· an A/New Caledonia/20/99 (H1N1)-like virus

· a B/Beijing/184/93-like virus*

or

a B/Shangdong/7/97-like virus

WHO recommends the vaccine composition for the northern hemisphere each year in February. Since 1998 WHO now also holds a Consultation each September to formulate a recommendation for the composition of inactivated influenza vaccines intended for the following southern hemisphere winter (May to September of the following year). The timing of WHO's recommendations is critical to allow sufficient time for companies to produce the vaccine before the new influenza season starts.

WHO strongly recommends the use of vaccine as an effective preventive measure against this potentially fatal disease. About 50% to 80% of vaccine recipients will be protected against the disease when there is good match between the vaccine and strains of influenza virus that are in circulation. In cases where the vaccine does not fully protect against the disease, severity of illness and the frequency of serious complications are reduced.

Most of the population has been previously infected with or exposed to influenza A(H3N2), influenza A(H1N1) and influenza B viruses and is known to have some degree of residual immunity. As a consequence, one dose of influenza vaccine should be sufficient for all ages except young children. Previously-unimmunized children should receive two doses of vaccine with an interval of at least four weeks.

The specific vaccine viruses used in each country should be approved by the national control authorities. National public health authorities are responsible for recommendations regarding the use of vaccines.

All WHO recommendations are published in WHO's Weekly Epidemiological Record (WER) and communicated to public health authorities, national control authorities and influenza vaccine manufacturers.