Influenza: prospect for prevention and control.

Influenza is an emerging and re-emerging disease. Since the late 1930s influenza viruses have been isolated yearly from different parts of the world during epidemics and pandemics. The "epidemiologic success" of influenza is due largely to rapid and unpredictable antigenic changes (antigenic drift) among human influenza viruses, and the emergence of new subtypes (antigenic shift), mostly from reassortment between human and avian influenza viruses. Antigenic shifts were attributed to the global pandemic viruses of 1957 (H2N2 Asian flu) and 1968 (H3N2 Hong Kong flu). Concern over possible new pandemics has been heightened by recent reports of human infection in Asia in 1997 with avian viruses (H5N1) and in 1999 (H9N2) and isolation of human-avian reassorted viruses from pigs and humans in Europe. Influenza has a high rate of inapparent infection, short incubation and high infectivity; epidemics usually start abruptly and spread rapidly to neighboring communities and countries. Isolation and quarantine are often unsuccessful in preventing the spread of the infection. Although not perfect, immunization and chemoprophylaxis are highly effective at minimizing the spread of influenza and reducing morbidity and mortality, social disruption and economic loss. Plans for future influenza epidemics and pandemics require national and international programs to be in place for the monitoring of influenza activity, the dissemination and exchange of information and the provision and delivery of sufficient quantities of vaccines and antiviral agents. This paper reviews and discusses the antigenic variations of the influenza virus, potential influenza pandemics, protective efficacy of inactivated vaccines and antiviral agents and preparation for control of future epidemics and pandemics.