The rigorous process of testing vaccines for safety and efficacy
Before any vaccine reaches the public, it must undergo rigorous testing in clinical trials. These trials answer critical questions: Is the vaccine safe? Does it work? Who benefits most? What are the side effects?
Clinical trials follow a stepwise process that has been refined over decades. Each phase builds on the previous, with increasingly larger and diverse populations studied. This methodical approach ensures that by the time a vaccine is approved, we have substantial evidence of its safety and effectiveness.
This section explains how clinical trials work, what happens at each phase, and why the process matters for vaccine confidence.
20–100 participants. First human testing. Primary goal: assess safety and identify optimal dosage. Participants are healthy adults with no prior exposure to the pathogen. Usually lasts several months.
Several hundred participants. Further evaluates safety and measures immune response (immunogenicity). May compare different formulations or dosing schedules. Usually lasts months to a couple of years.
Thousands to tens of thousands of participants. Compares vaccinated vs. unvaccinated (control) groups to measure efficacy against the target disease. Large enough to detect rare side effects. Usually lasts 1–4 years.
Ongoing after regulatory approval. Monitors safety in millions of real-world users. Identifies very rare adverse events and long-term outcomes that could not be detected in pre-approval trials. Continues indefinitely.
Vaccine trials use specific methodological approaches to ensure reliable, unbiased results:
Participants are randomly assigned to vaccine or control group using computer-generated lists. This ensures both groups are similar in age, health status, and other factors — preventing selection bias.
In single-blind trials, participants don't know if they get vaccine or placebo. In double-blind trials, neither participants nor researchers know — preventing both placebo effects and researcher bias.
The control group receives either a placebo (inactive substance) or an existing vaccine for comparison. This allows measurement of the vaccine's effect above any placebo response.
Researchers must specify in advance exactly how they'll analyze data and what results would constitute "success." This prevents cherry-picking favorable results after the fact.
An "endpoint" is the specific outcome a trial is designed to measure. The choice of endpoint critically affects what conclusions can be drawn:
As an example, the Pfizer-BioNTech COVID-19 Phase III trial reported 95% efficacy against symptomatic infection (Polack et al., 2020) — but this figure does not necessarily reflect efficacy against transmission or asymptomatic infection. Understanding what endpoints a trial measured is key to interpreting results correctly.
Learn more about trial endpointsFDA. "Vaccine Development: 101." U.S. Food and Drug Administration. https://www.fda.gov/vaccines-blood-biologics/development-approval-process-cber/vaccine-development-101
WHO. "Guidelines on Clinical Evaluation of Vaccines: Regulatory Expectations." World Health Organization. 2016. https://www.who.int/publications/i/item/guidelines-on-clinical-evaluation-of-vaccines-regulatory-expectations
Polack FP, et al. "Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine." New England Journal of Medicine. 2020;383:2603-2615. https://doi.org/10.1056/NEJMoa2034577
Plotkin S, Robinson JM, et al. "The complexity and cost of vaccine manufacturing — an overview." Vaccine. 2017;35(33):4064-4071. https://doi.org/10.1016/j.vaccine.2017.06.003
ClinicalTrials.gov. U.S. National Library of Medicine. https://clinicaltrials.gov