Mandatory Flu Vaccination or Mask for Health Care Workers October 4, 2019 General Alameda County, Flu The Alameda County Public Health Department has issued an alert, reminding health care providers and their employers that all health care workers must receive an annual influenza vaccination or, if they decline, must wear a mask during every influenza season while working in patient care areas. Influenza season dates are defined as November 1 to April 30 of the following year. This order is ongoing and applies to each influenza season. Read the full public health alert here.