Introduction to Epidemiology

• Epidemiology studies the distribution of health-related states/events and factors that determine them.
• The information gained is used to control health problems in populations.
• Epidemiology helps monitor the health of populations, identify determinants of health and disease, and evaluate interventions for disease prevention and health maintenance.
• Relevant concepts include clinical judgement, evidence, and health disparities.

Descriptive Epidemiology

• This studies the distribution of disease.
• This answers the questions of what, who, where, and when
• It considers person (demographics), place (geographic variation), and time(when symptoms started) in relation to a disease

Analytical Epidemiology

• This studies the origins and causal factors of diseases (how and why).
• It addresses determinants of health and disease like factors, exposures, characteristics, and behaviors that influence patterns.
• Analytical epidemiology assesses rate differences among groups, how a disease occurs, and differing impacts on populations.

Trends in Epidemiology

• Before the 20th century, the focus was on eliminating infectious diseases.
• During the 20th century, prevention and treatment became more possible.
• The focus shifted from infectious diseases to chronic conditions such as CV disease, malignant neoplasms, and unintentional injuries.
• Advancements in genetic and molecular techniques enable classification of individuals based on exposures or disease susceptibility.

Uses of Epidemiology in evaluating populations

• Diagnosing the health of a community.
• Evaluating health services.
• Examining the impact of disease over time.
• Predicting future health care needs.
• Estimating individual risk based on population data.
• Identifying syndromes.
• Determining causality.
• Enabling early intervention and preventing further mortality.

Community Nurses and Epidemiology

• Community health nurses collect data.
• They monitor and conduct surveillance.
• They identify disease patterns in groups.
• Community health nurses may investigate outbreaks.
• Investigation methods include client interviews, contact tracing, and education.

Steps for investigating disease outbreaks:

• Research recent disease rate stats.
• Verify the existence of an outbreak by comparing current and previous Dx rates.
• Define a working case, including lab data and S&S.
• Systematically find and collect cases.
• Conduct descriptive epidemiology based on person, place, and time.
• Develop hypotheses about the cause/source.
• Evaluate the hypotheses, refine if needed.
• Implement control and prevention measures.
• Relay findings to local health authorities.

Case Study: Toronto Mumps(2017-2018)

• Toronto, with a population of 2.7 million, typically sees 5 mumps cases per year, largely travel-related.
• Two unvaccinated siblings, ages 18 and 20, with lab-confirmed mumps, began an outbreak.
• Initial transmission may have occurred at a house party, with subsequent cases linked to bars/restaurants.
• Lab data and clinical symptoms identified mumps, including genotyping if possible.
• Data collection included lab specimens, case interviews, and iPHIS tool usage.
• Conducted Person, place and time analysis extended to demographics, geography, vaccine status, genotype, and symptoms, which ruled out social networking approach.
• Transmission among young adults with low vaccine rates caused the outbreak.
• Control measures included asking cases to self-isolate and excluding them from social gatherings and healthcare facilities.
• Control included identifying sites of transmission, notifying contacts, exclusion of susceptible student contacts, bar inspections and communication via social media to young adults.
• As of February 26, 2018, the outbreak was declared over (50 days after the last onset case.)
• 73% of cases were not or partially vaccinated.

Health Surveillance

• This involves the tracking and forecasting of any health event or health determinant.
• This includes collecting data, integrating, analyzing, interpreting it and disseminating surveillance products.

Basic Concepts

• It is important to look at the distribution of disease, health states & events.
• This distribution should consider context
• For example, 20 influenza cases in Town A (smaller community) is more significant than 20 cases in Town B (larger community)

Risk

• Risk is the probability of an event occurring within a specified period.
• High-risk populations have a greater probability of an event occurring.
• Risk factors are variables increasing disease rates (e.g., genetic disposition or infectious agent/high fat diet exposure).

Proportion

• Proportion is a type of ratio where the numerator is part of the denominator, expressed as percentages.
• Proportion shows the relationship between the total number and the frequency of occurrence.
• As an example, suppose 23 people in a class of 51 practice yoga; thus, 23/51 =.4509 or 45% of the class practice yoga.
• As another example, if there were 76,000 deaths due to heart disease in Canada, out of a total of 123,323 deaths, then the death rate is 76,000 / 123,323 = .62 or 62%

Rate

• A rate is similar to a ratio but differs by including a measure of time in the denominator.
• A rate measures how quickly a disease is developing or how rapidly people are dying.
• Rate is calculated by dividing total events in a given year by population at risk (same year), then multiply by 10^n
• The population at risk includes those at risk of experiencing the event
• For instance, to calculate the birth rate in NL, you divide the number of births in 2024 (3990) by the total population (525,604) which is 0.0076.
• Multiply by 1,000, gives 7.6/1000.

Measures of Morbidity and Mortality

• Morbidity reflects the occurrence of disease in a population.
• Mortality tracks the number of deaths in a population over a specific period.

Incidence Rate

• Incidence rate reflects the number of new cases developing in a population at risk over a specified time.
• It represents the risk of developing a disease within a specific time.
• It is calculated by Number of new cases or events occurring in the population over a specific period, divided by population at risk during that period, then multiplied by 10^n.
• For this calculation, existing cases are excluded from the population at risk.
• As an example, if 60 new cases of HIV were reported among 8,000 people in 2020, the rate is (60 / 8000) = 0.0075. Multiplying by 1000 gives an incidence rate of 7.5 per 1000.

Attack Rate

• Attack rate is a specialized form of incidence rate applicable when a population is exposed to an infectious disease at a certain time and place and documents the number of new cases of the disease.
• A common example is food poisoning: attack rate equals the number exposed/sick divided by number exposed to suspected food.
• If 10 of 50 people who ate suspected food got ill, then 10 / 50 = 0.20 or 20% is the attack rate.

Prevalence Rate

• Prevalence rate identifies the number of persons in a population that have a disease or event at a specific time.
• It is calculated by dividing Number of existing cases in a population at a specific point in time / Population at the same specific point in time and multiplying the dividend by 1,000.
• Suppose a community of 21,200 has 300 known diabetes patients plus 34 new cases; then, the rate is (300 + 34) / 21,200 (2023) = .0157 multiplied by 1,000 = 15.7.
• Alternatively, if, in a survey of clients at a STI clinic, 180 of 300 patients used a condom the past two months, then the rate is 180/300 = .60. In other words, 60% of the survey group used condoms.

Mortality Rates

• Mortality rates give information about deaths and fatal diseases.
• Commonly used mortality rates:
  • Crude mortality rate: the total number of deaths in a given year.
  • Age-specific rate: death count within an age group in a year.
  • Cause-specific rate: the count of deaths from a cause.
  • Infant mortality rate: number of deaths under 1 year per the number of births.

Measures of Association

• Risk indicates the probability that an event will occur in a specified period.
• Relative Risk compares the risk of a health condition between exposed and unexposed groups.
• Example: effects of smoking during pregnancy on birth weight.

Relative Risk Calculation

• Relative Risk (RR) equals the incidence rate among those exposed divided by the incidence rate among those not exposed.
• Relative risk helps determine whether the rate in the exposed population is higher than in the non-exposed population.
• A high RR implies that the factor is a risk factor for developing disease.
• A hypothetical example of diabetes risk for Aboriginal people living on reserves compared with Canadians generally gives RR = (8.5 per 100) / (3.2 per 100) = 2.67.

Attributable Risk

• Attributable Risk is a measure of the incidence of disease among people exposed to the risk factor, expressed as a percentage.
• It gauges the excess risk from a factor.
• Attributable Risk = (Incidence in exposed group – Incidence in unexposed group) / Incidence in exposed group.
• For example, CHD inactive - CHD active / CHD inactive -> 12.3 - 8.0 / 12.3 is equal to 0.3495 , or 35%.

Rates Of Disease Classification

• The endemic rate is the usual rate of occurrence in a population.
• An epidemic is when it clearly exceeds the usual level (increased incidence).
• A pandemic describes geographically widepsread populations
• A point epidemic is charted as a sharp peak, due to a concentration of cases in a short time.

Epidemiological Triangle

• Disease results from complex relationships among causal agents such as susceptible persons, and environmental factors.
• The three elements of agent, host, and environment are called the epidemiological triangle.
• Altering any of the elements can change the risk for disease.
• Agent: includes infectious, chemical, and physical agents, as well as nutritive elements.
• Host: Genetic susceptibility, immutable and acquired characteristics, lifestyle, and human behaviour
• Environment: Climate, plant/animal life, human distribution, socioeconomic factors, and working conditions