[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Epidemiology](https://durgical.com/en/epidemiology/) › [Study designs](https://durgical.com/en/epidemiology/areas/designs/) › Vaccine efficacy and evaluating preventive measures

# Vaccine efficacy and evaluating preventive measures

- Measure

Vaccine efficacy is the percentage by which a vaccine lowers the risk of disease in vaccinated people, compared with unvaccinated people.

Updated Nov 13, 2026

## Key points

- VE = (ARU − ARV) / ARU = 1 − RR, where ARU and ARV are the attack rates in unvaccinated and vaccinated people.
- It is a relative risk reduction; it does not say how many cases are prevented in absolute terms.
- Efficacy = in trials; effectiveness = in the population and in practice.
- Case-control: VE ≈ 1 − OR (also in the test-negative design).
- Screening method: compares the proportion of cases vaccinated with population coverage.
- With high coverage, most cases may be vaccinated even when the vaccine works well.

On this page

- The basics Key points
- What it is
- Formula and reading
- Worked example
- Markers
- How to read it
- Common pitfalls
- Memory aid
- Elsewhere in DurgiPedia

- Treatment How vaccine efficacy or effectiveness is estimated

- Study Test yourself

- Connections Connections

## What it is

Vaccine efficacy (VE) measures the proportional reduction in the risk of disease among vaccinated versus unvaccinated people: it is the relative risk reduction, VE = 1 − RR. Measured in a controlled trial it is called efficacy; measured in the population under real conditions, effectiveness. Outside trials it is estimated with cohorts, with case-control studies (VE = 1 − OR) or with the screening method, which needs only the coverage and the proportion of cases vaccinated.

## Formula and reading

- VE = (ARU − ARV) / ARU = 1 − ARV / ARU = 1 − RR.
- Screening method: VE = 1 − [PCV / (1 − PCV)] × [(1 − PPV) / PPV]; PCV = proportion of cases vaccinated, PPV = proportion of the population vaccinated.
- VE = 0 % no protection; VE = 100 % complete protection; negative VE: more cases among the vaccinated (look for bias).

## Worked example

- Trial: 10 000 vaccinated with 20 cases and 10 000 on placebo with 100 cases.
- ARV = 20 / 10 000 = 0.2 %; ARU = 100 / 10 000 = 1.0 %.
- VE = (1.0 − 0.2) / 1.0 = 0.80 = 80 %; the same as 1 − RR = 1 − 0.2 = 0.80.
- In the population (screening method): coverage PPV = 90 % and cases vaccinated PCV = 60 %.
- [0.60 / 0.40] × [0.10 / 0.90] = 1.5 × 0.111 = 0.167; VE = 1 − 0.167 = 0.833, rounded to 83 %.
- Reading: vaccinated people have 80 % less risk; 6 in 10 cases being vaccinated is no failure, since 9 in 10 people are vaccinated.

## How vaccine efficacy or effectiveness is estimated

| Method | Data needed | Formula | Limitation |
| --- | --- | --- | --- |
| Randomised trial | Cases among vaccinated and placebo | 1 − RR | Costly; selected population |
| Cohort | Follow-up of vaccinated and unvaccinated | 1 − RR | Confounding (people who get vaccinated look after themselves more) |
| Case-control (or test-negative) | Vaccination in cases and controls | 1 − OR | Control selection |
| Screening method | Coverage and % of cases vaccinated | 1 − [PCV/(1 − PCV)] × [(1 − PPV)/PPV] | Rough estimate; needs accurate coverage |

## Markers

- VE
- ARU
- ARV
- 1 − RR
- 1 − OR

## How to read it

- Vaccination programmes monitor effectiveness in the population to detect waning protection or cold-chain failures.
- The 1 − RR formula serves to evaluate any preventive measure (bed nets, prophylaxis, campaigns).
- Indirect protection of unvaccinated people (herd immunity) is not captured by individual VE.

## Common pitfalls

- Concluding a vaccine does not work because many cases are vaccinated, without accounting for coverage.
- Confusing efficacy (trial) with effectiveness (real practice).
- Reading VE as an absolute reduction: 80 % of a 1 % risk prevents 0.8 cases per 100 vaccinated.

## Memory aid

VE is the relative risk reduction in disguise: ‘one minus the RR’. And before panicking about vaccinated cases, check the coverage.

## Test yourself

Think of the answer, then tap the question to see it.

## If the attack rate is 5 % in unvaccinated and 1 % in vaccinated people, what is vaccine efficacy?

VE = (5 − 1) / 5 = 0.80 = 80 %; also 1 − RR = 1 − 0.2.

## In a case-control study, the odds ratio for vaccination is 0.30. What effectiveness does it estimate?

VE ≈ 1 − OR = 1 − 0.30 = 70 %.

## Why, with very high coverage, may most cases be vaccinated?

Because almost the whole population is vaccinated: even though each vaccinated person has lower risk, there are so many that they contribute many cases.

## What is the difference between vaccine efficacy and effectiveness?

Efficacy is measured in controlled trials; effectiveness in the population under real conditions of use.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/epidemiology/vaccine-efficacy-and-evaluating-preventive-measures/)

## Elsewhere in DurgiPedia

- [Vaccine types: live, inactivated, subunit, toxoid, mRNA, vector](https://durgical.com/en/immunology/vaccine-types-live-inactivated-subunit-toxoid-mrna-vector/)
- [Herd immunity and outbreak control](https://durgical.com/en/immunology/herd-immunity-and-outbreak-control/)
- [Colombia's PAI schedule (which vaccines, at what age)](https://durgical.com/en/immunology/colombia-s-pai-schedule/)

## Connections

## Related 5

- [Relative risk](https://durgical.com/en/epidemiology/relative-risk/)
- [Relative risk reduction](https://durgical.com/en/epidemiology/relative-risk-reduction/)
- [Odds ratio](https://durgical.com/en/epidemiology/odds-ratio/)
- [Case-control study](https://durgical.com/en/epidemiology/case-control-study/)
- [Basic reproduction number (R0) and herd immunity](https://durgical.com/en/epidemiology/basic-reproduction-number-and-herd-immunity/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Principles of Epidemiology | Lesson 3 - Section 6](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section6.html) (Centers for Disease Control and Prevention, 2012); [Chapter 6: Choosing effect measures and computing estimates of effect](https://www.cochrane.org/authors/handbooks-and-manuals/handbook/current/chapter-06) (Cochrane, 2024).

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/epidemiology/vaccine-efficacy-and-evaluating-preventive-measures/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/vaccine-efficacy-and-evaluating-preventive-measures/