Vaccine Effectiveness and Efficacy

Vaccine efficacy and effectiveness quantify how much a vaccine reduces the risk of an outcome, such as infection or disease, in vaccinated compared with unvaccinated people. Efficacy is the protection measured under the controlled conditions of a randomized trial, while effectiveness is the protection observed in routine, real-world use.

Definition

Vaccine efficacy and effectiveness express the proportional reduction in the risk (or rate) of an outcome among vaccinated relative to unvaccinated individuals, conventionally one minus the relative risk; efficacy refers to estimates from controlled trials and effectiveness to estimates from observational, real-world settings.

Scope

This entry distinguishes efficacy from effectiveness, defines the proportional-reduction measure used for both, and surveys the main study designs used to estimate them, including randomized trials, cohort and case-control studies, and the test-negative design. It is a methodological reference and does not recommend any specific vaccine or schedule.

Core questions

• What is the difference between vaccine efficacy and vaccine effectiveness?
• How is the proportional reduction in risk calculated and interpreted?
• Which study designs are used to estimate effectiveness once a vaccine is in routine use?
• How does the test-negative design control for health-seeking behaviour, and what are its assumptions?

Key concepts

• Vaccine efficacy
• Vaccine effectiveness
• Relative risk reduction
• Randomized controlled trial
• Test-negative design
• Confounding by indication
• Direct and indirect effects
• Correlates of protection

Key theories

Efficacy as one minus the relative risk

Both efficacy and effectiveness are commonly expressed as one minus the relative risk (or rate ratio, hazard ratio, or odds ratio) of the outcome comparing vaccinated and unvaccinated groups, giving the proportional reduction in risk attributable to vaccination.

Test-negative design

Among people seeking care for a compatible illness who are tested, those who test positive for the target pathogen are cases and those who test negative are controls; comparing vaccination odds between them estimates effectiveness while reducing bias from differential health-seeking behaviour.

Mechanisms

Efficacy is estimated by comparing outcome frequency between randomized vaccinated and placebo groups, where randomization balances confounders. After licensure, randomization is usually not feasible, so effectiveness is estimated from observational data that must address confounding, especially differences between people who do and do not get vaccinated. The test-negative design addresses one such bias by restricting the study to people who present for care and are tested, comparing vaccination status between those positive and negative for the target pathogen; under its assumptions this controls for differences in health-seeking and testing. Estimates can capture only direct protection of vaccinees or, in some designs, also the indirect protection that reduces transmission in the wider population.

Clinical relevance

Efficacy and effectiveness estimates underpin how the protective value of a vaccine is communicated and compared, and they explain why trial efficacy and field effectiveness can differ for the same product. This entry describes how those estimates are generated and interpreted; it is a reference framework and not a basis for individual vaccination decisions.

Epidemiology

Effectiveness varies with the outcome studied (infection, symptomatic disease, or severe disease), the population, the match between vaccine and circulating strains, and time since vaccination. For pathogens such as influenza, the test-negative design has become a widely used method for routine annual effectiveness monitoring across many surveillance networks.

History

Field assessment of vaccine protection was systematized in the late twentieth century, with Orenstein and colleagues codifying methods for estimating efficacy from observational data. As vaccines moved into routine use, observational designs became central, and the test-negative design, developed and formalized for influenza in the 2000s and 2010s, provided a practical and increasingly standard approach to monitoring effectiveness.

Debates

How well does the test-negative design remove bias?

The design controls for differential health-seeking behaviour under specific assumptions, but residual confounding, imperfect test sensitivity and specificity, and selection of the comparison illnesses can still bias estimates, and the conditions under which it is unbiased remain a subject of methodological analysis.

Key figures

• Walter Orenstein
• Geoffrey Weinberg
• Michael Jackson
• Sheena Sullivan
• Benjamin Cowling

Seminal works

• orenstein-1988
• weinberg-szilagyi-2010
• jackson-nelson-2013

Frequently asked questions

Is vaccine efficacy the same as vaccine effectiveness?

No. Efficacy is the protection measured under the controlled conditions of a randomized trial, while effectiveness is the protection observed in routine, real-world use; effectiveness is often somewhat lower because field conditions differ from trial conditions.

Why is the test-negative design popular for influenza vaccines?

It restricts the study to people who seek care and are tested, comparing vaccination between those who test positive and negative for the virus, which helps control for differences in health-seeking behaviour and makes routine, large-scale monitoring practical.