You could tell people "we want to know how people respond to flu shots, so we're going to give you two shots, one on September 1 and one on October 1, and either the first will be a saline solution and the second will be the flu shot, or the first will be the flu shot and the second will be a saline solution; either way, you'll be vaccinated against the seasonal flu by October".
(You could also have a control group where you don't give any shots at all, and don't claim to have given any.)
(You could also have a control group where you don't give any shots at all, and don't claim to have given any.)