Epidemiology Incidence vs. Prevalence: Exploring Two of the Most Impactful Concepts in Public Health

Epidemiology is an essential practice, yet many high-level concepts baffle the public and even highly knowledgeable epidemiologists. Two terms, in particular, are often mistakenly used interchangeably: incidence and prevalence. Capturing the public's fascination during the COVID-19 pandemic and typically entering the lexicon during flu season, these terms provide powerful insight into the total burden of disease. While closely linked, however, these describe distinct phenomena.

To fully understand how disease spreads within a population — and how potentially dangerous health conditions can be prevented or mitigated — public health professionals must be capable of navigating the divide between epidemiology incidence vs. prevalence. These are among the most critical concepts in epidemiology, providing a strong framework through which professionals can apply a wealth of data and, ideally, arrive at research-backed insights.

To clarify these concepts, below we address a couple of key questions: What is incidence in epidemiology, and how does it differ from prevalence? Keep reading to uncover in-depth definitions and easy-to-understand examples of how these concepts play out in the real world.

Understanding Epidemiology

Epidemiology represents the targeted study of disease from a public health perspective. Crucial for understanding everything from chronic diseases to full-blown pandemics, this discipline reveals how diseases or conditions are distributed throughout various populations — and which factors play into this distribution.

The Importance of Incidence and Prevalence

Concepts such as distribution (often described as the 'who,' 'when,' and 'where' underscoring health events) and determinants (biological, environmental, and social factors that shape health outcomes) tend to take center stage in epidemiological research, but these terms are less meaningful if not accompanied by the study of frequency. Revealing how often diseases take place, this broad concept involves two core components: incidence and prevalence.

What Is Incidence in Epidemiology?

Incidence highlights new cases of a given disease, along with the risk of developing that condition. Offering a helpful definition of incidence in epidemiology, the National Center for Health Statistics (NCHS) describes this as the "number of cases of disease having their onset during a prescribed period of time."

This is expressed as a specific rate — involving the number of new cases of a given disease (within a particular population) over a set period of time. With insights into incidence, epidemiology gains a reliable framework for understanding the risk of disease and identifying trends.

Types of Incidence Measures

Meant to quantify the occurrence of emerging diseases or conditions, incidence measures promote a data-driven approach to epidemiological research, public health interventions, and policy development. Common measures include:

• Incidence proportion (cumulative incidence) – Revealing the risk of an individual developing a particular disease over a certain time frame, the incidence proportion involves dividing the number of new cases by the population deemed at risk. This concept focuses on the general risk of the disease. Because it involves a fixed time period, it is preferred when entire populations are tracked for a specific amount of time.
• Incidence rate (incidence density) – Focused on the speed with which new cases arise, the epidemiology incidence rate relies on an estimate known as person-time, involving the total time in which study participants are at risk of developing specific conditions. This measure is more relevant when conducting studies across a longer period — or if different participants are potentially followed for different amounts of time.

Factors Influencing Incidence

Incidence can vary according to numerous factors, which often reflect individual characteristics or habits. Examples include:

• Demographics – Although age is commonly cited as a top concern underscoring increased incidence, many other demographic factors can come into play. Breast cancer provides a relatable example of how incidence can differ between populations; though it is possible for new cases to emerge among men, this condition is far more common in women.
• Lifestyle – Nutrition, physical activity, sleep, and even hand hygiene all have a major impact on whether certain individuals become ill — regardless of broader environmental concerns. Drug use can also increase the incidence of certain diseases, particularly when needle-sharing is involved.
• Infectivity and pathogenicity – Infectivity determines whether pathogens are able to enter hosts and multiply. Highly infectious diseases (such as measles) tend to prompt higher incidence rates. Revealing the ability of those pathogens to cause health problems, pathogenicity can have a substantial impact on the incidence of infectious diseases.

Applications of Incidence

Equipped with a thorough understanding of incidence, epidemiologists can take action to reduce the risk of outbreaks among at-risk individuals or within vulnerable populations.

• Identifying risk factors – Critical for revealing susceptible populations and understanding what exactly makes certain people more susceptible to disease, incidence ensures that all related risk factors are understood. After all, if these are not properly identified, they cannot be addressed.
• Evaluating preventative measures – Incidence can play a powerful role in determining whether various preventive measures prove effective. If the incidence rate increases dramatically, it can be assumed that current preventative measures are not sufficient for limiting the risk of contracting certain diseases in certain populations. This may indicate the need for new or adjusted preventative strategies.
• Public health interventions – In the midst of major outbreaks, incidence reveals the severity of the situation while also indicating which types of individuals are most susceptible to identified agents. This information, in turn, can shape public health interventions, which may involve everything from quarantines to vaccination campaigns.

What Is Prevalence in Epidemiology?

While incidence can provide valuable insight, it presents a few distinct limitations. For example, as NCHS points out, it is possible for the population in question to change during a given period. What's more, it can be difficult to determine whether particular cases are new — or, specifically, whether their onset occurred during a given time period. This is when a deep dive into disease prevalence may prove useful.

NCHS defines prevalence as "the number of cases of a disease, number of infected people, or number of people with some other attribute present during a particular interval of time." In general, prevalence looks into the overarching burden of disease at a given point in time.

Types of Prevalence Measures

As with incidence, prevalence can be quantified in different capacities, which provide a more nuanced understanding of the burden of disease. Common measures include:

• Point prevalence – Focusing on a specific point in time, the point prevalence highlights the proportion of individuals with a particular health condition (and at a particular time). This is often a crucial concept in the early stages of a disease outbreak, as it offers a valuable snapshot of the disease and its spread.
• Period prevalence – Utilizing a broader period of time (such as a month or a year), period prevalence can be useful for studying chronic diseases but also may be helpful when researching the patterns that underscore seasonal issues like influenza.

Factors Influencing Prevalence

Prevalence can differ between populations or at various points in time. The following are among the most significant factors that can lead to an increased (or decreased) prevalence of a particular illness or condition:

• Incidence – Yes, incidence and prevalence differ considerably, but they can still influence each other. If the incidence is high, more new cases will occur, and the overall number of cases will likely increase. Similarly, as fewer new cases arise, the prevalence will likely drop.
• Duration – This concept describes how long people are infected with (or otherwise impacted by) specific health conditions. While duration can differ from one person to the next, certain conditions tend to linger. In general, a longer duration leads to a higher prevalence.
• Public health interventions – Though insights into prevalence aim to inform public health interventions, this link goes both ways: Previously established interventions can reduce the prevalence. For example, proactive measures such as screenings can ensure that cases are detected early on — and with many conditions, early treatments are more likely to lead to a prompt cure.

Applications of Prevalence

Prevalence shapes many of the strategies and interventions mentioned previously, but there are also unique applications that may be less viable when drawing exclusively on the concept of incidence. Examples include:

• Resource allocation – One of the chief limitations in responding to (or preventing) outbreaks is limited funding and labor. This means that public health professionals may be forced to prioritize, focusing initially on populations deemed most vulnerable or those that consistently demonstrate a higher prevalence of relevant diseases or conditions. For example, communities dealing with a high prevalence of chronic conditions may require additional facilities or staffing — especially healthcare professionals well-versed in treating chronic concerns.
• Public policy development – Data on disease prevalence can help shape legislation designed to address the many factors that underscore both the spread of disease and related health outcomes. Returning to the prevalence-guided approach to addressing chronic disease, this strategy can guide educational or screening programs meant to help populations manage diseases highlighted in prevalence data.

Incidence vs. Prevalence – Key Differences and Relationships

At their most basic level, incidence and prevalence differ in how they deal with the timing of cases — as well as how that timing influences the collection and dissemination of data. Incidence focuses on new cases, while prevalence deals with total cases (including those thought to be new). The core differences between these concepts can have far-reaching implications, as outlined below:

Conceptual Differences

Simply put, incidence focuses on new cases, whereas prevalence involves a greater range of cases. Similarly, incidence is tied to the risk of infection, with prevalence revealing the total burden of diseases within a larger population.

Mathematical Relationship

Many epidemiologists find that mathematical applications grant them a deeper understanding of how incidence and prevalence play out in the real world. Two main formulas help epidemiologists quantify these concepts:

• Incidence Rate = Total person-time at risk ÷ Number of new cases

• Prevalence = Total population ÷ Number of existing cases
  

Meanwhile, an essential equation describes the mathematical relationship between these concepts: 

• Prevalence = (Incidence) x (disease duration)
  
  

Practical Implications

The key differences highlighted above have huge implications for the practice of epidemiology. However, when explored alongside each other, incidence and prevalence can provide a richer understanding of the spread of disease and its burden within a given population. Common patterns involving the interplay of incidence and prevalence include:

• High incidence, low prevalence – In some situations, many new cases of a given disease will emerge during a relatively short period of time. Despite this, the total number of cases may remain relatively low. This could occur for one of two main reasons. One is especially alarming: A high fatality rate reduces the prevalence because, when people die, they are no longer counted among the existing cases. However, if diseases are easily cured and people get better quickly, the prevalence could also remain low.
• Low incidence, high prevalence – Contrasting the above phenomena, a low incidence, high prevalence pattern of disease may occur if few new cases are reported, and yet the total number of cases remains high. The most easy-to-understand example of this involves chronic conditions such as diabetes or heart disease, which many people live with for years or decades.

Impact on Public Health Strategies

Both incidence and prevalence inform public health strategies, and often, it takes a blend of these two concepts to truly understand how diseases spread and what can be done to improve health outcomes. In certain situations, however, a deep dive into incidence may prove more relevant than examining prevalence (and vice versa).

Strategies can be carefully crafted to align with time-sensitive data on the incidence and prevalence of specific conditions, along with research-backed insights into the measures deemed most effective for addressing or preventing similar diseases. For example, during a severe outbreak, a high incidence may reveal the need for a rapid response, including short-term quarantines. Additionally, experts can look to prevalence data for guidance on how to reduce the burden of that disease over time.

Texas A&M's Online MPH in Epidemiology Program

As you strive to expand your understanding of incidence, prevalence, and other core epidemiological concepts, look to Texas A&M University for guidance. Offering the opportunity to earn an online master’s in public health, our flexible curriculum includes a detailed overview of biostatistics, data management, and beyond.