While the national vaccine mandate for health care workers remains mired in the federal courts, government officials say there is ample proof that requirements work — and do not cause a mass exodus of employees that some critics had feared.

Philadelphia, which issued a vaccine mandate in August, would seem to be a case in point. Virtually all hospital employees there are now fully vaccinated against Covid-19, according to a group of local hospitals that met last week with officials from the Centers for Medicare and Medicaid Services, which issued the national rule. About 95 percent of nursing home employees are also fully vaccinated, according to the city health department, compared with an average of 75 percent across the country.