Epidemiological Description

Epidemiological Description

The first step in investigating an epidemic is accurately describing the condition of the disease and related factors.

Descriptive Epidemiological Study:

• Social form of the disease: How does the disease manifest in society?

• Relationship of the disease to WHO, WHEN, WHERE: Who gets the disease? When does it occur? Where does it occur?

Objective of Descriptive Epidemiology:

• Describing health status: Provides information about the health status of the community.

• Disease trends: Analyzing changes in the disease over time.

• Providing information for public health planning: This information helps to plan disease prevention strategies.

• Forming hypotheses about the disease: Identifying factors associated with the disease.

Epidemiological variables in descriptive epidemiology:

• WHO: Who gets the disease?

• WHERE: Where does the disease occur?

• WHEN: When does the disease occur?

Purpose of data analysis in descriptive epidemiology:

• Describing the disease and its extent: Determining the incidence rate and severity of the disease.

• Disease risk factors and risk processes: Identifying factors associated with the disease and how they influence it.

Two characteristics about people needed in the description:

• Age: Age affects the likelihood of contracting the disease.

• Gender: Gender affects the likelihood of contracting the disease.

Factors that change with age:

• Different sensitivity and immunity: Children and the elderly are often more vulnerable.

• Cumulative exposure: Older people are often exposed to more risk factors.

• Health changes: Older people are often in poorer health.

• Different lifestyles: Different age groups have different lifestyles, leading to different disease risks.

Epidemiologists choose a large enough age group to:

• Narrow the scope: Minimize errors in analysis.

• 10 years: Analyze disease trends over time.

• Conceal social variations in the disease: Avoid the influence of social variations on the results of the analysis.

Gender factors are influenced by:

• Hormones: Sex hormone differences affect the likelihood of contracting a disease.

• Physical fitness: Women are often physically weaker than men.

• Society: The social roles of men and women are different, leading to different disease risks.

• Psychology: Men and women have different psychological responses to risk factors.

Pellagra is an example of the impact of social factors as described by Joseph Goldberger in people living in poorhouses due to a lack of nicotinic acid in their food.

Sickle cell anemia / living at >2000m is less likely to get malaria.

Diarrhea often occurs in the summer.

Respiratory diseases often occur in the winter.

Blood type O is more susceptible to peptic ulcer disease.

Describing the location answers the question:

• Where highest: Where is the incidence rate highest?

• Start where: Where did the disease start?

• Spread where: How did the disease spread?

Location has 3 scopes:

• Global: The disease occurs worldwide.

• National: The disease occurs in a specific country.

• Outbreak: The disease occurs in a specific geographic area.

Location descriptions are presented using dot maps.

Location description is a description in 4 groups of fields:

• Physical: The distribution of the disease according to topography, climate.

• Chemical: The distribution of the disease according to water quality, air.

• Biological: The distribution of the disease according to animals, plants.

• Nutritional: The distribution of the disease according to diet.

Time is presented using a graph, the vertical axis is the frequency of incidence, number of deaths, the horizontal axis is time.

An increase in the incidence rate over a short period suggests a common-source outbreak.

Analysis over time is particularly important for diseases related to the environment and occupation.

The epidemic progression is presented using a bar chart (vertical: cases, horizontal: time).

Measles, influenza A have a cycle of 2-3 years.

Cyclicity reflects changes in the level of epidemics after outbreaks and ineffective surveillance.

Cohort effect by age: A cohort of people born in the same year has common characteristics in interaction with risk factors.

There are 3 types of descriptive epidemiological studies:

• Correlational / ecological: Investigates the relationship between the factor of interest and disease / death in different communities.

• Case / Case series: Reports on individual cases of disease or a group of patients over a short period.

• Cross-sectional: Investigates the prevalence of disease and exposure to risk factors at a specific point in time.

Correlational studies investigate the relationship between the factor of interest and disease/death in different communities.

Characteristics of correlational studies:

• No information at the individual level: Only collects information about groups.

• Exposure level is considered representative of all individuals in the community: Does not distinguish the exposure level of each individual.

• Average group exposure <=> average group disease/death rate: Assumes a relationship between average exposure level and disease/death rate.

• Different from cross-sectional studies: Cross-sectional studies collect information at the individual level.

• Combined measurement using correlation coefficient r and scatter plot: Evaluates the relationship between two variables.

Data needs to be collected at the same time.

Advantages of correlational studies:

• Available information: Easy to collect data.

• Cheap: Low cost.

• Fast: Results are obtained quickly.

Disadvantages of correlational studies:

• Cannot conclude with certainty whether the factor of interest actually influences the disease or not: Only identifies a relationship, does not determine the cause.

• Cannot control confounding factors: Results can be affected by other factors.

Case reports use descriptions of characteristics, progression, therapeutic efficacy, and complications.

Case reports are common in medical literature, providing information about new diseases, rare clinical presentations.

Results of case reports help to form hypotheses about new diseases, new treatments, new side effects.

Disadvantages of case reports:

• Subjective: Based on individual observations.

• Inaccurate: Can be affected by errors.

Case series reports include information about cases in the same community over a short period.

Case series reports are useful because they help monitor epidemics (when there is an unusual increase in cases).

An example of a case series report is HIV/AIDS in 1981 in the United States, reporting 5 cases of Pneumocystis carinii pneumonia in gay men.

Advantages of case series reports help form hypotheses about the etiology of the disease.

The most common type of study is case/case series reports.

Cross-sectional studies describe the prevalence of disease and exposure at the same point in time.

Objectives of cross-sectional studies:

• Description: Provides information about the prevalence of disease and exposure.

• Determining the prevalence => prevalence level over time: Determining the prevalence of the disease in the community.

• Forming hypotheses about the association: Identifying factors associated with the disease.

Cross-sectional studies can identify the etiology in occupational health epidemiology.

To measure the strength of the association between disease and the factor of interest, use the prevalence ratio in groups with and without exposure (OR).

Advantages of cross-sectional studies:

• Fast: Results are obtained quickly.

• Easy: Easy to perform.

• Cheap: Low cost.

Advantages of cross-sectional studies:

• If the exposure factor is immutable, it is considered an analytical study (role as a validation tool).

Disadvantages of cross-sectional studies:

• Not suitable for studying rare diseases, rare exposures: Susceptible to errors.

• Exposure and disease are not measured at the same time, so it is not possible to draw a causal relationship: Only identifies a relationship, does not determine the cause.

• It is not possible to determine whether the consequence is due to the disease or to exposure: Cannot distinguish cause and effect.