How are vaccines tested to ensure they are safe and effective?

Vaccine candidates are tested many times in progressively larger clinical trials to ensure they are safe and effective.

Illustration of Phases 1, 2, and 3 followed by vaccine approval.

The first clinical studies in humans are phase 1 clinical trials which primarily assess the initial safety of the vaccine and identify the correct dose. Phase I clinical trials typically enroll tens or hundreds of healthy adults.

Vaccine candidates that show promise then move on to phase 2 clinical trials. Phase 2 clinical trials are larger studies designed to assess safety and to determine how well the vaccine activates the immune system response in healthy individuals. Typically hundreds or thousands of people participate in phase 2 trials. Sometimes phase 1 and phase 2 trials are combined into phase 1/2 trials where the aims of both the phase 1 and phase 2 trials are achieved. 

Phase 3 clinical trials are the final test of the vaccine prior to approval. In phase 3 trials, the vaccine is given to tens of thousands of healthy volunteers to determine whether the vaccine protects them from becoming sick. Typically, phase 3 trials enroll those at highest risk of the disease. If the vaccine proves to be safe and effective throughout these trials, all of the evidence will be reviewed by regulatory agencies who will determine whether to approve it for widespread use.

Once approved, vaccines continue to be monitored for safety and effectiveness. In addition, once approved, public health officials must implement detailed and transparent plans to deliver vaccines to individuals and communities who need them.

How long will it be until I can get a COVID-19 vaccine?

The complete process of testing a new vaccine in clinical trials to make sure that it is safe and effective often takes more than 10 years! 

However, COVID-19 vaccines are on an accelerated timeline because of the urgency of the pandemic.

How is it possible to accelerate vaccine approval WITHOUT compromising safety and efficacy?

This is a very important question! Never before have so many vaccine candidates been developed for a single disease in such a short period of time, using both existing and new technologies. In addition, an incredible amount of funding, time, and effort is being devoted to COVID-19 vaccine development at a scale far beyond the resources devoted to any previous vaccine. Learn more about developing COVID-19 vaccines at pandemic speed and accelerated vaccine development timelines.

While some estimates have suggested that safe and effective COVID-19 vaccines could be available in 12-18 months, more evidence is needed to confirm the expected timeline. It is also important to note that there won’t be enough vaccine doses to vaccinate the entire global population immediately.