What vaccine trials actually measure — and why it matters
When you hear that a vaccine "showed 95% efficacy," it's important to ask: 95% efficacy at doing what? The answer lies in the trial's endpoints — the specific outcomes the study was designed to measure.
A vaccine can have excellent efficacy at preventing symptomatic disease but much lower efficacy at preventing infection. It can prevent severe disease while allowing mild infections. Understanding endpoints helps you correctly interpret what trial results mean — and what they don't tell us.
This page explains the different types of endpoints used in vaccine trials and why the choice matters for interpreting results.
The main question the trial is designed to answer. Regulatory approval typically depends on meeting the pre-specified primary endpoint. Only one or a few primary endpoints are allowed to prevent cherry-picking favorable results.
Additional outcomes of interest that are measured but not the main basis for approval. These provide valuable additional information but are not used to determine regulatory success.
Critical Point: If a trial doesn't meet its primary endpoint, it generally cannot be approved based on secondary endpoints alone. This is why knowing what the primary endpoint was is essential for interpreting results.
Laboratory-confirmed infection WITH symptoms (fever, cough, etc.). This is the most common primary endpoint. A vaccine with 95% efficacy against symptomatic infection means 95% fewer vaccinated people develop symptoms compared to unvaccinated people.
The most important public health outcome. Preventing severe disease, hospitalization, and death is the primary goal of vaccination. This endpoint typically requires fewer cases to demonstrate efficacy but takes longer to accumulate.
Any laboratory-confirmed infection, regardless of symptoms. Requires intensive testing of all participants (not just those with symptoms). Measuring this is important for understanding transmission potential.
Whether vaccinated people can spread the pathogen to others. This is very difficult to measure directly and is usually inferred from infection data. COVID-19 vaccines initially showed high efficacy against symptomatic disease but lower efficacy against asymptomatic infection and transmission.
Immune responses measured in blood: antibody levels (titers), neutralization activity, T cell responses. Often used as "correlates of protection" — markers protection. Sometimes that predict clinical used as surrogate endpoints when clinical endpoints are impractical.
The choice of endpoint determines what we can conclude from a trial — and what we cannot:
| If Endpoint Was... | Efficacy Could Be... | But We CANNOT Conclude... |
|---|---|---|
| Symptomatic infection | 95% | 95% protection against hospitalization |
| Severe disease | 98% | 98% protection against mild infection |
| Infection (any) | 70% | 70% protection against transmission |
This is why it's essential to read beyond the headline efficacy number and understand what outcome was actually measured.
• FDA. "Development and Licensure of Vaccines to Prevent COVID-19." U.S. Food and Drug Administration, 2020.
• WHO. "Guidelines on Clinical Evaluation of Vaccines." World Health Organization, 2017.
• Mehrotra DV, et al. Clinical Endpoints for Evaluating Efficacy in COVID-19 Vaccine Trials. Ann Intern Med. 2021;174(2):221-228.