According to WHO ,as of today, 67 vaccine are the pre-clinical stage (laboratory and animal testing).
Two vaccine candidates have already started being tested on small groups of 20 to 100 healthy human volunteers (the so-called
Phase 1 of human clinical trials).
One vaccine candidate from Beijing is being run simultaneously in both
Phase 1 and
Phase 2 (100 to 500 volunteers) stages. I am no expert ,but presumably this is being done to cut development time.
https://www.who.int/blueprint/prior...ronavirus_Landscape_nCoV_11April2020.PDF?ua=1
Phases of Clinical Trials before Regulatory Approval
Phase 1:
Phase 1 clinical trials test the medicine or vaccine in a small group of 20 to 100 volunteers who are usually healthy, but not always. The trial often takes place in a hospital.
The goals are to determine:
- If the medicine or vaccine is safe
- If there are any side effects
- How the medicine is broken down by and discharged from the body
- How much medicine is needed and how often
- For a vaccine, researchers see if it causes the desired response from the body’s immune system
Phase 1 trials can take from six months to one year to complete
Phase 2:
Phase 2 clinical trials test the vaccine or medicine in approximately 100 to 500 volunteers. In the case of medicines, volunteers usually have the disease or condition the investigational medicine is designed to treat. In vaccine studies, the volunteers are usually healthy.
The goals of this phase are to determine:
- How well the medicine or vaccine works
- If the medicine or vaccine is safe
- If there are any side effects
- How much of the medicine or vaccine is needed and how often
Phase 2 trials can take from six months to one year or more to complete.
Phase 3:
Phase 3 clinical trials can test the medicine or vaccine in 1,000 to 5,000 patient volunteers. For medicines, volunteers have the disease or condition the medicine is designed to treat. In vaccine studies, the volunteers may be healthy or have diseases or conditions. Phase 3 trials take place in hospitals, clinics or physician offices.
Researchers closely monitor patients at regular intervals to:
- Confirm that the medication or vaccine is effective
- Identify and monitor side effects
- Compare the medicine or vaccine to commonly used treatments
Phase 3 trials can take from one to four years to complete, depending on the disease, length of study and the number of volunteers.
https://www.merck.com/clinical-trials/about-clinical-trials.html