WHO consultation on priority public health interventions
Executive summary (16-18 March 2004)

16-18 March 2004

Background

Table of contents Background The consultation Conclusions

In January 2004, health authorities in Viet Nam and Thailand reported their first human cases of infection with avian influenza, caused by an H5N1 strain. The cases in humans are directly linked to outbreaks of highly pathogenic H5N1 avian influenza in poultry initially reported in the Republic of Korea in mid-December 2003 and subsequently confirmed in an additional seven Asian countries (Viet Nam, Japan, Thailand, Cambodia, China, Laos, and Indonesia). As at end-March 2004, no countries other than Viet Nam and Thailand had reported human cases. The number of human cases has remained small to date, but treatment has been largely ineffective and case fatality rates have been high. Moreover, the situation has several disturbing features, including the historically unprecedented scale of the outbreak in poultry.

Of foremost concern is the risk that conditions present in parts of Asia could give rise to an influenza pandemic. Pandemics, which recur at unpredictable intervals, invariably cause high morbidity and mortality and great social disruption and economic losses. Conservative estimates based on mathematical modelling suggest that the next pandemic could cause from 2 million to 7.4 million deaths.

Conditions favourable to the start of a pandemic are now much better understood than in the previous century, which witnessed three pandemics. Influenza research was greatly stimulated in 1997, when the world’s first known cases of human infection with the H5N1 strain of avian influenza virus were documented in Hong Kong Special Administrative Region of China. Investigations launched by that outbreak, including studies in molecular biology and epidemiology, helped elucidate the mechanisms by which pandemic viruses could emerge and further clarified the conditions that favour such an event. These studies also demonstrated, for the first time, that the H5N1 strain can infect humans directly without prior adaptation in a mammalian host. On that occasion, the culling within three days of Hong Kong’s poultry population, estimated at 1.5 million birds, is thought to have possibly averted a pandemic.

Some experts believe that this improved understanding, when combined with efficient surveillance and immediate and aggressive action, might make it possible to detect events with pandemic potential and delay – or even prevent – their escalation and global spread. Research has identified three essential prerequisites for the start of a pandemic. First, a novel influenza subtype must be transmitted to humans. Second, the new virus must be able to replicate in humans and cause disease. Third, the new virus must be efficiently transmitted from one human to another; efficient human-to-human transmission is expressed as sustained chains of transmission causing community-wide outbreaks. Since 1997, the first two prerequisites have been met on four occasions: Hong Kong in 1997 (H5N1), Hong Kong in 2003 (H5N1), the Netherlands in 2003 (H7N7), and Viet Nam and Thailand in 2004 (H5N1). Of these outbreaks, those caused by H5N1 are of particular concern because of their association with severe illness and a high case fatality. Of even greater concern is the uniqueness of the present H5N1 situation in Asia. Never before has an avian influenza virus with a documented ability to infect humans caused such widespread outbreaks in birds in so many countries. This unprecedented situation has significantly increased the risk for the emergence of an influenza pandemic.

A pandemic virus capable of efficient human-to-human transmission could arise via two mechanisms: virus reassortment (the swapping of genetic material between viruses) when humans or pigs are co-infected with H5N1 and a human influenza virus, and adaptive mutation during human infection. The risk that either event will occur remains so long as H5N1 is present in an animal reservoir, thus allowing continuing opportunities for human exposure and infection. The level of risk is determined most directly by the prevalence of the virus in poultry and the frequency of its transmission to humans. The risk also depends on the co-circulation of human and avian influenza viruses and the inherent propensity of these viruses to reassort. Most experts agree that control of the present outbreaks in poultry will take several months or even years; some believe that the virus may have already established endemicity in domestic poultry. The recent detection of highly pathogenic avian influenza in wild birds adds another layer of complexity to control.

The world may therefore remain on the verge of a pandemic for some time to come. At the same time, the unpredictability of influenza viruses and the speed with which transmissibility can improve mean that the time for preparedness planning is right now. Such a task takes on added urgency because of the prospects opened by recent research: good planning and preparedness might mitigate the enormous consequences of a pandemic, and this opportunity must not be missed.

WHO consultation on priority public health interventions
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