BSc Medical Sciences: Epidemiology PDF

BSc Medical Sciences: Epidemiology Assistant Prof. Dr. Soza Th. Baban Learning objectives By the end of this session, students should be able to: 1. To compare different measures of morbidity, including incidence rates, cumulative incidence, attack rates, prevalence, and disease surveillance...

BSc Medical Sciences: Epidemiology Assistant Prof. Dr. Soza Th. Baban Learning objectives By the end of this session, students should be able to: 1. To compare different measures of morbidity, including incidence rates, cumulative incidence, attack rates, prevalence, and disease surveillance. 2. Understand and calculate the case fatality rate (CFR) and explain its importance in assessing disease outbreaks. 3. Interpret and analyse spot maps to identify disease patterns. 4. Apply epidemiological measures to real-world scenarios, including outbreak investigations. 5. Describe disease surveillance in human populations and its importance in providing information about morbidity from disease. 6. Understand challenges in calculating these rates and their importance in public health. Let’s go through each of these points in detail! Introduction and Review In Week 2, we discussed: How diseases are transmitted in populations. Define and distinguish between Endemic, Epidemic, and Pandermic and apply to real-world case senarios. Today, we will move forward by learning how to measure the impact of diseases in populations using core epidemiological measures like incidence and prevalence rates and disease surveillance. We'll also use these concepts to solve public health problems, much like real epidemiologists do.  What Is Epidemiologic Surveillance? The Centers for Disease Control and Prevention (CDC) defined epidemiologic surveillance as the “ongoing systematic collection, analysis, and interpretation of health data essential to the planning, implementation, and evaluation of public health practice closely integrated with the timely dissemination of these data to those who need to know (policy makers, Health officials etc.).”  Epidemiologic Surveillance Surveillance may be carried out to monitor changes in disease frequency or to monitor changes in the levels of risks for specific diseases. Disease surveillance programs provide most of our data on disease morbidity and mortality. Surveillance, once focused mainly on infectious diseases, now also monitors congenital malformations, noncommunicable diseases, environmental toxins, and health impacts from natural disasters.  Epidemiologic Surveillance Surveillance is also used to monitor for completeness of vaccination coverage and protection of a population and for the prevalence of drug-resistant organisms such as drug-resistant tuberculosis (TB) and malaria. Surveillance provides policy makers with guidance for developing and implementing the best strategies for programs for disease prevention and control.  PASSIVE AND ACTIVE SURVEILLANCE Passive surveillance is a public health data collection method in which available data on reportable diseases are used, or in which disease reporting is mandated or requested by the government or the local health authority, with the responsibility for the reporting often falling on the health care provider or district health officer.  Passive surveillance Passive surveillance is the most common type of surveillance and relies on the clinicians, laboratory staff or other related sources to either take the initiative or comply with mandatory rules to report data to public health authorities. This type of surveillance is able to more easily detect and report clinically sick individuals or outbreaks to be noticed. In practical terms, within veterinary public health, passive surveillance is useful for diseases that almost always produce clear clinical signs, such as foot-and-mouth disease in cows. Concomitantly, passive surveillance can be affected by under-reporting, for reasons that span from unreported asymptomatic cases or untested samples from clinically suspicions events.  Active surveillance An active surveillance system is a system that relies on the public health authorities' initiative to contact physicians, laboratory or hospital staff or other relevant sources to report data. Active surveillance denotes a system in which project staff are specifically recruited to carry out a surveillance program. They are recruited to make periodic field visits to health care facilities such as clinics, primary health care centers, and hospitals in order to identify new cases of a disease or diseases or deaths from the disease that have occurred (case finding).  Active surveillance An active surveillance system is a system that relies on the public health authorities' initiative to contact physicians, laboratory or hospital staff or other relevant sources to report data. Active surveillance denotes a system in which project staff are specifically recruited to carry out a surveillance program. They are recruited to make periodic field visits to health care facilities such as clinics, primary health care centers, and hospitals in order to identify new cases of a disease or diseases or deaths from the disease that have occurred (case finding). Homework Assignment: Active vs. Passive Surveillance Explore the advantages and disadvantages of active and passive surveillance in public health.  Measures of Morbidity Incidence rate The incidence rate of a disease is defined as the number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease. Incidence rate per 1,000 persons = No of NEW cases of a disease occurring in the population during a specified period of time ×1,000 No. of persons who are at risk of developing the disease during that period of time  Measures of Morbidity Incidence rate The incidence rate tells us how quickly a disease is spreading. Example: If 50 people develop a flu in a population of 1,000 over one year, the incidence rate is: Incidence Rate = (50/1000) x 100 = 5% per year.  Measures of Morbidity Incidence rate The incidence rate is a measure of risk. This risk can be looked at in any population group, such as a particular age group, among males or females, in an occupational group, or a group that has been exposed to a certain environmental agent, such as radiation or a chemical toxin.  Measures of Morbidity Incidence rate The incidence rate is a measure of risk. This risk can be looked at in any population group, such as a particular age group, among males or females, in an occupational group, or a group that has been exposed to a certain environmental agent, such as radiation or a chemical toxin. The incidence is calculated using a period of time during which all of the individuals in the population are considered to be at risk for the outcome, also called the cumulative incidence proportion, which is a measure of risk.  Measures of Morbidity Prevalence rate Prevalence rate refers to the total number of cases (new and existing) of a disease in a population at a specific time. It shows the overall burden of a disease on a population. Example: If there are 200 people with flu in a town of 1,000 people, the prevalence rate is: Prevalence Rate = (200/1000) x 100 = 20%.  Measures of Morbidity Prevalence rate Prevalence is defined as the number of affected persons present in the population at a specific time divided by the number of persons in the population at that time; that is, what proportion of the population is affected by the disease at that time? No of cases (old and new) of a disease present in the population at a specific time ×1,000 No. of persons in the population at that specified time  Measures of Morbidity Prevalence rate For example, if we are interested in knowing the prevalence of arthritis in a certain community on a certain date, we might visit every household in that community and, using interviews or physical examinations, determine how many people have arthritis on that day. This number becomes the numerator for prevalence. The denominator is the population in the community on that date. What is the difference between incidence and prevalence? For example, if we are interested in knowing the prevalence of arthritis in a certain community on a certain date, we might visit every household in that community and, using interviews or physical examinations, determine how many people have arthritis on that day. This number becomes the numerator for prevalence. The denominator is the population in the community on that date.  Types of Prevalence Point prevalence: Prevalence of the disease at a certain point in time. Period prevalence: How many people have had the disease at any point during a certain time period? The time period referred to may be arbitrarily selected, such as a month, a single calendar year, or a 5-year period. Some people may have developed the disease during that period, and others may have had the disease before and died or been cured during that period.  Types of Prevalence Examples of Point and Period Prevalence and Cumulative Incidence in Interview Studies of Asthma:  Example of incidence and prevalence Calculating incidence rate This figure shows five cases of a disease in a community in 2017. 1st case : occurred in 2016, and the patient died in 2017. 2nd case: developed in 2017 and continued into 2018. 3rd case: was a person who became ill in 2017 and was cured in 2017. 4th case: occurred in 2016, and the patient was cured in 2017. 5th case: occurred in 2016 and continued through 2017 and into 2018. What is the numerator for incidence in 2017? We know that incidence counts only new cases, and because two of the five cases developed in 2017, the numerator for incidence in 2017 is 2.  Calculating Point Prevalence in 2017 This figure shows five cases of a disease in a community in 2017. 1st case : occurred in 2016, and the patient died in 2017. 2nd case: developed in 2017 and continued into 2018. 3rd case: was a person who became ill in 2017 and was cured in 2017. 4th case: occurred in 2016, and the patient was cured in 2017. 5th case: occurred in 2016 and continued through 2017 and into 2018. What is the numerator for POINT PREVALENCE in 2017? This depends on when we do our prevalence survey. If we do the survey in May, the numerator will be 5. If we do the survey in July, the numerator will be 4. If we do the survey in September, however, the numerator will be 3 If we do it in December, the numerator will be 2. Thus, the prevalence will depend on the point during the year at which the survey is performed.  Attack Rate Attack rate is a special type of incidence rate used for outbreaks. It shows the proportion of people who become ill after being exposed to the disease. Formula: Attack Rate = (Number of Ill People / Number of Exposed People) x 100 Example: If 30 out of 100 people exposed to contaminated food get sick, the attack rate is: Number of people at risk in Attack Rate = (30/100) x 100 = 30%. whom a certain illness develops ×100 Total number of people at risk  Attack Rate Attack rate is a special type of incidence rate used for outbreaks. It shows the proportion of people who become ill after being exposed to the disease. Formula: Attack Rate = (Number of Ill People / Number of Exposed People) x 100 Example: If 30 out of 100 people exposed to contaminated food get sick, the attack rate is: Number of people at risk in Attack Rate = (30/100) x 100 = 30%. whom a certain illness develops ×100 Total number of people at risk  Spot Maps: Visualizing Disease Spread Spot maps are visual tools used to display the locations of disease cases. They help identify patterns of spread and the possible sources of outbreaks. Historical Example: John Snow’s famous cholera map in 1854. Spot maps continue to be valuable in modern epidemiology, especially during pandemics like COVID-19. Example: Imagine you are tracking cases of a flu outbreak in a city. You collect addresses of all reported cases and plot them on a city map. You notice a cluster of cases in one neighborhood, indicating a possible hotspot for the outbreak. Spot Maps: Visualizing Disease Spread  Case Fatality Rate (CFR) CFR measures the severity of a disease by calculating the percentage of people who die from it. What percentage of people who have a certain disease die within a certain time after their disease was diagnosed? It is useful for understanding how deadly an outbreak is. Formula: CFR =(Number of Deaths /Number of Confirmed Cases x100 Example: If there are 10 deaths out of 200 confirmed cases of a disease, the CFR is: Number of individuals dying during a specified CFR = (10/200) x 100 = 5%. period of time after disease onset or diagnosis ×100 No. of individuals with the specified disease  Recap and Q&A Let’s review: Incidence Rate: Measures new cases. Prevalence Rate: Measures all cases. Attack Rate: Measures the proportion of exposed people who get sick. CFR: Measures the percentage of cases that result in death. Spot Maps: Visual tools to track disease spread. Any questions before we move on to the next topic? Preparation for Next Session For next session, please: 1. Read Chapter 4 (Pages 65-93) from 'Gordis Epidemiology'. 2. Review Chapter 3 (Pages 62-70) from 'Parks Textbook of Preventive & Social Medicine'. 3. Research examples of mortality measures in different public health contexts. 4. Be prepared to discuss how these measures are used to evaluate population health outcomes.