Avian influenza: assessing the pandemic threat

January 2005

This publication evaluates the present pandemic threat on the basis of what we know about pandemics, influenza A viruses, and the H5N1 virus in particular. It draws together some current facts and figures, evidence from the past, and some best-guess speculations useful in assessing the present situation and understanding its multiple implications for human health. Basic information on human cases detected to date is set out in tabular form (please see note below).

Contents

1. The H5N1 outbreak in 2004: a pandmemic in waiting?
2. Lessons from past pandemics
3. Understanding the outbreaks in poultry
4. Action in the face of an uncertain threat

Tables and Boxes
- Influenza A viruses:sloppy, capricious, and promiscuous
- Investigations of human-to-human transmission
- Human cases, Viet Nam
- Human cases, Thailand
- The origin of pandemic viruses
- The WHO global influenza programme: a network of flu "detectives"
- Previous outbreaks of pathogenic avian influenza worldwide
- Documented human infections with avian influenza viruses
- Non-medical interventions at the national level
- Non-medical interventions at the international level

Note on official numbers of cases
For human cases in Viet Nam, some confusion has arisen concerning the official number of laboratory-confirmed cases. A case announced by WHO as confirmed in an update issued on 28 January 2004 was subsequently discarded. The case, listed as the eighth in Viet Nam, concerned a four-year-old boy from Vinh Phuc Province, with date of onset given as 22 December 2003. Viet Nam's Ministry of Health initially announced the case as confirmed on the basis of a single positive serology result. When subsequent tests involving PCR and viral culture produced negative results, the case was reclassified by the Ministry of Health and WHO as "probable". The child has since fully recovered.
For both Viet Nam and Thailand, the tables in this document show some slight differences from initially reported information on individual cases, mainly in the ages and onset dates of some patients. These differences reflect the results of case investigations following the first announcement of confirmed cases. As a result, the accuracy of the present tables has improved and the data are more complete.
As the situation continues to evolve, situation updates and the table showing the cumulative number of confirmed human cases should be consulted for the latest information .