Core Functions of Epidemiology Essay
Define the core functions of epidemiology. Select one of the epidemiologic core functions and provide an example of how the core function might be demonstrated in clinical practice by a Masters prepared Registered Nurse. Can you relate this or one of the other functions to an example or content in your text readings?
Epidemiology is the study of how often diseases occur in different groups of people and why. Epidemiological information is used to plan and evaluate strategies to prevent illness and as a guide to the management of patients in whom disease has already developed.
Like the clinical findings and pathology, the epidemiology of a disease is an integral part of its basic description. The subject has its special techniques of data collection and interpretation, and its necessary jargon for technical terms. This short book aims to provide an ABC of the epidemiological approach, its terminology, and its methods. Our only assumption will be that readers already believe that epidemiological questions are worth answering. This introduction will indicate some of the distinctive characteristics of the epidemiological approach. Core Functions of Epidemiology Essay.
All findings must relate to a defined population
A key feature of epidemiology is the measurement of disease outcomes in relation to a population at risk. The population at risk is the group of people, healthy or sick, who would be counted as cases if they had the disease being studied. For example, if a general practitioner were measuring how often patients consult him about deafness, the population at risk would comprise those people on his list (and perhaps also of his partners) who might see him about a hearing problem if they had one. Patients who, though still on the list, had moved to another area would not consult that doctor. They would therefore not belong to the population at risk.
The importance of considering the population at risk is illustrated by two examples. In a study of accidents to patients in hospital it was noted that the largest number occurred among the elderly, and from this the authors concluded that “patients aged 60 and over are more prone to accidents.” Another study, based on a survey of hang gliding accidents, recommended that flying should be banned between 11 am and 3 pm, because this was the time when 73% of the accidents occurred. Each of these studies based conclusions on the same logical error, namely, the floating numerator: the number of cases was not related to the appropriate “at risk” population. Had this been done, the conclusions might have been different. Differing numbers of accidents to patients and to hang gliders must reflect, at least in part, differing numbers at risk. Epidemiological conclusions (on risk) cannot be drawn from purely clinical data (on the number of sick people seen).
Implicit in any epidemiological investigation is the notion of a target populationabout which conclusions are to be drawn. Occasionally measurements can be made on the full target population. In a study to evaluate the effectiveness of dust control measures in British coal mines, information was available on all incident (new) cases of coal workers’ pneumoconiosis throughout the country.
More often observations can only be made on a study sample, which is selected in some way from the target population. For example, a gastroenterologist wishing to draw general inferences about long term prognosis in patients with Crohn’s disease might extrapolate from the experience of cases encountered in his own clinical practice. The confidence that can be placed in conclusions drawn from samples depends in part on sample size. Small samples can be unrepresentative just by chance, and the scope for chance errors can be quantified statistically. More problematic are the errors that arise from the method by which the sample is chosen. A gastroenterologist who has a special interest in Crohn’s disease may be referred patients whose cases are unusual or difficult, the clinical course and complications of which are atypical of the disease more generally. Such systematic errors cannot usually be measured, and assessment therefore becomes a matter for subjective judgement.
Systematic sampling errors can be avoided by use of a random selection process in which each member of the target population has a known (non-zero) probability of being included in the study sample. Core Functions of Epidemiology Essay.However, this requires an enumeration or censusof all members of the target population, which may not be feasible.
Often the selection of a study sample is partially random. Within the target population an accessible subset, the study population, is defined. The study sample is then chosen at random from the study population. Thus the people examined are at two removes from the group with which the study is ultimately concerned:
Target population – study population – study sample
This approach is appropriate where a suitable study population can be identified but is larger than the investigation requires. For example, in a survey of back pain and its possible causes, the target population was all potential back pain sufferers. The study population was defined as all people aged 20-59 from eight communities, and a sample of subjects was then randomly selected for investigation from within this study population. With this design, inference from the study sample to the study population is free from systematic sampling error, but further extrapolation to the target population remains a matter of judgement.
The definition of a study population begins with some characteristic which all its members have in common. This may be geographical(“all UK residents in 1985” or “all residents in a specified health district”); occupational(“all employees of a factory,” “children attending a certain primary school”, “all welders in England and Wales”); based on special care(“patients on a GP’s list”, “residents in an old people’s home”); or diagnostic (“all people in Southampton who first had an epileptic fit during 1990-91”). Within this broad definition appropriate restrictions may be specified – for example in age range or sex.
Oriented to groups rather than individuals
Clinical observations determine decisions about individuals. Epidemiological observations may also guide decisions about individuals, but they relate primarily to groups of people. This fundamental difference in the purpose of measurements implies different demands on the quality of data. An inquiry into the validity of death certificates as an indicator of the frequency of oesophageal cancer produced the results shown in Table 1.1
Inaccuracy was alarming at the level of individual patients. Nevertheless, the false positive results balanced the false negatives so the clinicians’ total (53 + 21 = 74 cases) was about the same as the pathologists’ total (53 + 22 = 75 cases).Core Functions of Epidemiology Essay. Hence, in this instance, mortality statistics in the population seemed to be about right, despite the unreliability of individual death certificates. Conversely, it may not be too serious clinically if Dr. X systematically records blood pressure 10 mm Hg higher than his colleagues, because his management policy is (one hopes) adjusted accordingly. But choosing Dr. X as an observer in a population study of the frequency of hypertension would be unfortunate.
|Table 1.1 Cause of death diagnosed clinically compared with at necropsy|
|Diagnosis of oesophageal cancer||No.|
|Diagnosed by clinician and confirmed by pathologist||53|
|Diagnosed by clinician and not confirmed by pathologist||21|
|First diagnosed post mortem||22|
Conclusions are based on comparisons
Clues to aetiology come from comparing disease rates in groups with differing levels of exposure – for example, the incidence of congenital defects before and after a rubella epidemic or the rate of mesothelioma in people with or without exposure to asbestos. Clues will be missed, or false clues created, if comparisons are biased by unequal ascertainment of cases or exposure levels. Of course, if everyone is equally exposed there will not be any clues – epidemiology thrives on heterogeneity. If everyone smoked 20 cigarettes a day the link with lung cancer would have been undetectable. Lung cancer might then have been considered a “genetic disease”, because its distribution depended on susceptibility to the effects of smoking.
Identifying high risk and priority groups also rests on unbiased comparison of rates. The Decennial Occupational Supplement of the Registrar General of England and Wales(1970-2) suggested major differences between occupations in the proportion of men surviving to age 65:
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.Core Functions of Epidemiology Essay.
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. Core Functions of Epidemiology Essay. 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.Core Functions of Epidemiology Essay. 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. Core Functions of Epidemiology Essay.
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 . Core Functions of Epidemiology Essay. 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) Core Functions of Epidemiology Essay.
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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De Clercq, E. (2006) ‘Antiviral agents active against influenza A viruses’, Nature Reviews Drug Discovery, vol. 5, no. 12, pp. 1015-1025.
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Lusted, P. and Greene, A. (2015) Childcare rebates could be denied to anti-vaccination parents under new Federal Government laws. ABC News [Online], Available: http://www.abc.net.au/news/2015-04-12/parents-who-refuse-to-vaccinate-to-miss-out-on-childcare-rebates/6386448 [12 Feb 2015].
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