On the Problems and Strategies of Fighting Pandemics in Light of the 2005 Avian Flu

Theme: This analysis focuses on the importance of implementing a strategy for combating contagious diseases that takes into consideration the patterns of human interaction when trying to fight a pandemic in modern society.

Summary: This analysis discusses the importance of taking into consideration the social contact patterns of individuals in a region, both in deciding which vaccination strategy to use and when trying to limit contacts in the region in the event of a large pandemic. The analysis gives a brief introduction to recent outbreaks of new contagious diseases such as AIDS and SARS. We conclude that taking into consideration social interaction patterns –targeting long-distance travellers, individuals with many daily contacts and individuals with an increased risk of coming into contact with infectious individuals, such as healthcare workers– is more effective when designing a vaccination strategy to impede the further diffusion of a disease than a more random approach, or an approach that focuses on the most vulnerable people.

Analysis: During the last 30 years we have repeatedly been reminded of the threat that contagious disease pose for modern society. The first HIV/AIDS case was discovered in 1981 and to this day it has cost approximately 30 million lives. Around 40 million people are living with HIV/AIDS, with an estimated 5 million new cases in 2004 alone. As HIV/AIDS is particularly common in poor and underdeveloped areas of the world, the hope of survival is minuscule despite the advances in medical treatment. Three years ago, in early 2003, SARS put the western world on alert, with early symptoms similar to those of common flu, as it spread from South East Asia to Europe and North America. The disease emerged in China in December 2002, and in 2003 774 people the world over died from it. A total of 8,098 became ill the same year, amounting to approximately a 10% death rate. In the autumn of 2005 it is now avian (bird) influenza that is attracting our attention, and this time the health authorities around the world are treating the threat of a deadly pandemic as something highly possible.

What is the difference between an epidemic and a pandemic, you might ask. An epidemic is the outbreak of a disease. For instance, most countries in the western world have a low prevalence (the relative number of cases in the population) of gonorrhoea. The prevalence is strongly related to the incidence (the relative number of cases in the population that become ill in a given period of time) and as long as the prevalence is low and the incidence is constant there is no epidemic. The word epidemic itself is formed from the Greek words for “among” (epi) and “people” (demos), and it should be noted that in itself an epidemic does not necessarily have to have any lethal consequences. We usually reserve the concept epidemic for a large, non-local outbreak, and for “local epidemics”, for instance when limited to one city, we prefer to talk about an outbreak. However, when we deal with really large and potentially all-encompassing epidemics we talk about a pandemic. The word pandemic is also from the Greek and is composed of the words for “all” (pan) and “people” (demos).

As suggested by its name, bird flu attacks and kills birds. The disease passes from one bird to another, and as many birds seasonally migrate over long distances, a small outbreak in one corner of the world will rapidly spread as birds migrate in the autumn and spring. The risk of being infected by bird flu is minimal at most for most people. There are reported cases of the virus transferring from bird to man in Asia, and although people that live or work close to poultry are at risk, the chief consequences of bird flu are primarily economic. As domestic and wild birds are hard to isolate from each other, the effect is particularly dramatic for poor self-subsistent households and for poultry farmers. The agricultural consequences are real and sometimes dramatic for every country that is hit by bird flu. But what is really causing the alert surrounding bird flu, is the risk that the virus (known as H5N1) should mutate and begin to spread from human being to human being. The risk is very small, although such mutations are by no means rare. One can assume that the evolutionary pressure on viruses actually drives such a development. For example, it is believed that the Corona-virus that causes SARS evolved from within a wild-life population in China.

Recall that, simply put, a pandemic denotes a global epidemic. Throughout the history of mankind, a number of pandemics have had a considerable impact on society. To Europeans, the Black Death is perhaps the most famous example of an epidemic that had a dramatic and far-reaching influence on the future of Europe. The Cholera epidemics that hit Europe in the 19th and early 20th century are also well-known examples of pandemics. A current Cholera epidemic in Africa has lasted for over 30 years, indicating that epidemics can be both short- and long-term experiences, depending on the disease. Indeed, the 20th century saw a number of pandemics, and some even suggest that a number of new diseases that evolved in the 20th century suggest we can talk about an epidemic of epidemics. From the perspective of the current bird flu scare, Spanish flu is particularly interesting since researchers now believe that it evolved from a bird virus similar to the bird flu that is currently spreading across the world. There are good reasons to believe that a mutated H5N1 virus would spread very rapidly and with severe effects.

Faced with a worst-case scenario, the question is; what can society do to protect itself and its populations from a flu pandemic? In order to suggest an answer to this question, let us briefly consider the way in which a contagious disease spreads in a population. We completely disregard the viral microbiological aspects here, as this is beyond our competence, but concentrate on the role of social interaction in disease transfer. Our basic point of departure is that the importance of the actual contact pattern is a function of the infectiousness of the disease. Infections spread from person to person, either directly or by air, or through food and water. The more intimate the contact has to be for a transmission to take place, the more important the contact pattern. Figure 1 shows some known infectious diseases on a one-dimensional scale, ranging from high infectiousness and non-intimate transmission on the left hand side to low infectiousness and intimate transmission on the right hand side. For the forthcoming discussion it is enough to state that the lower the infectiousness the more attention has to be paid to social contact patterns. On the other hand, the higher the infectiousness, the greater the threat, and the higher the cost of misrepresenting the importance of social interaction at all levels. Let us focus on the three cases we have already introduced: HIV, SARS and Influenza. HIV/AIDS spreads mainly through intimate sexual contact. Therefore, in principle, it is an easily contained disease. If people who have already contracted the disease refrain from sexual contacts, the incidence (the relative number of cases in the population that fall ill in a given time) of HIV/AIDS should decline rapidly. In fact, it would be quite enough if infected persons and persons in the risk group were to use condoms consistently. SARS, on the other hand, spreads through close inter-personal contact. Flu, in turn, spreads through droplets and is thus more contagious than SARS because inter-personal contacts need not be very close and certainly not intimate for the virus to transfer from one person to another.

Figure 1. Some well known infectious diseases arranged according to the approximate degree of intimacy in transmission

Conclusions: There are two basic avenues that society can take to mitigate the effects of a flu pandemic: (a) to vaccinate the population; or (b) to limit the contact surface within the population. As already indicated, limiting contact between people is very efficient when dealing will not highly infectious diseases, but we argue that it is also worth considering strategies in that direction when facing very highly infectious diseases if there are insufficient vaccines available.

The first thing to consider with respect to vaccination is that there is not, and cannot exist, a vaccination for a virus that has not yet evolved. Thus, the vaccination strategy only becomes feasible after the virus has been identified and enough vaccines have been manufactured. If there is no influenza vaccine available, antivirals, such as Tamiflu, can be used. Tamiflu is commonly used to treat people with early influenza symptoms. But because the person who is treated with Tamiflu is less likely to contract influenza, it can also be prescribed as a preventive measure to reduce the chance of flu spreading in the population. The real risk with applying such measures too early is that the virus can evolve immunity against the antiviral, thereby rendering it useless. Once the virus is identified, however, we can assume that a vaccine will be produced as rapidly as possible. But we can also safely assume that there will not be enough vaccine ‘for everyone’, and thus that the vaccine will be a scarce resource.

For a vaccination strategy to be successful the essential thing is to keep the population below the epidemic threshold. The epidemic threshold defines the ‘either-or’ transition from no epidemic into an epidemic. A parallel is the boiling point of water; as long as the required temperature is not reached, the water does not boil. Similarly, vaccine is used to keep the epidemic below this point.

When vaccine is a scarce resource the authorities are faced with the dilemma of a trade-off between saving lives and maximising societal utility. It is well known that infants, the elderly, and people in a frail state of health are most vulnerable to the influenza virus, with death being a likely outcome. Thus, if saving lives is the penultimate goal, vaccination should be given first to individuals in those groups. On the other hand, persons in those groups are probably not the primary spreaders of the virus, nor do they occupy key positions in society. The functions of society would be better served, and the scope of the epidemic reduced, if a more selective vaccination strategy were to be employed. Such a ‘vaccination hierarchy’ suggests that vaccination should be given to (1) first responders and key personnel, (2) large workplaces and schools and (3) long-distance travellers.

First responders in this context are first and foremost hospital personnel and other healthcare workers. Some other key functions are included here as well, such as ambulance personnel, policemen, and fire fighters, as well as people in jobs supplying crucial infrastructure such as electricity, water, drainage, etc. To vaccinate those involved in large workplaces and schools offers both advantages of economies of scale and strategic intervention in epidemic dynamics. First, at a large-scale workplace or a school you can distribute vaccine to many people at one place and at one time. Secondly, these places are natural hotbeds for the transmission of influenza and the offensive vaccination of such places implies a relatively significant reduction in the communication of the virus. Generally it is a good strategy to vaccinate people with many contacts if they can be identified. Examples include mundane positions such as bus drivers and other personnel in public transport as well as supermarket cashiers. Long-distance travellers, such as lorry drivers, can rapidly transmit the disease over long distances within a country as well as take the virus from one country to another.

The fact that one avenue for slowing down and possibly isolating the virus involves dealing with contact surfaces is evident in what we have said about vaccination strategies. First responders are critical targets for vaccination because they are exposed to the virus at a very early stage. Therefore, vaccination of first responders implies that the contact surface between infected and potentially infected is reduced. We will suggest two other approaches in the battle against pandemics that deal explicitly with the manipulation of contact surfaces: (a) distance and (b) large-scale areas for social interaction. It is important to bear in mind that some routine strategies for tracking the avenues of disease transmission in a population are less relevant when dealing with influenza. Contact tracing, for instance, is only feasible and efficient if there is a limited number of infected persons in the region and the rate in which infected persons enter it is low. Outbreak isolation, which was a reasonably successful strategy against SARS for instance, is also less likely to be successful with a disease that is highly transmissible and when a lot of import cases can be expected.

As noted above, long-distance travellers are dangerous because they transfer infectious diseases relatively rapidly from one geographical area to another, intra- and internationally. Therefore reducing travelling distances is one way to also slow down the pandemic. Limiting the maximum distance for commuting would buy time because the speed of the epidemic would decrease. This would be desirable in advance of the vaccine being produced.

Finally, we noted previously that large workplaces are good targets for vaccination campaigns partly because they contain a lot of people, and that they are therefore ‘hotbeds’ for disease transmission. This suggests that the scope of a pandemic can be reduced by limiting the contacts within large workplaces or even closing them down. The key here is to target people that have many contacts, as in the case with vaccination.

Some other important implications from this are also worth noting. In a flu pandemic, children should be kept at home and not sent to school. On the other hand they should not be sent away to relations, and especially not to elderly relations. In addition, influenza care wards need to be isolated from all other healthcare facilities. SARS is the recent most frightening illustration of the fact that one of the most dangerous places to be during an epidemic is the hospital. This is even truer in the case of flu unless influenza care is isolated.

Christofer Edling and Fredrik Liljeros
Professors of Sociology at Stockholm University