Not OP but from the reports I have read, people who have been given placebos will be informed if the actual vaccine is found be really effective. This is for ethical concerns (in the sense that it would be unethical to not vaccinate people who have got a placebo if the real vaccine is really effective)
For some history on the matter of ethics in medical research, the 'Tuskegee Experiments' was among the most infamous biomedical research study in U.S. history [0].
