Written by Chandana Balasubramanian
Whether you are a student of nursing, health sciences, public health, or another specialty, you may be interested in expanding your knowledge of epidemiology. Today, the study of infectious diseases is not limited to specialists in the field or researchers. This is because, in our ever-shrinking world, an emerging disease can turn from a small outbreak in one country to a global pandemic in a few weeks. Additionally, shrinking forests and greater urbanization increase the risk of zoonotic transmission of pathogens (transfer of an infectious virus or bacteria to humans from animals).
Understanding the spread of diseases and epidemiological data can help clinicians and nurses detect and curtail infections early, and public health and government agencies prevent the spread of emerging infectious diseases on their shores.
Here are the most common epidemiological metrics to help you get started.
Incidence refers to the probability of a disease occurring in a given population in a specific time period. The incidence measures new cases that develop or are diagnosed in a month or a year.
For example, compare the two GIDEON maps below color-coded based on incidence. The first map is an outbreak map for the plague and the second is that for SARS-COV-2 or COVID-19. Between the two, even at first glance, we can identify the higher incidence of COVID-19 versus the current endemicity of the Plague.
Prevalence includes all the people who suffer from a specific condition in a population during a specified period.
Incidence Versus Prevalence
Incidence refers to newly developed cases of a disease or illness within a specific time frame, while prevalence tells us the total number of individuals who are living with it.
Let’s look at infections from COVID-19 (SARS-COV-2) as an example. In December 2019, when the first outbreak happened, incidence and prevalence numbers worldwide were low. By March 2020, incidence numbers (newly diagnosed cases) were high. Global prevalence numbers were still low because the virus hadn’t spread to the larger population as yet. But by July 2021, due to highly infectious variants, worldwide incidence and prevalence numbers were high.
Seroprevalence refers to the number of people in a population who test positive antibodies to a pathogen (infectious virus or bacteria). Seroprevalence to an antibody to a virus or pathogen can give us an idea of the number of people infected with the virus.
Seroprevalence is expressed as the percentage of people with antibodies to an infectious agent.
Seroprevalence Survey or Serosurvey
A seroprevalence survey involves blood serum testing of a population and monitoring whether a particular substance is present or absent. These seroprevalence surveys can be tailored based on how widespread or localized infection is. These studies help public health officials and epidemiologists understand how an infectious agent or pathogen is spreading across a specific population over time.
Morbidity means disease. Morbidity Rate measures how often a specific disease or illness occurs in a given population and includes acute and chronic conditions. The metric is used by public health officials, governments, healthcare systems, epidemiologists, and many more to estimate the overall health of a given population. A higher morbidity proportion indicates a greater number of people suffering from illnesses or diseases.
The Morbidity Rate is expressed as a percentage of the number of cases of a specific disease or condition in a given population.
Attack Rate (AR) or Incidence Proportion
The Attack Rate measures the proportion of people who develop a specific condition or illness in a population that was initially free from that disease or illness. This metric is commonly used in epidemiology to track disease outbreaks. Public health officials use the metric to estimate how many individuals may be infected during an outbreak, epidemic, or pandemic.
The Attack Rate or Incidence Proportion is calculated as the number of new cases of an infection in the total at-risk population. It is expressed as a percentage.
Secondary Attack Rate (SAR)
The Secondary Attack Rate estimates the spread of infection from an individual to people in their household, residential area, or to another specified group of people who are at risk of getting infected (like hospital staff). SAR is an important metric during contact tracing for infectious diseases.
Just like the Attack Rate, the Secondary Attack Rate is expressed as a percentage. It is the number of new cases of an infection developed among a person’s contacts out of their total number of contacts.
Incidence Rate (IR) or Incidence Density Rate or Person-Time Incidence Rate
Incidence rate measures the number of new cases of a disease occurring in a given population during a specified timeframe.
Incidence Rate is often expressed as the number of new cases per 100,000 population.
Mortality Rate (MR)
Mortality refers to death. The Mortality Rate or Death Rate helps us understand how frequently death occurs in a given population in a specific amount of time.
The metric is expressed as deaths per 1,000 individuals per year or 100,000 individuals per year.
Crude Mortality Rate
Crude Mortality Rate measures the total number of deaths in a population during a specific period of time. It is called ‘crude’ because the number includes all deaths and is used to provide a high-level estimate of deaths during a humanitarian crisis.
It can be expressed as deaths per 1,000 or 100,000 individuals per year.
Case-fatality Rate (CFR) or Case-fatality Proportion
The case-fatality rate is used to understand the severity of a disease, evaluate new therapies, and predict how a disease will continue to affect a given population. It estimates the percentage of individuals who die from a specific disease among all people diagnosed with the disease for a given period. It is best applicable for diseases that have specific starts and stops, like acute infections or outbreaks. In reality, it is not a rate but the proportion of deaths due to a specific disease condition.
It is expressed as a percentage. A higher percentage indicates greater disease severity due to more deaths from the disease.
Cause-Specific Mortality Rate
Cause-specific Mortality Rate is the number of deaths from a particular cause in a population during a given time interval (usually a calendar year).
It is expressed as deaths per 100,000 individuals.
Note: It is different from the case-fatality rate which is not exactly a rate but a proportion of deaths in a population from a particular disease.
Age-Specific Mortality Rate
The Age-specific Mortality Rate gives us the mortality rate for a specific age group of individuals in a given population. It is also often known as the age-adjusted mortality rate. It is the best way to understand diseases that are highly influenced by age and their impact on a population. This metric is commonly used to understand the rates of heart disease conditions, strokes, diabetes, cancer, and more in a population.
Age-specific mortality is expressed as deaths per 100,000 in an age range.
Maternal Mortality Rate (MMR)
The Maternal Mortality Rate is a ratio that helps us understand how many women die from pregnancy-related complications in a population. It is the number of maternal deaths by the total number of live births in a given period.
The MMR is an important health indicator for a country or a given population because most pregnancy-related deaths are preventable. The biggest pregnancy-related complications include bleeding after childbirth, infections, high blood pressure during pregnancy, and issues that arise during delivery.
MMR is expressed as deaths per 100,000 live births.
Infant Mortality Rate (IMR)
The Infant Mortality Rate measures the probability of a child born in a specific period of time dying before it turns one. It helps us understand the likelihood of a newly born infant surviving in a particular population. Since the probability of survival is closely linked to the social structures, economic, and health conditions of a country, it is often a significant indicator of a nation’s health.
It is expressed as deaths per 1,000 live births.
The term endemic is used to refer to anything that naturally occurs and is confined to a particular location, region, country, or population. For example, from the malaria outbreak map below, we can see that chloroquine-resistant malarial strains are endemic to certain regions in South Asia, Africa, and South America.
An epidemic affects many people at the same time. In an epidemic, there is a sudden increase in disease infections that spreads in a region or locality where the disease condition is not naturally found.
An outbreak is a sudden increase in the number of people infected by or diagnosed with a specific disease condition. It is similar to an epidemic but usually restricted to a much smaller locality or geographic location.
A pandemic is an epidemic that has spread to several countries or continents. It usually affects a much larger group of people than an epidemic.
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