Influenza vaccine: the challenge of antigenic drift.

Influenza continues to have a major worldwide impact, resulting in considerable human suffering and economic burden. The regular recurrence of influenza epidemics is thought to be caused by antigenic drift, and a number of studies have shown that sufficient changes can accumulate in the virus to allow influenza to reinfect the same host. To address this, influenza vaccine content is reviewed annually to ensure protection is maintained, despite the emergence of drift variants; however, it is not always possible to capture every significant drift, partly due to the timing of the recommendations. Vaccine mismatch can impact on vaccine effectiveness, and has significant epidemiological and economical consequences, as was seen most apparently in the 1997-1998 influenza season. To meet the challenge of antigenic drift, vaccines that confer broad protection against heterovariant strains are needed against seasonal, epidemic and pandemic influenza. In addition to the use of vaccine adjuvants, emerging research areas include development of a universal vaccine and the use of vaccines that exploit mechanisms of cross-protective immunity.