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The role of epidemiology in infection control and the use of immunisation programs in preventing epidemics
The discipline of epidemiology is broadly defined as "the study of how disease is distributed in populations and the factors that influence or determine this distribution" (Gordis, 2009: 3). Among a range of core epidemiologic functions recognised (CDC, 2012), monitoring and surveillance as well as outbreak investigation are most immediately relevant to identifying and stopping the spread of infectious disease in a population.
Most countries perform routine monitoring and surveillance on a range of infectious diseases of concern to their respective jurisdiction. This allows health authorities to establish a baseline of disease occurrence. Based on this data, it is possible to subsequently discern sudden spikes or divergent trends and patterns in infectious disease incidence. In addition to cause of death which is routinely collected in most countries, many health authorities also maintain a list of notifiable diseases. In the UK, the list of reportable diseases and pathogenic agents maintained by Public Health England includes infectious diseases such as Tuberculosis and Viral Haemorrhagic Fevers, strains of influenza, vaccine-preventable diseases such as Whooping Cough or Measles, and food borne infectious diseases such as gastroenteritis caused by Salmonella or Listeria. (Public Health England, 2010) At the international level, the World Health Organization requires its members to report any "event that may constitute a public health emergency of international concern" (International Health Regulations, 2005). Cases of Smallpox, Poliomyelitis, Severe Acute Respiratory Syndrome (SARS), and new influenza strains are always notifiable. (WHO, undated) These international notification duties allow for the identification of trans-national patterns by collating data from national surveillance systems. Ideally, the system would enable authorities to anticipate and disrupt further cross-national spread by alerting countries to the necessity of tightened control at international borders or even by instituting more severe measures such a bans on air travel from and to affected countries.
As explained in the previous paragraph, data collected routinely over a period of time allows authorities to respond to increases in the incidence of a particular disease by taking measures to contain its spread. This may include an investigation into the origin of the outbreak, for instance the nature of the infectious agent or the vehicle. In other cases, the mode of transmission may need to be clarified. These tasks are part of the outbreak investigation. Several steps can be distinguished in the wake of a concerning notification or the determination of an unusual pattern. These include the use of descriptive epidemiology and analytical epidemiology, the subsequent implementation of control measures, as well as reporting to share experiences and new insights. (Reintjes and Zanuzdana, 2010)
In the case of an unusual disease such as the possibility of the recent Ebola outbreak in West Africa to result in isolated cases in Western Europe, it might not be necessary to engage in further epidemiological analysis once the diagnosis has been confirmed. Instead, control measures would be implemented immediately and might include ensuring best practice isolation of the patient and contact tracing to ensure that the infection does not spread further among a fully susceptible local population. Similarly, highly pathogenic diseases such as meningitis that tend to occur in clusters might prompt health authorities to close schools to disrupt the spread. In other types of outbreak investigations identifying the exact disease or exact strain of an infectious agent is the primary epidemiologic task. This might, for instance, be the case if clusters of relatively non-specific symptoms occur and need be confirmed as linked to one another and identified as either a known disease/infectious agent or be described and named. In the same vein, in food-borne infectious diseases, the infectious organism and vehicle of infection may have to be pinpointed by retrospectively tracing food intake, creating comparative tables, and calculating measures of association between possible exposures and outcome (CDC, 2012). Only then can targeted control measures such as pulling product lots from supermarket shelves and issuing a pubic warning be initiated.
Beyond identifying and controlling infectious disease outbreaks, monitoring and surveillance also plays a role in ensuring that primary prevention works as effectively as possible: collecting information on behavioural risk factors in cases such as sexually transmitted diseases can help identify groups that are most at risk and where Public Health interventions may yield the highest benefit. In another example, monitoring immunization coverage and analysing the effectiveness of vaccines over the life course may predict epidemics in the making if coverage is found decreasing or immunity appears to decline in older populations. In addition, the ability to anticipate the potential spread of disease with a reasonable degree of confidence hinges not only on good data collection. Advanced epidemiological methods such as mathematical modelling are equally instrumental in predicting possible outbreak patterns. Flu vaccines, for instance, need to be formulated long before the onset of the annual flu season. Against which particular strains the vaccines are to provide immunity can only be determined from past epidemiological data and modelling. (M'ikanatha et al., 2013) Mathematical models have also played a role in determining the most effective vaccine strategies, including target coverage and ideal ages and target groups, to eliminate the risk of epidemic outbreaks of infectious diseases (Gordis, 2009).
In addition to controlling outbreaks at the source and assuring the key protective strategies such as mass immunisation are effectively carried out, epidemiology is also a tool that allows comprehensive planning for potential epidemics. A scenario described in a research article by Ferguson and colleagues (2006) has as its premise a novel and therefore not immediately vaccine-preventable strain of influenza that has defied initial attempts at control and reached pandemic proportions. The large scale simulation of the theoretical epidemic assesses the potential of several intervention strategies to mitigate morbidity and mortality: international border and travel restrictions, a measure that is often demanded as a kneejerk reaction by policy-makers and citizens is found to have minimal impact, at best delaying spread by a few weeks even if generally adhered to (Ferguson et al., 2006). By contrast, interventions such as household quarantines or school closures that are aimed at interrupting contact between cases, potential carriers, and susceptible individuals are much more effective. . (Ferguson et al., 2006) Time sensitive antiviral treatment and post exposure prophylaxis using the same drugs are additional promising strategies identified. (Ferguson et al., 2006) The latter two potential interventions highlight the role of epidemiological risk assessment in translating anticipated spread of infectious disease into concrete emergency preparedness. For instance, both mass treatment and mass post exposure prophylaxis require advance stockpiling of antivirals. During the last H1N1 epidemic, public and political concern emerged over shortages of the antiviral drug oseltamivir (brand name Tamiflu). (De Clerq, 2006). However, advance stockpiling requires political support and significant resources at a time when governments are trying to reign in health spending and the threat is not immediate. Thus, epidemiologists also need to embrace the role of advocates and advisors that communicate scientific findings and evidence-based projections to decision-makers.
That being said, immunisation remains the most effective primary preventive strategies for the prevention and control of epidemics. As one of the most significant factors in the massive decline of morbidity and mortality form infectious disease in the Western world over the last century, vaccination accounts for an almost 100% reduction of morbidity from nine vaccine-preventable diseases such as Polio, Diphtheria, and Measles in the United States between 1900 to 1990. (CDC, 1999) Immunisation programmes are designed to reduce the incidence of particular infectious diseases by decreasing the number of susceptible individuals in a population. This is achieved by administering vaccines which stimulate the body's immune response. The production of specific antibodies allows the thus primed adaptive immune system to eliminate the full strength pathogen when an individual becomes subsequently exposed to it. The degree of coverage necessary to achieve so called herd immunity- the collective protection of a population even if not every single individual is immune- depends on the of the infectivity and pathogenicity of the respective infectious agent. (Nelson, 2014) Infectivity, in communicable diseases, measures the percentage of infections out of all individuals exposed, whereas pathogenicity is the percentage of infected individuals that progress to clinical disease. (Nelson, 2014). Sub-clinical or inapparent infections are important to take into account because, even though they show no signs and symptoms of disease, people may still be carriers capable of infecting others. Polio is an example of an infectious disease where most infections are inapparent, but individuals are infectious. (Nelson, 2014).
Gauging infectivity is crucial to estimating the level of coverage needed to reach community immunity. The so called basic reproductive rate is a numerical measure of the average number of secondary infections attributable to one single source of disease, e.g. one infected individual. The rate is calculated by taking into account the average number of contacts a case makes, the likelihood of transmission at each contact point, and the duration of infectiousness. (Kretzschmar and Wallinga, 2010). The higher the reproductive rate, i.e. the theoretical number of secondary cases, the higher the percentage of the population that needs to be immunised in order to prevent or interrupt an outbreak of epidemic proportions. For instance, smallpox, which was successfully eradicated in 1980 (World Health Organization, 2010), is estimated to have a basic reproduction number of around 5, requiring a coverage of only 80% of the population to achieve herd immunity. By contrast, the estimated reproduction number for Measles is around 20 and it is believed that immunisation coverage has to reach at least 96% for population immunity to be ensured. (Kretzschmar and Wallinga, 2010). Once the herd immunity threshold is reached, the remaining susceptible individuals are indirectly protected by the immunised majority around them: in theory, no pathogen should be able to reach them because nobody else is infected or an asymptomatic carrier. Even if the unlikely event of an infection among the unvaccinated eventuated, the chain of transmission should be immediately interrupted thanks to the immunised status of all potential secondary cases. Vaccinating primary contacts of isolated cases is also an important containment strategy where a cluster of non-immune individuals was exposed to an infected individual. Such scenarios may apply, for example, where groups of vaccine objectors or marginalized groups not caught by the regular immunisation drive are affected or an imported disease meets a generally susceptible population.
However, epidemic prevention does not stop with having reached vaccination targets. Instead, constant monitoring of current coverage is required and adaptations of the immunisation strategy may be needed to ensure that epidemics are reliably prevented. Recent trends underscore the enduring challenge of permanently keeping at bay even diseases that are officially considered eradicated or near eradication: in the United Kingdom, a marked spike in the number of confirmed measles cases has been observed in the last decade, with an increase from under 200 cases in 2001 to just over 2,000 cases in 2012. (Oxford Vaccine Group, undated) The underlying cause is evident from a comparison of case numbers with data from vaccine coverage monitoring: indeed, the number of children receiving the combination Measles vaccine decreased in the 2000s roughly in parallel with the increase in Measles incidence. (Oxford Vaccine Group, undated) Other countries have seen similar trends and have responded with measures intended to increase vaccine uptake: for instance, in Australia, the government recently decided to enact measures that would withhold child benefit payments to parents who refuse to have their children vaccinated. (Lusted and Greene, 2015)
In conclusion, epidemiology, and in particular routine monitoring and surveillance, is a potent tool that enables health authorities to anticipate, detect, and contain the spread of infectious disease. Over the last century, immunisation has proven itself as one of the key interventions to curb infectious disease morbidity and mortality. However, with vaccine-preventable diseases again on the rise in UK and other industrialised countries, epidemiologic monitoring of vaccine coverage and disease incidence remains critically important. Where vaccines are not available or vaccine-induced immunity is short-lived, an effective system to detect cases and contain outbreaks is even more instrumental to the effort of preventing infectious disease epidemics.
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