9. When the place of an appointed or elected member of a Board becomes vacant by his removal, resignation or death, a new member shall be appointed or elected in the manner provided in section 3, and shall hold office so long as the member whose place he fills would have been entitled to hold office if such vacancy had not occurred:Provided that no act of the Board, or of its officers, shall be deemed to be invalid by reason only that the number of members of the Board at the time of the performance of such act was less than the number provided by section 3.