The composition of the vaccine for 1999-2000 (northern hemisphere influenza season) has been published today and communicated to vaccine manufacturers by the World Health Organization (WHO), following agreement on its content by international experts at a WHO meeting held at WHO headquarters in Geneva, Switzerland. The experts recommended that the influenza vaccine for 1999-2000 (northern hemisphere) contain the following three components:

• An A/Sydney/5/97 (H3N2)-like virus
• An A/Beijing/262/95 (H1N1)-like virus
• A B/Beijing/184/93-like virus

or

A B/Shangdong/7/97-like virus.

Decisions on the most appropriate B component should be made by national control authorities on the basis of local epidemiological data.

These three strains were chosen because influenza A(H3N2), A(H1N1) and influenza B viruses continued to circulate widely during the 1998-1999 influenza season. Influenza A(H3N2) and influenza B viruses predominated while influenza A(H1N1) viruses were isolated occasionally.

The influenza virus is constantly changing and the use of influenza vaccines is increasing worldwide. Each year, therefore, WHO holds a Consultation in mid-February to formulate a recommendation for the composition of inactivated influenza vaccines intended for the following northern hemisphere winter (November of that year to April of the following year). WHO also holds a consultation in September to determine the content of the influenza vaccine for the upcoming southern hemisphere winter, which runs from June to September.

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. The northern hemisphere flu season started in October 1998 and various countries have already reported tens of thousands of cases, which is not unusually high. 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 is likely to have previously been infected with influenza A(H3N2), influenza A(H1N1) and influenza B viruses and 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 manufacturers.

The detection of new influenza viruses is made possible through the WHO network for influenza surveillance and control composed of 110 National Influenza Centres in 83 countries and the four WHO Collaborating Centres for Virus Reference and Research in Australia, Japan, the United Kingdom and the USA. This network helps WHO monitor influenza activity in all regions of the world and ensures that virus isolates and information are sent rapidly to the WHO Collaborating Centres for Virus Reference and Research for immediate strain identification.

For further information, journalists can contact Gregory Hartl, Office of Press and Public Relations, WHO, Geneva. Telephone (41 22) 791 44 58. Fax (41 22) 791 48 58. E-Mail: hartlg@who.int