Phase 1
Tests safety in a small group of volunteers, typically a few dozen to about 100 people. Researchers watch closely for side effects and work out a safe dose range before moving any further.
Clinical Evidence & Safety
Before a vaccine reaches the public, it moves through a series of tests, each one asking a different question. Here's what each phase actually checks.
From First Trial to Ongoing Watch
Every vaccine moves through the same sequence of checkpoints, each testing something the one before it didn't.
Phase 2
Expands to a larger group, often several hundred people, and starts studying how well the vaccine trains the immune system — not just whether it's safe.
Phase 3
Tests efficacy and safety across thousands, sometimes tens of thousands, of people from different backgrounds and health conditions, comparing outcomes against a control group.
Post-Approval Monitoring
Ongoing safety surveillance systems continue collecting real-world data after a vaccine reaches the public, watching for patterns that only show up at larger scale or over a longer time than any trial could capture.