It probably won't even show up. There have been some unintentional experiments with the flu vaccine, such as the year production got botched or that the people in charge of predicting which strains to include guessed wrong. There were no noticeable increases in sickness or morbidity in those years.

Don't take this too far; the data is very noisy and maybe it was there and we just didn't see it. Flu vaccines do seem to work in labs and in small community experiments.