Source: Tammie L. Nelson, MPH, CPH, Epidemiologist at the Marion County Public Health Department, Ryan White/HIV Services Program
Published May 8, 2017
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An epidemiologist’s job is to study the cause, effect, distribution, pattern, and control of population health and disease. The methods used by an epidemiologist are important in HIV quality management and improvement; however, the terms used are often confusing. This article seeks to describe many of the ideas, measures, and vocabulary used by these professionals. This information should not be seen as a comprehensive list of epidemiological terms because it is far from comprehensive. Instead, this article touches upon those terms most frequently seen in epidemiological data or reports.
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Surveillance & Investigation
Medical surveillance occurs when a potentially exposed individual is monitored for disease occurrence in order to detect it early. Two related terms that are often confused are quarantine and isolation.
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Quarantine is medical surveillance of those who may have been exposed to determine if they are infected; whereas, isolation is used to separate those who are infected from those who are not to prevent secondary transmission.
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Public health surveillance is systematic collection, analysis, and dissemination of public health data to maintain vital records and immunization history, and to monitor patterns of injury and disease. One form of public health surveillance is sentinel surveillance, in which a network of pre-arranged clinicians (i.e., emergency departments, primary care providers) agrees to test for and report all diagnoses of a specific notifiable condition. This is often used by public health agencies to identify severity and trends during seasonal flu.
Injury or Disease
Infectious diseases are those caused by agents that replicate in their host and cause disease. The most common agents are bacterial, viral, or fungal; however, protozoa and helminths (worms) are also considered to be infectious agents. When an infection can pass from one person or animal to another, it is a communicable disease.
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Chronic diseases are those lasting at least three months (i.e., type 2 diabetes and obesity).
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The incubation (infectious) or latency (chronic) period of a disease is the period of time between exposure and disease onset when only subclinical changes occur.
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Pathogenicity is the proportion of individuals infected following exposure to an infectious agent. Similar measures are attack rate, the frequency of infection among those exposed, and secondary attack rate, the frequency of infection among those who were in contact with the infected.
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Virulence is the proportion of individuals who become severely ill or die after becoming infected.
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An epidemic, or outbreak, occurs when disease or injury occurs at a higher than expected rate in a given area and varies by condition. A flu outbreak may not be declared until thousands are diagnosed; whereas, one small pox diagnosis is an outbreak. A pandemic is an epidemic affecting a large proportion of the population over a wide geographic area.
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Active immunity is resistance to an infection developed in response to an antigen. This can follow vaccination against, or infection with, an infectious agent (e.g., measles). In comparison, passive immunity is acquired from an external source such as antibodies passed from mother to infant (transplacentally or in breast milk) or by administration of immune globulin.
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Herd immunity is group resistance to an infection based on the proportion of group members resistant to infection. For instance, pertussis vaccination among a high proportion of residents helps to prevent outbreaks of the disease among those too young to have been vaccinated.
Measures of Risk
A rate is the frequency of an event over time in a population such as the number of flu deaths per month. Either crude or adjusted rates can be reported. An adjusted rate has been standardized to eliminate the effect of a certain characteristic (e.g., age, race) when comparing different populations using rate ratios (RR). RR could be used to determine risk of HIV by gender.
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Relative risk (RR) is used to compare risk between groups. RR could be used to determine risk of HIV by gender. Often confused with RR, odds ratios (OR) are used to compare occurrence of injury or disease among a sample of the population with known exposure status.
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Incidence is the number of new diagnoses made during a set period of time in a population. Incidence is often reported as a rate of occurrences per 100,000 residents per year.
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Prevalence is the number of individuals in a population who have a particular injury or disease at a specific point in time. Prevalence is often expressed in terms of the number of affected per 100,000 residents.
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Morbidity is injury or disease in a population.
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Mortality is death in a population. Mortality rates can be calculated for a specific condition over time such as the number of lung cancer deaths per 100,000 Indiana residents during 2016. Mortality is usually reported per 100,000 in adults and per 1,000 live births in infants.
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Years of life lost (YLL) is a measure of years of life lost due to premature death in a population. It is calculated by summing differences between life expectancy and age of death of individuals who died earlier than expected due to injury or disease.
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Years lost due to disability (YLD) is a measure of the impact of disability in a population. It is calculated by summing the number of years lived with a disability, weighted by specific injury or disease.
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Disability-adjusted life year (DALY) is a measure of the overall burden of an injury or disease in a population in terms of years of healthy life lost. It is calculated as: YLL+YLD=DALY.
Statistical Terms
P-value is a number between 0 and 1 that indicates the strength of a test statistic. The smaller the P-value, the stronger the result. A test statistic is considered statistically significant if the P-value is less than a pre-determined significance level. A finding is usually considered to be significant if the P-value is ≤.05.
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A confidence interval (CI) is the range of values surrounding an estimate that fall within a specified probability of including the true value. The lower and upper bounds of the CI are called confidence limits. This statistic is often used to identify the range that falls within a 95% probability.
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Correlation is a statistical relationship or association between two or more events or characteristics. These relationships can be positive or negative. There is a strong, positive correlation between smoking and emphysema.
Conclusion
This article barely scratches the surface of the epidemiological vernacular. If you encounter a term that you need to understand better, you can turn to the Centers for Disease Control and Prevention (CDC) to look it up. The CDC’s epidemiological glossary can be found at http://bit.ly/2ove1rj.
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The Marion County Public Health Department’sRyan White/HIV Services Program (RWSP) oversees Ryan White Part A, C, and Minority AIDS Initiative (MAI) funding for the Indianapolis-Carmel Metropolitan Statistical Area (MSA). These programs serve the needs of persons newly infected or living with HIV and uninsured, underinsured, or medically underserved.