Influenza should not be dismissed as a trivial disease. It kills thousands of people every year at a very high cost to the economy, hits hardest the young and the elderly, and is most dangerous for people over the age of 65. Influenza is mainly a seasonal illness of the winter months, though in tropical and subtropical areas of Asia and the Pacific it can occur all the year round. The damaging effects of influenza can be prevented by immunization, but constant changes of antigenic specificity of the virus necessitate a different composition of the vaccine (疫苗) from one year to another. The network of WHO surveillance activities to monitor the evaluation of influenza virus strains, and WHO hold an annual consultation at the end of February to recommend the composition of the vaccine for the forthcoming epidemiological season. These recommendations are published immediately in the weekly epidemiological record. Vaccination each year against influenza is recommended for certain high-risk populations. In closed or semi-closed settings, maximum-benefit from immunization is likely to be achieved when more than three-quarters of the population are vaccinated so that the benefit of 'herd immunity' can be exploited. Special care should be taken of the following groups: —adults and children with chronic disorders of the pulmonary or cardiovascular systems requiring regular medical follow-up or who had been hospitalized during the previous year, including children with asthma —residents of nursing homes and other establishments for patients of any age with chronic medical conditions —all people over the age of 65. Physicians, nurses, and other personel in primary and intensive care units, who are potentially capable of transmitting influenza to high risk persons, should be immunized visiting nurses and volunteer workers providing home care to high-risk persons should also be included. This passage ______.