What vaccine trials measure — and why the choice of endpoint matters for interpreting results
A clinical trial endpoint is a pre-specified outcome that is measured to determine whether a vaccine (or other treatment) has achieved its intended effect. Endpoints must be defined before the trial begins — changing endpoints after seeing data is considered scientific misconduct and is prohibited by regulators.
Endpoints fall into two main categories: primary endpoints (the main outcome the trial is designed to measure) and secondary endpoints (additional outcomes tracked alongside the primary).
The single most important outcome the trial is powered to detect. The trial's sample size is calculated specifically to detect a meaningful difference in this outcome. For most vaccine trials, the primary endpoint is disease prevention.
Additional outcomes tracked alongside the primary. May include severity of disease in breakthrough cases, duration of protection, or specific adverse event rates. Results are informative but the trial is not statistically powered to confirm these.
Adverse events are tracked systematically throughout all trial phases. Serious adverse events (SAEs) must be reported to regulators within 7–15 days of occurrence regardless of trial phase.
In some trials, an immune response measure (e.g. antibody titre) is used as a surrogate for disease prevention when direct efficacy measurement would take too long. Surrogate endpoints require prior validation that the immune marker predicts protection.
The most common primary efficacy endpoints in vaccine trials:
The most common endpoint: does the vaccine reduce the rate of confirmed symptomatic disease compared to placebo? Used in most COVID-19, influenza, and RSV vaccine trials.
Does the vaccine reduce hospitalisation, ICU admission, or death? Often a secondary endpoint. Considered more clinically meaningful than symptomatic disease prevention for high-risk populations.
Does the vaccine prevent infection entirely — including asymptomatic infection? Harder to measure, requires active screening. Few vaccines achieve complete sterilising immunity.
Does the vaccine produce a measurable antibody or T-cell response? Used when direct efficacy measurement is not feasible. Must be validated against disease prevention data.
Vaccine efficacy (VE) is calculated as:
Where Risk Ratio = attack rate in vaccinated group ÷ attack rate in placebo group.
Example: If 1% of placebo recipients get disease vs. 0.1% of vaccinated recipients:
A 90% efficacy means vaccinated people are 90% less likely to develop disease than unvaccinated people under trial conditions.
Important: Trial efficacy does not equal real-world effectiveness. Real-world effectiveness is measured separately in Phase 4 studies.
FDA and WHO guidance on minimum acceptable efficacy:
For COVID-19 vaccines, FDA required a minimum primary efficacy of 50% with a lower bound of the 95% confidence interval above 30%. This was set as the minimum bar for Emergency Use Authorization.
WHO's preferred vaccine efficacy threshold for most vaccines is ≥50%, with country-specific guidance varying by disease burden.
A point estimate of 70% efficacy with a 95% CI of 20%–90% is less reliable than 70% efficacy with a 95% CI of 60%–78%. Wide confidence intervals indicate uncertainty — often due to insufficient endpoint events.
FDA. "Guidance for Industry: COVID-19 Vaccines." fda.gov, 2020. View source →
WHO. "WHO preferred product characteristics for COVID-19 vaccines." who.int, 2020. View source →
Prentice RL. "Surrogate endpoints in clinical trials." Statistics in Medicine, 1989.
Plotkin S et al. Vaccines. 7th edition. Elsevier, 2018.
ICH E9 Statistical Principles for Clinical Trials. ich.org, 1998. View source →
Learn how vaccine trials progress from pre-clinical testing through Phase 1, 2, 3, and post-market surveillance.
The full picture: how vaccine clinical trials are designed, regulated, and what results mean for public health.
Understand the critical difference between what a vaccine does in a controlled trial and how it performs in the real world.