You need both groups to behave in the same way. Phase III trials are enrolling enough people (tens of thousands) so that a sizeable number will likely contract

You need both groups to behave in the same way. Phase III trials are enrolling enough people (tens of thousands) so that a sizeable number will likely contract the virus within a few months. They specifically run these trials in places where the virus is spreading (this is why Chinese vaccines are being tested outside China, in places like Brazil). > Would the researchers not want people to go out and live their normal lives? That would be great for the trials (assuming both the vaccinated and placebo groups behaved identically). But most people would consider it unethical for researchers to encourage the trial participants to increase their likelihood of exposure to the virus. People who sign up for vaccine trials are not signing up to get CoVID-19.