A health care worker administers a dose of a COVID-19 vaccine at the Javits Center in New York in March. When companies began announcing tentative return-to-office plans this spring, there was a sense of optimism behind the messages. COVID cases were dwindling in the United States as the vaccine rollout picked up pace. Employers largely hoped their workforces would get shots on their own, motivated by raffle tickets, paid time off and other perks, if not by the consensus of the medical community. In recent days, that tone suddenly has shifted.