Introduction
The world has experienced three cases of pandemic influenza over the past century, all of which are thought to have originated in East Asia. More recently there has been a serious outbreak of severe acute respiratory syndrome (SARS) in South-East Asia. Because of very consistent action, the SARS outbreak was contained before it reached a pandemic scale but, nevertheless, it managed to affect people in countries across the globe in a matter of weeks. Now, only a couple of years after the SARS scare, the WHO has issued a new pandemic alert. This time the threat is known as avian influenza. Hence, it is fair to say that East Asia, and particularly South-East Asia, is a breeding ground for new types of severe human illness. This paper assesses the potential threat of pandemic influenza and the measures adopted for its prevention. We conclude by pointing out that even if avian influenza does not cause a human influenza pandemic, it is probably time to invest more resources in Asian (and African) countries if our aim is to prevent such pandemics from emerging in the future.
Understanding the Threat of a Pandemic Influenza
Although a novel influenza virus could emerge anywhere in the world at any time, scientists across the globe are particularly concerned about the possibility of the avian influenza virus (H5N1) giving rise to the next pandemic among human beings.
Outbreaks of H5N1 have occurred among poultry in several countries in Asia since 1997. Avian influenza, as its popular name indicates, mainly affects poultry and wild birds. As is the case of human influenza, it can appear in multiple forms, although there are two main variants: (1) the so-called ‘low pathogenic’; and (2) the ‘high pathogenic’. While the former causes mild symptoms and might even go undetected, the latter is far more virulent. In its highly pathogenic version the virus spreads extremely rapidly in poultry flocks and has a mortality rate approaching 100%, often within 48 hours. One of the virus subtypes causing avian influenza in its highly pathogenic form is known as H5N1, and is probably one of the most terrible bird-to-bird influenza viruses of all time. More than 150 million birds have died of the disease or been culled to control it. More recently the virus has spread to Europe and Africa, transmitted by migrating wild birds. At present, the virus is primarily transmitted from bird to bird and the only real victims of the avian influenzas are the world’s bird populations. However, on very rare occasions infected birds have passed the virus on to human beings, with very severe consequences. According to the WHO, from 2003 to February 2006 there have been 173 known cases of people who have contracted the illness after having been in close contact with birds infected with the H5N1 virus, of which 93 have died.[1]
Although the seriousness of the possibility of the virus’s bird-to-human diffusion should not be underestimated, neither should it be exaggerated. The fact that birds can pass on the virus means that the necessary precautions should be taken to avoid such a possibility. Simply put, the logic behind this is that the more infected birds there are, the higher the risk of bird-to-human transmission.
The second reason why human beings should beware an avian influenza pandemic among the world’s bird population is still only theoretical, but of far greater concern. Researchers say that if the H5N1 virus undergoes genetic changes and gains the ability to spread quickly among people, it could touch off a lethal worldwide epidemic, or pandemic. The question is how likely it is for the virus to undergo such a change, making human-to-human transmission possible.
It is impossible to know at this stage whether the H5N1 virus poses any real and significant threat to human health. However, the fact that all three pandemics in the 20th century –the Spanish flu in 1918, the Asian flu in 1958 and the Hong Kong flu in 1968– were the result of the genetic transformation of a bird flu virus, provides sufficient empirical evidence to suggest that it could indeed be very likely for the H5N1 virus to become a serious human health problem.
There are two possible outcomes as regards the H5N1 virus:
(1) The virus never mutates and, hence, there is no immediate threat of a pandemic.
(2) At some point in the future the virus acquires human-to-human transmission capacity, through either:
(a) An exchange with a human influenza virus.
(b) Gradual adaptation.
Both transformation scenarios gain credibility as the incidence of infection in birds increases. In addition, the likelihood of a genetic combination rises in line with the possibility of contact between infected birds and other species, which follows naturally from the first condition. If the virus ends up exchanging genetic codes with a human influenza virus, the human population is likely to have some resistance to the ‘new’ resulting virus, since many people might have been exposed to at least one of the new H and N virus subtypes. This is the reason given by some experts as to why the pandemics in 1957 and 1968 were far less severe in terms of mortality than the 1918 pandemic, which according to recent research was caused by a virus that mutated gradually by itself without the interference of a human influenza virus. The way the H5N1 virus is currently evolving does not rule out the risk of a severe pandemic similar to those of 1918 and 1919.
The Damage to Health and the Economy by a Pandemic Influenza
To deal effectively with the pandemic threat, the important question is: what is the potential damage that can be expected from an influenza pandemic?
Given the uncertainty regarding the virus’s capacity to inflict damage once –and if– it has acquired a human-to-human transmission capacity; it is extremely difficult to forecast the damage it can cause to society. The WHO, in assessing the potential lethal impact of a new pandemic, usually considers the death rates of the 1957 and 1968-69 pandemic as a point of reference. However, since (a) the H5N1 virus has a demonstrated capacity to kill people when they are infected by birds, (b) there is probably not going to be an effective vaccine until several months after the onset of the new pandemic and (c) there is no assurance that existing anti-viral drugs will be effective against the new virus, there is a possibility that a pandemic caused by a mutated H5N1 could be as deadly as –or even deadlier than– the virus that caused the 1918 pandemic.
However, it is not only mortality that is an issue when assessing the possible damage of a new pandemic. Other issues, such as the number of people falling ill and the number of people that require some form of medical assistance, are also important to consider. Studies aimed at assessing the general impact of an influenza pandemic typically cite 30% as a likely overall attack rate [2]. Of course, the number could be much higher. Attack rates of between 30% and 70% have been observed in particular countries in all three pandemics. It is important to note that the attack rate is likely to be high regardless of the virus’s severity in terms of mortality.
Furthermore, studies aimed at assessing the impact of pandemics on the healthcare system estimate that between 40% and 50% of those affected require some form of medical attention/consultation, and that 2% to 3% of those requiring medical attention also require hospitalisation.[3] [4] However, hospitalisation rates are dependent on the virus’s virulence and some experts do not rule out a ten-fold increase in the hospitalisation rate should a pandemic be caused by a virus with a similar lethal capacity to the 1918 strain.[5]