The problem, as I understand it, is that a bad vaccine may end up causing people to become even more susceptible/vulnerable to the virus (or other illnesses). In that event, there is a LOT to lose by giving them something like this. Here is an article that touches on that theme: https://www.reuters.com/article/us-health-coronavirus-vaccines-insight/as-pressure-for-coronavirus-vaccine-mounts-scientists-debate-risks-of-accelerated-testing-idUSKBN20Y1GZ