Vaccine strains changed every year is

Influenza virus shows changing antigenic characteristics due to antigenic shift and antigenic drift, hence new vaccines are constantly required. WHO makes recommendations every year as to what strains should be included in the vaccine. FLU VACCINES FOR 2020 Southern hemisphere H1N1 (Brisbane strain) (Influenza type A) H3N2 (South Australia) (Influenza type A) Influenza type B (Washington strain of Victoria lineage) Influenza type B (Phuket strain of Yamagata lineage) Nohern Hemisphere H1N1 (Brisbane strain) (Influenza type A) H3N2 (Kansas) (Influenza type A) Influenza type B (Colarado strain of Victoria lineage) Influenza type B (Phuket strain of Yamagata lineage)