Immunization, Vaccines and Biologicals


The burden of influenza in the USA is currently estimated to be 25–50 million cases per year, leading to 150 000 hospitalizations and 30 000–40 000 deaths. If these figures are extrapolated to the rest of the world, the average global burden of inter-pandemic influenza may be on the order of ~1 billion cases of flu, ~3–5 million cases of severe illness and 300 000–500 000 deaths annually. Epidemics and outbreaks of influenza occur in different seasonal patterns depending on the region in the world. In temperate climate zones, seasonal epidemics typically begin in the late fall and peak in mid- to late winter. In tropical zones, seasonal patterns appear to be less pronounced, with year-round isolation of virus.

In developed countries, annual influenza epidemics infect about 10–20% of the population each season, and cause febrile illnesses that range in severity from mild to debilitating and can lead in some instances to hospitalization and even cause death. The latter mostly occur as a consequence of primitive fulminant influenza virus pneumonia or of secondary respiratory bacterial infections and are facilitated by underlying pulmonary or cardiopulmonary pathologies. The risk of developing serious complications is aggravated in the very young and in the elderly. Data collected in Michigan (USA) and in Japan indicate that the mass vaccination of school-aged children correlates with a reduced rate of respiratory illness in all age groups, suggesting that larger-scale immunization in childhood could favourably affect influenza epidemics. More

WHO Position papers

Further information

Last updated: 25 January 2008