An influenza pandemic occurs when a new influenza virus appears against which the human population has no immunity, resulting in several, simultaneous epidemics worldwide with enormous numbers of deaths and illness. With the increase in global transport and communications, as well as urbanization and overcrowded conditions, epidemics due the new influenza virus are likely to quickly take hold around the world.

A new influenza virus: how it could cause a pandemic

Annual outbreaks of influenza are due to minor changes in the surface proteins of the viruses that enable the viruses to evade the immunity humans have developed after previous infections with the viruses or in response to vaccinations. When a major change in either one or both of their surface proteins occurs spontaneously, no one will have partial or full immunity against infection because it is a completely new virus. If this new virus also has the capacity to spread from person-to-person, then a pandemic will occur.

Outbreaks of influenza in animals, especially when happening simultaneously with annual outbreaks in humans, increase the chances of a pandemic, through the merging of animal and human influenza viruses. During the last few years, the world has faced several threats with pandemic potential, making the occurrence of the next pandemic just a matter of time.

Consequences of an influenza pandemic

In the past, new strains have generated pandemics causing high death rates and great social disruption. In the 20th century, the greatest influenza pandemic occurred in 1918 -1919 and caused an estimated 40–50 million deaths world wide. Although health care has improved in the last decades, epidemiological models from the Centers for Disease Control and Prevention, Atlanta, USA project that today a pandemic is likely to result in 2 to 7.4 million deaths globally. In high income countries alone, accounting for 15% of the worlds population, models project a demand for 134–233 million outpatient visits and 1.5–5.2 million hospital admissions. However, the impact of the next pandemic is likely to be the greatest in low income countries because of different population characteristics and the already strained health care resources.

If an influenza pandemic appears, we could expect the following:

• Given the high level of global traffic, the pandemic virus may spread rapidly, leaving little or no time to prepare.



• Vaccines, antiviral agents and antibiotics to treat secondary infections will be in short supply and will be unequally distributed. It will take several months before any vaccine becomes available.



• Medical facilities will be overwhelmed.



• Widespread illness may result in sudden and potentially significant shortages of personnel to provide essential community services.



• The effect of influenza on individual communities will be relatively prolonged when compared to other natural disasters, as it is expected that outbreaks will reoccur.

Detecting a new pandemic virus

Continuous global surveillance of influenza is key. WHO has a network of 112 National Influenza Centres that monitors influenza activity and isolates influenza viruses in all continents. National Influenza Centres will report the emergence of an “unusual” influenza virus immediately to the WHO Global Influenza Programme or to 1 of the 4 WHO Collaborating Centres. Rapid detection of unusual influenza outbreaks, isolation of possible pandemic viruses and immediate alert to the WHO system by national authorities is decisive for mounting a timely and efficient response to pandemics.

Preparing for an influenza pandemic

Contingency planning for an event sometime in the future is often difficult to justify, particularly in the face of limited resources and more urgent problems and priorities. However, there are two main reasons to invest in pandemic preparedness:

1. Preparation will mitigate the direct medical and economic effects of a pandemic, by ensuring that adequate measures will be taken and implemented before the pandemic occurs.

2. Preparing for the next influenza pandemic will provide benefits now, as improvements in infrastructure can have immediate and lasting benefits, and can also mitigate the effect of other epidemics or infectious disease threats.

A major component of pandemic preparedness is to strengthen the capacity to respond to yearly epidemics of influenza. A surveillance network for human and animal influenza and a targeted influenza vaccination programme are the cornerstones of a national influenza policy.

Ensuring an adequate system for alert, response and disaster management, should be the basis of every national pandemic preparedness plan. Depending on the available resources, more specific preparations can be made, such as developing specific contingency plans, stockpiling of antivirals, strengthening risk communications, investing in pandemic vaccine research and promoting domestic production of influenza vaccines.

WHO has developed an Influenza Pandemic Preparedness Plan, which defines the responsibilities of WHO and national authorities in case of an influenza pandemic. This plan is being updated to incorporate new scientific data and experience obtained during recent outbreaks that had pandemic potential. WHO also offers guidance tools and training to assist in the development of national pandemic preparedness plans.