Concept Architecture
Concept
Theoretically, Prevented Fraction (PF) is an epidemiological measure that quantifies the proportion of potential disease occurrence prevented by a protective exposure or intervention among exposed individuals. It represents the fraction of disease that would have occurred in the absence of the protective factor but has been prevented because of the intervention or exposure. The concept is founded on causal inference and comparative risk assessment and exists to quantify the effectiveness of preventive interventions.
Mathematically, the Prevented Fraction is represented as the proportional reduction in disease risk among exposed individuals relative to an unexposed or comparison group. It is a dimensionless measure ranging from 0 to 1 or equivalently 0% to 100%. Higher values indicate greater preventive effectiveness and larger reductions in disease occurrence attributable to the protective intervention.
In practice, Prevented Fraction is estimated using cohort studies, randomised controlled trials and intervention evaluations in which a protective exposure reduces disease incidence. It is widely applied in epidemiology, vaccination studies, preventive medicine and health economic evaluation to quantify intervention effectiveness and estimate preventable disease burden.
Purpose
Used to quantify the proportion of disease prevented by a protective intervention, evaluate preventive effectiveness, estimate avoidable disease burden and support public health and health economic decision-making.
Mathematical Formulae
Primary Formula
PF = (I? ? I?) / I?
where:
- PF = prevented fraction
- I? = incidence in the exposed or protected group
- I? = incidence in the unexposed or comparison group
Supporting Formulae
PF = 1 ? RR
PF% = (1 ? RR) ? 100
RR = I? / I?
where:
- RR = relative risk
Related Mathematical Methods
- Relative Risk
- Absolute Risk Reduction
- Attributable Fraction
- Population Attributable Fraction
- Vaccine Effectiveness
- Comparative Risk Assessment
Example
A vaccination programme reports an incidence of influenza of 3% among vaccinated individuals and 12% among unvaccinated individuals.
Relative Risk:
RR = 0.03 / 0.12 = 0.25
Prevented Fraction:
PF = 1 ? 0.25 = 0.75
PF = 75%
The vaccination programme prevents approximately 75% of influenza cases that would otherwise have occurred among vaccinated individuals.
Excel Implementation
| Function | Example Formula | Health Economics Application |
|---|---|---|
| Subtraction | =(C2-B2)/C2 | Calculates the Prevented Fraction using incidence in protected and comparison groups. |
| Subtraction | =1-D2 | Calculates the Prevented Fraction from the Relative Risk. |
| Percentage | =(1-D2)*100 | Expresses the Prevented Fraction as a percentage. |
| IF | =IF(D2<1,1-D2,0) | Calculates the Prevented Fraction only when the intervention reduces disease risk. |
VBA (Optional)
A VBA macro can automatically calculate Prevented Fractions for multiple preventive interventions and generate comparative effectiveness summaries across population subgroups.
Sources
- Rothman KJ, Greenland S, Lash TL. Modern Epidemiology. 4th ed.
- Gordis L. Epidemiology. 6th ed.
- Halloran ME, Longini IM, Struchiner CJ. Design and Analysis of Vaccine Studies.
- Bonita R, Beaglehole R, Kjellstr�m T. Basic Epidemiology. 2nd ed.
- Drummond MF, et al. Methods for the Economic Evaluation of Health Care Programmes. 4th ed.
Related Concepts (2)
Library
Publications
1
Statistical Analysis of Cost-Effectiveness Data — Willan & Briggs, 1st Edition ed., 2006 (John Wiley & Sons)
A synthesis of statistical methods for analysing cost-effectiveness data, including net-benefit regression, confidence intervals for the ICER, cost-effectiveness acceptability curves, and covariate adjustment. Part of the Wiley Statistics in Practice series.
BookView source →
Frequently Asked Questions (6)
What is the prevented fraction?
The proportion of disease cases avoided in a population due to a protective exposure or intervention, the counterpart to the attributable fraction.
Source: Levin 1953
What does the prevented fraction measure about a protective factor?
The prevented fraction is the share of disease cases avoided in a population thanks to a protective exposure or intervention, the mirror image of the attributable fraction that measures harm. It measures how much illness a beneficial factor, such as a vaccine, has kept from occurring, showing the good it does across a population. A large prevented fraction marks an intervention that spares many cases, which helps quantify the value of prevention. The disease averted by protection is what it captures. Rothman and colleagues (2008) describe this measure.
Source: Rothman et al. 2008
How is the prevented fraction calculated?
The prevented fraction is calculated by comparing the disease frequency that would have occurred without the protective factor with the frequency that actually occurred with it, expressing the averted cases as a proportion of what would have happened. For a population measure it also reflects how widespread the protective factor is. So the prevented fraction is calculated from the reduction in disease attributable to the protective exposure relative to the disease expected in its absence, giving the proportion of cases prevented, which, like attributable measures, combines the strength of the protective effect with, for population measures, the prevalence of the protective factor.
Source: Levin 1953
How does the prevented fraction relate to the attributable fraction?
The prevented fraction relates to the attributable fraction as its mirror image: the attributable fraction is the proportion of cases caused by a harmful exposure, and the prevented fraction the proportion of cases avoided by a protective one. Both express the impact of an exposure as a fraction of disease, one for harm and one for benefit. So the prevented fraction and the attributable fraction are complementary measures of an exposure's impact, applied to protective and harmful exposures respectively, and both depend on the strength of the exposure's effect and, for population measures, its prevalence, sharing the same logic in opposite directions.
Source: Levin 1953
Why is the prevented fraction useful?
The prevented fraction is useful because it quantifies the benefit of a protective exposure or intervention at the population level, showing how much disease has been or could be averted, which is valuable for evaluating preventive measures such as vaccination or public health programmes. So the prevented fraction is useful for assessing and communicating the impact of prevention, since it expresses the share of potential cases avoided, helping to judge the value of a protective factor and to compare interventions, and it complements the attributable fraction by focusing on disease prevented rather than disease caused.
Source: Levin 1953
What are the limitations of the prevented fraction?
The limitations of the prevented fraction include that it assumes the protective association is causal, so it misleads if confounding or bias inflates the apparent benefit; that it depends on accurate estimates of the protective effect and, for population measures, the prevalence of the protective factor; and that it requires a valid estimate of the disease that would have occurred without the factor. So the prevented fraction is interpreted with attention to causality and to the soundness of the estimates, since attributing prevented cases to a factor that is not genuinely protective, or mis-estimating the counterfactual disease frequency, would overstate the benefit, and its value depends on these assumptions holding.
Source: Levin 1953
Trust Record
Verified by Dr Darrin Baines
British health economist
Professional identity: darrinbaines.org
Verification date: 10 Dec 2025
Content version: 1.0.0
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- Term code
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