PHILADELPHIA — Philadelphia city workers will be required to be vaccinated or get an exemption by mid-January, city health officials said Friday.

The announcement reverses course from previous directives that allowed unvaccinated workers to wear double masks while at work in enclosed offices. City officials had mandated this summer that new employees be vaccinated as a condition of employment.

The new mandate covers workers represented by four city labor unions, as well as some city contractors, and requires two doses or an approved exemption by Jan. 14.

Officials said employees who fail to comply will be considered unable to fulfill their duties and will be placed on unpaid leave for no longer than 15 days. Continued refusal could result in firing, city officials wrote.