Some have suggested we can end vaccine trials early because of good results:

Efficacy is not the real issue: Most of the vaccines will probably work, because they are based on the spike protein with the RNA sequence published by the Chinese. The real issue is safety. Once people are immunized, how will their immune system react to another exposure? In the case of the toxic shock tampon staph bacteria, menstruating women gained natural immunity during one period and then the next month their immune systems went completely haywire. Most of them developed an autoimmune disease like lupus or MS.

So far two people in the Oxford trial have developed a spinal symptom that is often a part of MS. What happens months later when the vaccinated people are exposed again? This is why Phase III trials take a long time. If you stop it based only on efficacy, you can miss long-term safety problems. It would be horrible if we end up with 30,000 people who are doomed to lupus or MS upon later virus exposure.