OTTAWA, Aug 18 (Reuters) - Canada’s annual inflation rate accelerated to 3.7% in July, the fastest rate since 2011, up from a year-over-year increase of 3.1% in June, due to both the base-year effect and higher shelter costs, Statistics Canada said on Wednesday. Analysts polled by Reuters had expected the annual rate to rise to 3.4% in July. (Reporting by Julie Gordon in Ottawa and Steve Scherer; Editing by Alex Richardson) Our Standards: The Thomson Reuters Trust Principles.