Evaluating the case for trivalent or quadrivalent influenza vaccines.

Influenza viruses circulate widely throughout the world and it is estimated that they affect between 5 and 15% of the population annually. Since 1977, four viruses co-circulate - two A Viruses (H1N1 and H3N2) and two B viruses (B Yamagata and B Victoria). Type A viruses generally cause up to two thirds of annual infections, although single country studies have shown that B infections may be the predominant virus in the one year in four. Influenza vaccines have traditionally included the hamagglutinins and neuraminidases from the two circulating A viruses and either B Yamagata or B Victoria - however, selecting the B strain for inclusion in these trivalent vaccines has variable success. The alternative approach is to include both B strains in a quadrivalent vaccine. Immunological studies of such vaccines show non-inferiority with a trivalent vaccine comparator, and significant superiority to the additional B strain. Quadrivalent vaccines are more expensive than trivalent preparations but theoretical evidence would suggest they are likely to be more effective and therefore play a much greater role in national immunisation programmes in the future.