References:
- Community and Public Health Nursing, 2nd Philippine Edition, ISBN 978-981-48-6503-6, by Earl Francis R. Sumile
Epidemiological Studies
Descriptive Epidemiology focuses on describing disease distribution by characteristics relating to time, place, and people. This could be done through review of records, routine data collection, or ecological surveys. It examines factors like age, sex, education, socioeconomic status, or availability of health services. The primary considerations for descriptive epidemiology are frequency and pattern. Frequency evaluates the rate of occurrence, and pattern helps analytical epidemiologists suggest risk factors.
- Describing the person means looking into inherent characteristics (sex, age), acquired characteristics (marital status, immune status), activities (occupation, tobacco use, alcohol, exercise), and other conditions (socio-economic status, access to care).
- Time could be about secular trends, or seasonality or days of the week of the health related event.
- Place means geographic location, urban or rural, and others that give orientation to space.
Analytic Epidemiology aims to understand the quality and amount of influence that determinants have on the occurrence of disease or health-related event. This is usually done by group comparisons. The epidemiologist looks for factors that might have preceded the disease or event.
- Cross-sectional Study: an examination of the relationship between health problems and other variables of interest as they exist in a defined population at one particular time. Exposure and outcome are measured at the same time. This creates temporal ambiguity; hence, causal relationship cannot be established. This is used to get baseline data, determine the magnitude of the health problem or disease, or evaluate medical care or health service delivery.
- Cohort Study: a longitudinal study. Subjects are selected based on exposure status, being disease-free at the beginning of the study. The cohort is followed through time to determine the later outcome status.
- Case-control Study: a retrospective study. Subjects are selected based on disease status, then compared to subjects without such a disease status. The past exposures or risk factors of these two groups are then compared.
Experimental Epidemiology uses an experimental design to confirm a causal relationship suggested by observational studies. Experimental epidemiologic studies control factors to reveal unbiased relationships between exposures and outcomes, e.g., exposure to smoking relating to an outcome measure of hypertension. These test a hypothesis about a health problem or treatment.
- Randomized Control Trials (RCT) are experiments with patients as subjects to evaluate a potential cure for a disease and prevent death and disability. These are the most stringent way of determining the cause-effect relationship
- Field Trials: experiments done in the field involving subjects free of the disease or health condition, but at a high risk of contracting them. These tests are done to determine if interventions are effective in reducing the risk of developing the disease or condition. These are more expensive due to the locality and number of subjects.
- Community Trials: an extension of field trials, involving the whole community as the unit of assignment. For example, water treatment is done at the community level through water supplies. This intervention can be evaluated by involving entire communities who were assigned the water treatment.
Sources of Data
Epidemiologists collect and analyze data in doing the different types of epidemiologic studies. These may be primary or secondary:
- Primary data: original data collected for a specific purpose by a researcher. An example of this is with water samples and interviews collected during a cholera outbreak.
- Secondary data: data already collected by other individuals and/or institutions for some specific purpose. Examples of these include population census, birth and death certificates, disease registries, patients’ medical records, health insurance claims, health surveys, etc. Many of these are from government agencies such as the Department of Health.
- Population census: the total process of collecting, compiling, evaluating, analyzing, and publishing or otherwise disseminating demographic, economic and social data pertaining, at a specified time, to all persons in a country or in a well delimited part of a country. In the Philippines, this is done every five years (previously 10 years, amended by E.O. 352) and the results are used to allocate congressional seats and government program funding. As of writing, the next census is scheduled for the year 2020.
- Civil Registration and Vital Statistics (CRVS) are “those systems which produce births, deaths and causes of death information, which are of fundamental strategic importance for countries’ development process and governance” (Mikkelsen, 2012). It is important that CRVS involve continuous, timely, accurate, consistent and complete registration and reporting of all human beings’ life vital events such as birth, marriage and death in a given population using globally accepted standards. This helps in (1) defining patterns, trends, and impact inclusive of the causes of mortality; (2) understanding emerging health issues and challenges; (3) defining appropriate and relevant health-sector reforms, poverty reduction and development efforts; and (4) defining baseline levels and monitor and track progress towards both national and international development goals and assess effectiveness of health programs.
- The population growth was 1.5% by the year 2022, and is generally decreasing.
- Disease or Patient Registries: collections of secondary data related to patients with a specific diagnosis, condition, or procedure. They are “organized systems that use observational study methods to collect uniform data (clinical and other) to evaluate specified outcomes for a population defined by a particular disease, condition, or exposure and that serve predetermined scientific, clinical or policy purposes” (Agency for Healthcare Research and Quality, 2019)
- Disease surveillance and notification may be on an individual, local, national, and international level. It is used for strategizing for the prevention and control of diseases most especially epidemic prone diseases. Disease surveillance is the continuous scrutiny of occurrence of disease and health-related events to enable prompt intervention for the control of diseases. These are primarily developed for monitoring diseases of high burden, detecting disease outbreaks that could escalate into epidemic proportions, and monitoring progress toward attainment of targets for the control, elimination, or eradication of a specific disease. It may be passive (reports are received by the health agency) or active (the agency actively contacts the population to obtain information). Disease notification is an integral part of disease surveillance, involving the official and timely reporting of the occurrence of specific diseases and conditions to designated public health authorities by clinicians and other health personnel for action using designated reporting tools. There are four critical diseases which are deemed always to be unusual or unexpected and may have serious public health impact, and hence must be notified to WHO in all circumstances: smallpox, poliomyelitis due to wild type poliovirus, human influenza caused by a new subtype, and severe acute respiratory syndrome (SARS). Prompt reporting of cases of infectious diseases and other events of potential public health threat could prevent potential outbreaks which would result in a high rate of morbidity and mortality.
- The Philippine Integrated Disease Surveillance and Response (PIDSR) is an enhanced surveillance that monitors notifiable diseases and other health-related events of public health importance utilizing an integrated approach. It integrates health statistics generated through the major disease surveillance systems in the country; name: Notifiable Disease Reporting System (NDRS), Field Health Services Information System (FHSIS), National Epidemiology Sentinel Surveillance System (NESSS), Expanded Program on Immunization Surveillance System (EPI Surveillance), and HIV-AIDS Registry.
- The Unified Registry Systems on Chronic Noncommunicable Diseases, Injury Related Cases, Persons with Disabilities, and Violence Against Women and Children serve as tools and mechanisms to collect information on reportable cases on chronic noncommunicable diseases, injuries, violence, and disabilities that have been diagnosed or confirmed in the country.
- Population-based surveys are useful for disease surveillance when they ask people about information for which they may be the most valid and reliable source; e.g., their own private behaviors, attitudes, or mental health status, or for which they can report with reasonable reliability, even if they are not the only or most valid source of information. Some examples of health surveys in the U.S. are:
- Behavioral Risk Factors Surveillance System (BRFSS)
- Youth Risk Behavior Surveillance System (YRBSS)
- National Health Interview Survey (NHIS)
- In the Philippines, the National Nutrition Survey (NNS) is the key source of data for the national government not just on nutrition-related information but on health matters as well. These determine and evaluate the food intakes, nutrition and health status of Filipinos, and provide official statistics on food, nutrition and health situations of the country.
Conducting Epidemiologic Investigations
- Defining Disease Outbreak: “the occurrence of cases of disease in excess of what would normally be expected in a defined community, geographic area or season” (WHO, 2012). The following may be utilized to determine an outbreak:
- Review of surveillance data - an outbreak may be detected during regular conduct and timely analysis of surveillance data.
- Observation of single events or clusters - cases of communicable diseases are reported by laboratories and healthcare providers to health departments.
- Reports from patients or members of the community - individuals that report to the hospital, e.g. a sudden spike in patients reporting diarrhea after attending a party, or a local citizen calling about a mysterious disease affecting their neighborhood.
- Factors Affecting Disease Outbreak Investigation: investigation is needed to identify and eliminate the source of disease/health condition and prevent occurrence of more cases.
- Factors related to the disease or health problem - these include the severity of illness, the number of cases, the source, mode or ease of transmission, and the availability of prevention and control measures. That is, investigation is more likely if there are a large number of affected persons, when the disease is severe, when effective control measures exist, and when the outbreak has the potential to affect others unless prompt control measures are taken.
- Factors related to the health department - availability of staff and resources, and competing priorities; if a health department is experienced in investigating outbreaks.
- Factors related to external concerns - other concerns include (1) control or prevention of the health problem, (2) opportunity to learn or do research, (3) public, political, or legal concerns, (4) public health program considerations, and (5) training in field epidemiology.
Steps in Disease Outbreak Investigation
The CDC (2012) outlines the steps in disease outbreak investigation. The steps listed are ordered conceptually though, in practice, some of the steps may be done at the same time; or a different ordered may be followed; such as, in cases where diagnosis and existence of an outbreak is established first before an outbreak investigation is decided upon, or control measures are implemented as soon as the source and mode of transmission are known.
- Prepare for field work: gain background knowledge on the disease or condition, supplies, and equipment required for the investigation. Plan with a team and coordinate with the local area.
- Establish the existence of an outbreak: the expected number is usually the number from the previous few weeks or months, or from a comparable period during the previous few years. For a notifiable disease, the expected number is based on surveillance records. Local data may also be used for other diseases. A survey of the community may be conducted to establish the background of the disease or condition.
- Verify the diagnosis: this ensures the disease has been identified, and to ensure that laboratory error is not the source of increasing cases.
- Construct a working case definition: a case definition is a tool for classifying someone as either having or not having the disease of interest. These include clinical criteria and restrictions by time, place and person. In early investigation, case definitions are usually operationalized: (a) suspect case has a few typical clinical features, (b) probable case has typical features, and (c) confirmed case has typical features and laboratory verification. This minimizes false-positive and false-negative error cases.
- Find cases systematically and record information: passive surveillance (send out information about the outbreak and ask for reports) or active surveillance (visit facilities to collect information about the outbreak). Forms, questionnaires, or snowball sampling may be used.
- Perform descriptive epidemiology: characterize the outbreak by time, place, and person (descriptive epidemiology). This can be the basis for testable hypotheses, and provide directions for prevention and intervention measures.
- Develop hypotheses: may address the source of the agent, the mode of transmission, or the exposures that caused the disease, depending on what is known.
- Evaluate hypotheses epidemiologically: evaluate hypotheses by (a) comparing them with established facts, or (b) using analytic epidemiology to quantify relationships and assess the role of chance. The necessity of analytical epidemiology (retrospective cohort studies, case-control studies) depends on the amount of evidence supporting the hypotheses.
- As necessary, reconsider, refine, and re-evaluate hypotheses: if the results of analytic epidemiology are unrevealing, rethinking the hypothesis may be necessary.
- Compare and reconcile with laboratory and/or environmental studies: laboratory and environmental studies can correlate with the analytic epidemiology studies. Together, they provide a complete picture of the outbreak investigation and guide appropriate public health action.
- Implement control and prevention measures: measures that target areas of the chain of infection are set in place, sometimes even before investigation begins.
- Initiate or maintain surveillance: continuing surveillance is important to determine whether the prevention and control measures are working in terms of declining number of cases; as well as checking whether the outbreak has spread to other areas where control measures would need to be initiated too.
- Communicate findings: summarization of the findings and its outcomes in a report, and communication of this report through oral briefing to local authorities and as a written report with a scientific format of introduction, background, methods, results, discussion, and recommendations. It is also published in public health literature to contribute to the knowledge base of epidemiology and public health.