[nshepperd]

To add to the other comments: because giving a placebo is essential for the experimental results to have any validity, not giving a placebo would be imo way more unethical, since it would mean you are exposing people to a medical risk (experimental vaccine) for no scientific benefit.

(Needless to say, running an invalid vaccine trial also has very bad consequences: it would delay the actual deployment of a working vaccine to the general public, resulting in millions of unnecessary person-hours of potential virus exposure.)