Epidemiological and evolutionary dynamics of influenza B virus in coastal Kenya as revealed by genomic analysis of strains sampled over a single season by Nyasimi, FM, Owuor, DC, Ngoi, JM, Mwihuri, AG, Otieno, GP, Otieno, JR, Githinji, G, Nyiro, JU, Nokes, DJ, Agoti, CN