Principles of Healthcare Epidemiology

Principles of Healthcare Epidemiology

Frances J. Boly

Victoria J. Fraser

Jennie H. Kwon

Epidemiology is defined as the study of the factors determining the occurrence of disease in human populations. Epidemiology is an indispensable tool for characterizing infectious diseases in medical institutions, communities, countries, and worldwide. Epidemiologic methods are necessary to determine the exposure-disease relationship and modes of acquisition and transmission that are critical for treatment, control, and prevention of infectious disease. Clinicians, microbiologists, and other personnel involved in public health and disease prevention professions use epidemiologic methods for disease surveillance, outbreak investigations, infectious diseases outcome measurements, and observational studies to identify risk factors for various infectious diseases. Knowledge of these risk factors is essential for making decisions regarding further epidemiologic or microbiological investigations, directing research activities, implementing relevant prevention and control measures or interventions, and establishing public health policies. In the pharmaceutical and biomedical industries, the application of epidemiologic methods is integral to the investigation of product contamination, ascertainment and characterization of risk factors for contamination, and maintenance of quality assurance practices in the laboratory or manufacturing operations to ensure safe distribution of products.

The use of epidemiology and the use of statistical methods to analyze epidemiologic data grew out of attempts to understand, predict, and control the great epidemics of the past; the diseases associated with those early epidemics were largely infectious. The study and implementation of infection control and prevention practices and interventions in hospitals were developed to understand and control the acquisition of infectious diseases that were initially described due to Staphylococcus aureus outbreaks in hospitals in the 1950s.1,2 Thus, discussion of the principles of epidemiology begins with examples of methods that were first formalized in the study of transmissible microorganisms, many of which continue to cause problems today.

The term hospital epidemiology was coined by professionals in the United States,3 as was the recognition of the use of epidemiologic methods for the study and control of noninfectious diseases in hospitals.4 The term nosocomial infection has traditionally defined acute infections acquired in hospital inpatient settings.5 However, in the current era of managed care, healthcare systems in the United States have evolved from traditional acute care hospital inpatient settings to more integrated, extended care models that now encompass hospitals, outpatient clinics, ambulatory centers, long-term care facilities, and the community. As expected, infections may be acquired at any of these levels of care. For this reason, the term nosocomial infection has been replaced by healthcare-associated infection (HAI).6

The terms hospital epidemiology and infection control and prevention remain synonymous in the minds of many, and both the terms and their associated programs have grown in definition and scope over the past five decades. With advances in the medical field and technology, interest in infection control and prevention has broadened from an initial focus on puerperal sepsis and surgical site infections to full, scientifically tested programs of surveillance, prevention, and control of all types of HAIs. Hospital epidemiology programs were among the first hospital disciplines to demonstrate the utility of the scientific method and statistics to characterize and analyze infectious diseases data and use the results of these analyses to improve the quality of healthcare and patient outcomes. In the special environment of the acute care hospital, a natural repetition of
earlier studies of population-based infectious disease outbreaks provided the basis for epidemiologic investigations.

Surveillance data generated from epidemiologic studies may be used to determine the need for clinical or public health action; assess the effectiveness of prevention, intervention, or control programs, or diagnostic algorithms; or set priorities for rational or appropriate use of limited microbiology resources, planning, and research. An understanding of epidemiology is important to quantify and interpret microbiology and pharmaceutical data and to apply these data to clinical practice, quality assurance, and hypothesis generation during investigation of outbreaks and other adverse events. Epidemiology is also an important foundation for rational medication prescribing policies, antimicrobial stewardship, and other public health measures.

Data from epidemiologic and microbiological studies can inform diagnostic and therapeutic practice and indicate areas for allocation of scarce resources. For example, one of the perennial problems that clinicians and microbiologists face is how to differentiate between a true bloodstream infection (BSI) and blood culture contamination resulting from coagulase-negative Staphylococci, which are the most frequently isolated microorganisms in blood cultures.7,8,9 Blood culture contamination can occur during venipuncture with inadequate antisepsis of the skin, after the blood draw at the time of inoculation of blood into the culture bottle, or during processing of blood culture bottles in the microbiology laboratory. To make an informed decision about whether a positive blood culture is a true BSI vs contamination, clinicians and microbiologists need to be familiar with the epidemiology of BSIs in different clinical settings and be able to integrate these data with relevant clinical and microbiology information so that a decision can be made whether or not to initiate antimicrobial therapy.


Classic epidemiologic methods are essential for the study, characterization, and understanding of the various infections that occur in healthcare settings, communities, and broad geographic regions. Epidemiologic methods are used to determine the exposure-disease relationship in humans; establish the modes of acquisition, mechanisms of transmission, and spread; identify risk factors associated with infection and disease; characterize and relate causal factors to an infectious disease; determine or select appropriate methods of prevention and control; or guide rational application and practice of clinical microbiology methods. These epidemiologic methods were developed in an attempt to control common errors in observational studies
that occur when individuals study the association of one event (a risk or causal factor) with another later event (the outcome or disease).13,14,15,16

Epidemiologic study methods are grouped as either observational or experimental. Observational epidemiologic methods are further classified as either descriptive or analytic. Observational studies are conducted in natural, everyday community or clinical settings, where the investigators observe the appearance or an outcome but have no control over the environment or the exposure of people or product to a risk factor or suspected etiologic agent, a specific intervention or preventive measure, or a particular therapeutic regimen.

Descriptive Epidemiology

Observational descriptive studies establish the case definition of an infectious disease event by obtaining data for analysis from available primary (eg, medical records) or secondary (eg, infection control surveillance) sources. These data enable the characteristics of the population that has acquired the infection to be delineated according to (1) “person” (age, sex, race, marital status, personal habits, occupation, socioeconomic status, medical or surgical procedure or therapy, device use, underlying disease, or other exposures or events), (2) “place” (geographic occurrence of the health event or outbreak, medical or surgical service, place of acquisition of infection, or travel), and (3) “time” (preepidemic and postepidemic periods, seasonal variation, secular trends, or duration of stay in hospital). The information from descriptive studies might provide important clues regarding the risk factors associated with infection, and in each case, it is hoped that an analysis of the collected data might be used to generate hypotheses regarding the occurrence and distribution of disease or infection in the population(s) being studied.

Analytic Epidemiology

Observational analytic studies are designed to test hypotheses raised by the findings in descriptive investigations. The objectives of these studies are (1) to establish the cause and effects of infection in a population and (2) to determine why a population acquired a particular infection in the first place. The three most common types of observational analytic studies are cohort studies, case-control studies, and prevalence or cross-sectional studies.

Cohort Studies In cohort studies, hypotheses that have been generated from previous (descriptive) studies are tested in a new population. A population of individuals (a cohort) that is free of the infection or disease of interest is recruited for study. The presence or absence of the suspected (hypothesized) risk factors for the disease is recorded at the beginning of the study and throughout the observational period. All members of the cohort population (eg, all premature infants admitted to a neonatal intensive care unit [ICU] during a defined time period) are followed over time for evidence or appearance of the infection or disease and classified accordingly as exposed or unexposed to specific risk factors. If the observation period begins at the present time and continues into the future or until the appearance of disease, the study is called a prospective cohort study. If the population studied is one that in the past was apparently free of the markers of disease on examination of records or banked laboratory specimens, it may be chosen for study if data on exposure to the suspected risk factors for disease are also available. The population may be followed to the present or until the appearance of disease. This type of study, common in occupational epidemiology, is called a historical or retrospective cohort study.

A key requirement of a cohort study is that participants be reliably categorized into exposed and unexposed groups. Relative risk, that is, the ratio of the incidence of the outcome in the exposed group to the incidence in the unexposed group, is used to measure the strength of an association between exposures or risk factors and disease. Cohort studies have the advantage of enabling identification and direct measurement of risk factors associated with disease, determination of the incidence of infection and disease, and ascertainment of the temporal relationship between exposure and disease. In cohort studies, observational bias may be less of a limitation on the validity or results, since the information on the presence of risk factors is recorded before the outcome of disease is established. To ensure sufficient numbers for analysis, cohort studies require continual follow-up of large populations for long periods unless the disease under investigation is one of high incidence. Cohort studies are, in general, more expensive and time-consuming to conduct and are not suitable for the investigation of uncommon infections or conditions. However, they render the most convincing nonexperimental approach for establishing causation.

Case-Control Studies In a case-control study, individuals (cases) who are already infected, ill, or meet a given case definition are compared with a group of individuals (controls) who do not have the infection, disease, or other outcome of medical interest. In contrast to cohort studies, participants in a case-control study are selected by manifestation of symptoms and signs, laboratory parameters, or a specific condition, disease, or outcome. Thus, the search for exposure of case and control subjects to potential risk factors remains a retrospective one. For case-control studies, the measure of association between exposures or risk factors and health outcomes are expressed as an odds ratio, that is, the ratio of the odds of an exposure, event, or outcome occurring in a population to the odds in a control group, where the odds of an event is the ratio of the probability of it occurring to the probability of it not occurring.

The presence of significant differences in the exposure to risk factors among cases vs control subjects suggests an etiologic (causal) association between those factors and the infection or disease defined by cases. Case-control methods are useful for studying infections, events, or outcomes likely associated with multiple risk factors or low incidence rates; for investigating situations in which there is a long lag-time between exposure and outcome of interest; and for establishing etiologic associates or causation of a disease, infection, or other outcome when there is no existing information about the cause or source. In an attempt to reduce bias, control subjects might be selected from individuals matched with cases for selected characteristics, such as age, gender, socioeconomic status, or other variables not suspected or under investigation as risk factors. Compared with cohort studies, case-control studies may be conducted in relatively shorter time, are
relatively less expensive, or may require a smaller samples size to execute. Limitations of case-control studies include selection bias in choosing cases and control subjects; recall bias in which study subjects might have difficulty in remembering possible exposures; incomplete information on specific exposures; or risk factor data may be difficult to find (or remember). Case-control studies are not used to measure incidence or prevalence rates and, generally, are not capable of establishing temporal relationships between an exposure and outcome.

Prevalence or Cross-sectional Studies In prevalence studies, the presence of putative risk factors and the disease under investigation is recorded in a survey of a study population at a specific point in time or within a (short) time period. The rates of disease among those with and without the suspected risk factors are compared. Thus, cross-sectional studies can establish association but not causation for suspected risk factors. Prevalence studies are relatively inexpensive and can be carried out rapidly if well planned. However, they do not allow the ascertainment of risk factors at the beginning of disease nor do they enable one to establish a temporal sequence of risk factors preceding the infection or other outcome of interest. Point prevalence, period prevalence, and seroprevalence surveys are examples of cross-sectional studies.

Experimental Epidemiology

In experimental studies, the investigator compares an exposure of individuals in a population to a suspected causal factor, a prevention measure, a therapeutic regimen, or some other specific intervention. These exposure modalities are randomly allocated to comparable groups, thereby minimizing confounding factors. Both the exposed and unexposed groups are monitored thereafter for specific outcomes (eg, appearance of infection or disease, evidence of effective prevention or control of the disease, or cure). Experimental studies often are used to evaluate antimicrobial or vaccine treatment regimens and are generally more expensive to conduct than observational studies. Within healthcare settings, studies that examine restriction of certain antimicrobials or promotion of use of alternative antimicrobials for the control of antimicrobial resistance could be considered under the category of experimental. For ethical reasons, it is rarely possible to expose human populations to potential pathogens or to withhold a preventive measure that could potentially be beneficial to patients. Unfortunately, many animal hosts are not naturally susceptible to many agents of human disease. Thus, one has to be careful when extrapolating epidemiologic findings in animal experimental studies to the control of infections in human subjects.

Quasi-experimental studies: This type of study shares the design characteristics of experimental studies but lacks random assignments of study subjects. Quasi-experimental studies are useful where randomization is impossible, impractical, or unethical. The main drawbacks of quasi-experimental studies are their inability to eliminate confounding bias or establish causal relationships. The most common quasi-experimental design is the beforeafter study in which there is a baseline measurement of the outcome (prospectively or retrospectively) followed by an intervention with measurement of the outcome and an assessment whether the intervention altered the outcome.


The epidemiology of infectious disease presents two processes for discussion: (1) the epidemiology of the determinants leading to infections in hosts and (2) the epidemiology of the appearance and extent of disease related to the infection in those hosts. It is common to discuss health and disease as the result of a series of complex interactions between an infectious agent, the host that is the target of the infectious agent’s actions, and the mutual environment in which the host and infectious agent exist. In studies of HAIs, the infectious agents are the microorganisms associated
with the infections, the hosts are the patients or healthcare personnel (HCP), and the common environment is the acute care hospital, ICU, outpatient, home, or other healthcare setting.

FIGURE 1-1 The “epidemiologic triad” of infectious disease summarizes the factors that influence an infection and the measures you might take to combat the infection. (From Johnson Y, Kaneene J. Epidemiology: From Recognition to Results. 2018:1-30. Ref.21)

The interactions determining the probability of a microbiologic agent causing infection in a host may be simply presented by an equation of infection:

where Ip is the probability of infection, D is the dose (number of microorganisms) transmitted to the host, S is the receptive host site of contact with the agent, T is the time of contact (sufficient for attachment and multiplication or not), and V represents virulence, the intrinsic characteristics of microorganism that allow it to infect. The denominator in the equation (Hd′) represents the force of the combined host defenses attempting to prevent the infection.

Any reduction in host defenses (represented by the denominator) in such an equation allows infection to take place with a similar reduction in one or more of the infectious agent factors in the numerator. Infection may take place with a smaller dose of microorganisms. Infection may take place at an unusual site. The contact time for a microorganism to attach to an appropriate surface may be shorter, or infection may take place with an agent of lesser virulence, one that does not cause infection in a normal host. Reductions in the host defense characteristics, represented by the denominator, and the reduction of requirements to infect for the agent are typical of the interactions that facilitate opportunistic infections in compromised hosts, increasing numbers of whom receive medical and surgical care in modern hospitals. In the equation of infection, the environment might be considered the background on which the infectious agent-host interaction takes place.20 A number of additional models of the interaction of agent, host, and environment have been suggested to help understand these processes. The model in Figure 1-1, the triangle model, attempts to visualize the interplay between the three components of the host pathogen and environment.


There are often multifactorial causes that lead to outcome events (infection or disease). For some infectious diseases, a single unique factor or agent is necessary and sufficient for the disease to appear. This is exemplified by measles or rabies. It is only necessary for the host to be exposed to and infected by an agent (the measles virus or rabies virus) for that disease to develop. For other infectious diseases, the single factor of infectivity of the agent is necessary but not sufficient to cause disease in the host. Mycobacterium tuberculosis (M tuberculosis), polio virus, hepatitis A, and many other agents necessary for specific disease in a human host can infect hosts without causing disease in many cases. Within the hospital setting, exposure to a specific microorganism or colonization of an inpatient with an agent, such as vancomycin-resistant enterococci (VRE) or S aureus, may be necessary but not sufficient to generate disease. For VRE and S aureus, disease generally develops as a result of complex interactions between contributing factors, such as age, state of debilitation, immune or nutritional status, device use, invasive procedures, antimicrobial usage, or susceptibility of the microorganism to available antimicrobials. The presence of the infection in these cases is not always sufficient to produce disease in the host without the contribution of additional host or environmental factors.


The agents causing healthcare-associated infectious diseases are diverse microorganism ranging in size and
complexity from viruses and bacteria to protozoa and helminths. Bacteria, fungi, and certain viruses have been the most recognized and studied causes of HAIs.22 For transmission to take place, microorganisms must remain viable in the environment until contact with the host has been sufficient to allow infection. Reservoirs that allow the agent to survive or multiply may be animate, as exemplified by HCP carriage of S aureus in the anterior nares or throat,23 or inanimate as occurs in the environment, as demonstrated by Pseudomonas spp. colonization of sink areas, Legionella in hot and cold water supply systems,24 C difficile spores on computer keyboards,25 or Serratia marcescens growing in contaminated soap or hand lotion preparations.26

Certain intrinsic and genetically determined properties of microorganisms are important for their survival in the environment. Intrinsic and genetic properties that enhance survival include the ability to resist the effects of heat, drying, ultraviolet light, or chemical agents, especially antimicrobials, and the ability to independently multiply in the environment or to develop and multiply within another host or vector. Intrinsic infectious agent factors important to the production of disease include infectivity; pathogenicity; virulence; the infecting dose; the infectious agent’s ability to produce toxins; its immunogenicity and ability to resist or overcome the human immune defense system; its ability to replicate only in certain types of cells, tissues, or hosts (vectors); its ability to persist or cause chronic infection; and its interaction with other host mechanisms, including the ability to cause immunosuppression (eg, HIV).

Once transferred to a host surface, infectious agents may multiply and colonize without invading or evoking a measurable host immune response.27 The presence of an infectious agent at surface sites in the host does not define the presence of an infection. Nonetheless, colonized patients may act as the reservoir and/or source of transmission to other patients or HCP.28

If infection takes place, a measurable immune response will develop in most hosts even if the infection is subclinical. The likelihood of infectious agents causing infections is increased in nonimmune hosts, and infections are most likely to occur in nonimmune, immunocompromised hosts. A microorganism’s ability to infect another host vector (eg, yellow fever virus in mosquitoes) or another nonhuman reservoir (eg, yellow fever virus in monkeys) is important in the epidemiology of certain infectious diseases globally but plays little role in the epidemiology of HAIs where vector-borne infectious diseases are uncommon and where hospital settings have sealed windows/doors and controlled heating, ventilation, and air conditioning (HVAC) systems in the United States.


Infection depends on exposure of a susceptible host to an infecting agent. Exposure of the susceptible host to such agents is influenced by age, behavior, family associations, occupation, socioeconomic level, travel, avocation, access to preventive healthcare, vaccination status, or hospitalization. Whether or not disease takes place in the infected host and the severity of disease when it occurs depends not only on the intrinsic virulence factors of the infectious agent but also more importantly on the pathogenicity of the infectious agent and host pathogen interactions. The host immune defenses exist to prevent infection. Thus, any reduction in host defenses may allow infection to take place with a smaller dose of microorganisms or at a body site that is not usually susceptible to infection. A combination of reductions in host defense characteristics and the requirements for an agent to cause infection are typical of the interactions that allow acquisition of opportunistic infections in immunocompromised patients.29,30

Host factors important to the development and severity of infection or disease may be categorized as intrinsic or extrinsic. Intrinsic factors include the person’s age at infection; birth weight; sex; race; nutritional status31; comorbid conditions (including anatomic anomalies) and diseases; genetically determined immune function; immunosuppression associated with other infections, diseases, or therapy; vaccination or immunization status; previous exposure or infection with an infectious agent or similar agents; and the psychological state of the host.32,33 Colonization of the upper and lower respiratory tracts is more likely when the severity of illness increases in critically ill patients. This, along with other host impairments (eg, reduced mucociliary clearance or changes in systemic (pH), allows colonization to progress to invasive infection. Moreover, other clinical conditions may lead to an alteration in epithelial cell surface susceptibility to binding by bacteria, leading to enhanced colonization.27 Extrinsic factors include invasive medical or surgical procedures, the presence of medical devices, such as intravenous catheters or mechanical ventilators, specific behaviors such as sexual practices and contraception, duration of antimicrobial therapy and hospitalization, and exposure to hospital personnel.


The environment serves as the background on which infectious agent-host interactions take place. Environmental factors can positively or negatively influence the spread of infection. Environmental factors that influence the spread of infection include (1) physical factors such as climate conditions of heat, cold, humidity, seasons, and the built environment (eg, ICUs, outpatient clinics, long-term care [LTC] facilities, or water reservoirs); (2) biologic factors (eg, the presence or absence of intermediary hosts such as insect or snail vectors, presence of biofilms in sinks or devices); and (3) social factors (eg, socioeconomic status, types of food and methods of preparation, and availability of adequate housing, potable water, adequate waste disposal, and healthcare amenities).34 These environmental factors influence both the survival and the multiplication of infectious disease agents in their reservoirs and the behavior of the host in housing, occupation, and recreation that relate to exposure to pathogens. Food- and waterborne diseases flourish in warmer months due to better incubation temperatures for microorganism multiplication and the recreational exposures of the host, whereas respiratory agents appear to benefit from increased opportunities for airborne and droplet transmission in the reduced humidity and closer living environments during the winter. In U.S. hospitals, the frequency of hospital-acquired Acinetobacter spp. infections is increasing in ICUs. The seasonal variation in the incidence of Acinetobacter spp. infections is thought to be due to changes in climate since the number of Acinetobacter spp. infections identified increases in the summer months.35

Within healthcare settings, the components of the agent, host, and environment triad interact in a variety of ways to produce HAIs. For example, ICUs are now considered the areas with the highest risk for the transmission of healthcare-associated pathogens in U.S. hospitals.36,37 Moreover, methicillin-resistant S aureus (MRSA), VRE, extended spectrum β-lactamase (ESBL) producing E coli and Klebsiella, and β-lactam-resistant Pseudomonas aeruginosa are endemic in many ICUs in these hospitals.36,37 The emergence of vancomycin-resistant S aureus (VRSA) in U.S. institutions in 2002 highlighted the unwelcome but inevitable reality that VRSA may become endemic in acute care settings.38,39 A complex interaction of contributory factors, such as inadequate hand hygiene and suboptimal infection control practices among HCP, fluctuating staffing levels, an unexpected increase in patient census relative to staffing levels in the ICU, or an unprecedented increase in the number of severely ill patients with multiple invasive devices, could all contribute to the acquisition of hospital or long-term care facility infections caused by one of these endemic microorganisms.40,41,42,43,44 Adding to the complexity of the process would be the unquantifiable mechanism of transmission of the agent from the host to HCP, HCPto-HCP, and host-to-environment. Thus, acceptable measures for the prevention and control of HAIs dictate that healthcare epidemiologists study and analyze the interrelationships among all components of the triad of agent, host, and environment.36,37

It is well known that the social environment is important in influencing personal behavior that affects the direct transmission of agents, such as HIV transmission to infants from nursing mothers via breast milk in regions of high HIV endemicity, Gram-negative microorganisms transmitted via artificial nails worn by HCP in U.S. ICUs,43,45 and increased risk of sexually transmitted infections among vulnerable populations who cannot always use condoms or other barrier protection or do not have access to preexposure prophylaxis. What must be understood to be equally relevant is the impact of other factors in the social environment, such as the distribution of and access to medical resources; the use of preventive services46,47,48,49,50; the enforcement of codes in food preparation, infection prevention practices, or occupational health practices; the extent of acceptance of breast-feeding for children51,52,53; and the acceptance of advice on the appropriate use of antimicrobials.54,55,56,57,58 Also, there must be an appreciation by patients, relatives, and healthcare providers alike that at-risk patients (eg, those born very prematurely, the very elderly, or those with premorbid end-stage cardiac or pulmonary disease) who have numerous indwelling catheters or invasive devices, or who have undergone multiple invasive or surgical procedures will be particularly susceptible to HAIs that are likely nonpreventable.

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Jun 8, 2021 | Posted by in INFECTIOUS DISEASE | Comments Off on Principles of Healthcare Epidemiology
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