La donación de tres millones de tratamientos con oseltamivir a la OMS ayudará a responder rápidamente a una pandemia incipiente de gripe
H5N1 avian influenza: background for journalists
Overview of the present situation
Outbreaks of highly pathogenic H5N1 avian influenza in poultry are now known to have begun in parts of South-east Asia in mid-2003. These outbreaks have been historically unprecedented in their geographical size and the number of birds affected. So far, around 150 million birds have died during the outbreaks or been destroyed as part of the containment effort.
While some of the initially affected countries have successfully contained the disease in poultry, the virus is now considered entrenched in many parts of Viet Nam and Indonesia and in some parts of Cambodia, China, Thailand, and possibly also Laos.
To date, human cases have been confirmed in four countries: Viet Nam, Thailand, Cambodia, and Indonesia.
During early August 2005, highly pathogenic H5N1 avian influenza was confirmed in poultry in parts of Siberia, Russia and in adjacent parts of Kazakhstan. Both countries have reported deaths of migratory birds in the vicinity of poultry outbreaks. These events mark the first detection of highly pathogenic H5N1 avian influenza in the two countries. Last week, avian influenza of the H5 subtype was confirmed in dead migratory birds in Mongolia. No human cases have been reported in conjunction with any of these newer outbreaks.
Assessment of the pandemic threat
WHO considers the present risk of a pandemic great, but unpredictable in terms of its timing and severity. All conditions for the start of a pandemic have been met save one: changes in the virus that would make it contagious among humans, thus allowing easy and sustainable human-to-human transmission. The likelihood that this will happen is a matter of opportunity and probability.
The expanding geographical range of the virus increases opportunities for human cases to occur and these, in turn, increase opportunities for the virus to improve its transmissibility. The fact that the virus is now endemic in poultry populations in several countries increases the probability that this will occur.
In response to the pandemic threat, WHO has recommended a series of strategic actions to be undertaken by affected and at-risk countries, by WHO, and by the international community. These actions are phase-wise according to escalating levels of risk, and pertain to the present pre-pandemic situation, the first emergence of a contagious virus, and the declaration of a pandemic and subsequent international spread. In the present situation, recommended strategic actions aim to reduce opportunities for human infection (by controlling the outbreaks in poultry and avoiding contact between humans and infected birds), and to strengthen the early warning system.
Experts anticipate that a virus with improved transmissibility will announce itself in the form of clusters of human cases, closely related in time and space. Surveillance and reporting systems in all countries experiencing outbreaks in birds need to be strong enough to detect such clusters of human cases.
Internal stockpile of antiviral drugs
If the first signs of improved transmissibility are picked up quickly, there is a chance that rapid intervention, involving mass prophylactic administration of antiviral drugs, might contain the pandemic at its source or at least delay international spread, gaining time to intensify preparedness. An international stockpile of antiviral drugs is needed for this purpose. The prospect of halting a pandemic at its source or delaying its international spread is attractive, but untested, as no attempt has ever been made to alter the natural course of a pandemic. Successful intervention requires that at least 5 conditions be met:
- The first viruses that show an ability to sustain transmission among humans will not yet be highly contagious.
- The emergence of such viruses will be limited to a small geographical area.
- The first clusters of human cases caused by the virus will be rapidly detected and reported.
- Antiviral drugs will be rapidly mobilized from the stockpile, made available to the affected population, and administered to sufficiently large numbers of people.
- Movement of people in and out of the area will be effectively restricted.
Given the unpredictable nature of influenza viruses, it is impossible to know in advance if the first two conditions will be borne out in reality when a pandemic virus emerges. The remaining conditions require excellent surveillance and logistics capacity in the initially affected area, combined with an ability to enforce movement restrictions.
While mass intervention with antiviral drugs has no guarantee of success, it nonetheless needs to be undertaken as it represents one of the few preventive options for an event with predictably severe consequences for every country in the world. As drugs in the stockpile can also be used for treatment purposes, having such a stockpile provides the best guarantee that populations affected at the start of a pandemic and thus in greatest need will have drugs available for treatment.
Once the virus has become fully contagious, its spread to all parts of the world is considered unstoppable. However, some non-medical interventions, such as quarantine, movement restrictions, and the banning of public gatherings, could potentially delay introduction of the virus to new areas.
Vaccines, if available early enough and in sufficient quantities, can reduce the high morbidity and mortality typically experienced during influenza pandemics.