Epidemiology and control of influenza.

Influenza causes frequent epidemics and periodic pandemics, and is a major public health problem. Pandemics occurred in 1918 (swine influenza), 1957 (Asian influenza), 1968 (Hong Kong influenza) and 1977 (Russian influenza) due to major antigenic variation of the type A influenza virus. Frequent epidemics occur after pandemics as a result of minor antigenic variation of the pandemic virus strains. Minor antigenic variant strains of type A (H1N1), A (H3N2) and type B influenza viruses are currently circulating globally, causing frequent epidemics. The World Health Organization (WHO) has established a network of National Influenza Centres all over the world to study the epidemiology and ecology of influenza, and collaborating centres for updating the influenza vaccines and other research activities. As a part of this programme, it has set up the WHO Flunet for disseminating updates on the global influenza situation, current vaccines and antiviral drugs. Some National Influenza Centres in India have investigated and reported pandemics and epidemics caused by global influenza virus strains during the past 50 years. There is a need to expand influenza surveillance in our country, as only a few centres are conducting these studies.