The vaccine against seasonal influenza is called trivalent, because it contains viruses representing three influenza strains: one A/H1N1, one A/H3N2, and one B. the single B-lineage antigen included is the one that is predicted to circulate in the related season. However, this means that some mismatch between the B antigen in the vaccine and the B antigen actually circulating may occur, this being a problem since cross-protection by immunization against the other lineage is expected to be low.