It's how all vaccines work. The antigens in your body that are produced because of the vaccine do nothing until you get infected. Once you are infected the antigens get to work producing an immune response to kill the virus. Without the antigens it takes the body a much longer time for the body to produce that response allowing the virus to replicate out of control.

How well the vaccine works against symptoms and infectivity depend on the amount of antigens produced, the strength of the response and the strength and reproduction capability of the virus.

Why Gupta is peddling misinformation is anybody's guess.