Pandemic influenza

Why is there so much concern about the current outbreaks?

How likely is a new pandemic of influenza?

Can a pandemic be averted

Why is there so much concern about the current outbreaks?

Public health officials are alarmed by the unprecedented outbreaks in poultry for several reasons. First, most - but not all - of the major outbreaks recently reported in Asia and elsewhere have been caused by the highly pathogenic H5N1 strain. There is mounting evidence that this strain has a unique capacity to jump the species barrier and cause severe disease, with high mortality, in humans.

A second and even greater concern is the possibility that the present situation could give rise to another influenza global epidemic (a pandemic) in humans. Scientists know that avian and human influenza viruses can exchange genes when a person is simultaneously infected with viruses from both species. This process of gene swapping inside the human body can give rise to a completely new subtype of the influenza virus to which few, if any, humans would have natural immunity. Moreover, existing vaccines, which are developed each year to match presently circulating strains and protect humans during seasonal epidemics, would not be effective against a completely new influenza virus.

If the new virus contains sufficient human genes, transmission directly from one person to another (instead of from birds to humans only) can occur. When this happens, the conditions for the start of a new influenza pandemic will have been met. Most alarming would be a situation in which person-to-person transmission resulted in successive generations of severe disease with high mortality.

This was the situation during the great influenza pandemic of 1918-19, when a completely new influenza virus subtype emerged and spread around the globe, in around 4 to 6 months. Several waves of infection occurred over 2 years, killing an estimated 40-50 million persons.

How likely is a new pandemic of influenza?

Pandemics arise when a new virus emerges which is capable of spreading in the world wide population. In the current outbreak of avian influenza the majority of cases described have had contact with poultry from which the infection is assumed to have spread. If the current avian influenza H5N1 adapts and can be spread from person to person then a pandemic would be possible.

Can a pandemic be averted?

No one knows for sure. Influenza viruses are highly unstable and their behavior defies prediction. However, WHO believes that if the right actions are taken quickly, an influenza pandemic might be averted. This is WHO's foremost objective at present.

The first priority, and the major line of defence, is to reduce opportunities for human exposure to the largest reservoir of the virus: infected poultry. This is achieved through the rapid detection of poultry outbreaks and the emergency introduction of control measures, including the destruction all infected or exposed poultry stock, and the proper disposal of carcasses.

All available evidence points to an increased risk of transmission to humans when outbreaks of highly pathogenic avian H5N1 influenza are widespread in poultry. As the number of human infections grows, the risk increases that a new virus subtype could emerge, triggering an influenza pandemic. This link between widespread infection in poultry and increased risk of human infection is being demonstrated right now in Asia. The majority of human cases and deaths detected since 2004 have been detected in three countries - Viet Nam, Indonesia and Thailand - with very widespread outbreaks in poultry.

WHO stresses the urgency of the situation and the need for rapid action in the animal and agricultural sectors. For example, the culling in 1997 of Hong Kong's entire bird population - an estimated 1.5 million chickens and other birds - was done in 3 days. Again in 2003, the culling of nearly 30 million birds (out of a total bird population of 100 million) in the Netherlands was done within a week. Rapid action in both of these situations is thought by many influenza experts to have averted an influenza pandemic in humans.

More information about pandemic influenza is available on our pandemic influenza pages, including frequently asked questions.

Last reviewed: 20 August 2008