In addition to the provisions of law now in force, in all counties of the State of Oklahoma, where any member of the board of county commissioners shall become incapacitated physically or mentally and thereby be unable by reason of such disability to perform the duties of his office for the remainder of the term to which such officer was elected, a vacancy in said office is hereby declared to exist and such vacancy shall be filled by election in the manner provided by law for the filling of such vacancies; provided, however, that where such vacancy occurs after the election and prior to the beginning of the term of the county commissioner-elect, such county commissioner so elected shall be entitled to qualify and enter upon the discharge of his duties.