How we know vaccines work: The phases of a clinical trial
20 October 2021
Healthy Pilipinas

All the available COVID-19 vaccines approved by the Food and Drug Administration (FDA) have been proven to be safe and effective. How do we know this?

Like all medical products, every vaccine is subjected to an extensive series of screenings and evaluations before it can be given to the population. This rigorous process is called a clinical trial.

Before any clinical trial of a vaccine, scientists first carry out preclinical research on animals. This is meant to determine which antigen—an immune response trigger—will work against a disease or virus.

Only when the proposed vaccine is able to produce an immune response in animals will it begin undergoing clinical trials in humans. The trials are typically carried out in three to four phases, each one building on the results of previous phases.


In the first phase of clinical trials, a small group of volunteers receives the vaccine. This confirms that the vaccine is safe for human use and causes an immune response. It also helps determine the correct dosage. Phase 1 trials typically involve around 20 to 100 participants, and the volunteers are healthy, young adults. They are asked to report any issues and side effects.


Several hundred volunteers participate in Phase 2 trials. In this phase, the researchers continue to test the vaccine’s safety, side effects, and ability to produce an immune response. Multiple tests are performed on different age groups using different vaccine variations. Usually, about half of the participants are given a placebo vaccine in order to see whether any changes in the vaccinated group were really caused by the vaccine.


In this phase, the vaccine is given to thousands of volunteers with the same goal of determining whether it is safe and effective. What sets this phase apart is that the trials are now performed across countries and multiple demographics within a country. This ensures that the vaccine works well in the much larger population. Similar to Phase 2, different groups of participants are given either the real vaccine or a placebo, and they won’t know which one they received until the trial is over.

Even after a vaccine finishes clinical trials and results are available, there are still processes and regulatory measures countries follow before approving its use. Health officials in each country review the data and decide whether to authorize the vaccine. And as the vaccines are rolled out around the world, researchers continue to monitor their safety and effectiveness, including long-term side effects and risk factors (this can be part of Phase 4, or post-marketing surveillance trials). This continued monitoring ensures that the vaccine is still safe to use.

Johann Ulrik Go/ CONTRIBUTED