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.

When and where can I get a COVID-19 vaccine?

There are currently many approved vaccines that can prevent COVID-19. Although some international organizations, such as the World Health Organization (WHO) and the Africa Regulatory Taskforce (ART) might pre-approve COVID-19 vaccines for emergency use to speed up approval by their member countries, vaccines are most typically approved on a country-by-country basis, which means different countries have different vaccines approved for use in their country at different times.

Due to the limited availability of doses at present, many countries have established priority groups that will be vaccinated first before sufficient vaccination doses are available to all who need them. Frequent examples of priority groups include health care workers, essential workers (police officers, firefighters, teachers), elderly people, and people living with a chronic disease. The composition and ranking of those priority groups varies by country and different areas of the same country might have different priority groups.

To find out more about what is happening country-by-country, click on any country on the approvals map to see a list of vaccines that are approved/authorized or made available under any regulatory mechanism in that country. Then check with the ministry of health, national health authority, local health department, or health care team in your area to find out when you will be eligible for vaccination. Unfortunately, we do not have information about who is eligible for vaccination in each country, so be sure to check with local authorities.

Interested in how many COVID-19 vaccine doses have been given in each country? Check out this excellent resource from Our World in Data to explore trends in vaccination globally.

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 lightning-fast 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.