, allowing for something approximating pre-pandemic life to resume.

There is also, regrettably, a far bleaker scenario for the coming months: The virus remains implacable, mutating in ways that either end-run the protection of vaccines or keep it contagious enough to sicken a large portion of the population, including younger people at lower risk from previous strains and the millions who refuse to be vaccinated. As drug makers race to tailor vaccines to the variants, the country might remain trapped in a purgatory of stressed hospitals, shuttered businesses, and remote learning.

Dr. Edward Nardell, a professor at Harvard Medical School who studies infectious diseases, prefers to think we’re heading toward the rosy outcome. He is already planning to attend a cabaret festival in Provincetown in June, and hopes to resume organizing concerts this fall at his condominium complex in Brookline.