Section 26B. In a city the mayor, with the approval of the city council, unless otherwise provided in the city charter, and in a town the board of selectmen, if authorized by a vote of the town, shall appoint, and may remove for cause, a commissioner of health who shall be a citizen of the United States who has been graduated from a medical school approved by the state authority for the approval of medical schools, and either shall be the holder of a degree of public health with at least two years’ full-time experience in a responsible position in public health service, or shall have had four years’ full-time experience in a responsible position in such service. He shall be eligible to be registered to practice medicine under the laws of the commonwealth. Said commissioner shall devote his entire time to the performance of his duties and the supervision of the employees of the department. He shall be appointed in January for a period of five years beginning the first Monday of the following February, and until the qualification of his successor. Any vacancy in office shall be filled for the balance of the unexpired term in the same manner as the original appointment. His salary shall be such as the city council or selectmen may determine.