How are vaccines tested and developed?
Vaccines are first tested in people during phase 1 clinical trials. Phase 1 clinical trials assess the initial safety of the vaccine candidate and often compare different vaccine doses. Phase 1 clinical trials typically enroll dozens or hundreds of healthy adult volunteers.
If the results of the phase 1 clinical trials are promising, then the vaccine candidate will move on to phase 2 clinical trials. Phase 2 clinical trials are larger studies designed to continue to assess safety and to test the immune response to the vaccine in healthy people, often comparing those who are randomly assigned to receive the vaccine to those who remain unvaccinated. Typically, hundreds or thousands of volunteers participate in phase 2 clinical trials. Sometimes, phase 1 and phase 2 trials are combined into phase 1/2 clinical trials where the aims of both the phase 1 and phase 2 trials are achieved.
If the results of the phase 2 clinical trials are promising, then the vaccine candidate will move on to phase 3 clinical trials. Phase 3 clinical trials are so important, because they can provide the strongest evidence about how safe and protective the vaccine candidate is. In phase 3 clinical trials, thousand, tens of thousands, or sometimes more than 100,000 eligible volunteers participate and are randomly assigned to receive the vaccine or remain unvaccinated. Phase 3 clinical trials allow researchers to determine whether the vaccine protects those who are vaccinated from becoming sick compared to those who remain unvaccinated. By evaluating the evidence from phase 3 trials, we can understand a vaccine candidate’s initial safety profile and how well the vaccine protects. Sometimes phase 2 and phase 3 trials are combined into phase 2/3 clinical trials where the aims of both the phase 2 and phase 3 trials are achieved.
If the evidence from phase 1, 2, and 3 clinical trials demonstrates that the vaccine is safe and effective, this evidence will be reviewed by national authorities and regulatory agencies who will decide whether the vaccine should be approved and authorized for use.
Even after completing phase 1, 2, and 3 trials, and even after vaccines are authorized for use, vaccines continue to be monitored to ensure that they remain safe and effective.
COVID-19 vaccine clinical trials are accepting volunteers in dozens of countries around the world. Find out which trials are underway in your country .