Immunocompomised people can be mobile and still lack the defenses necessary to tolerate a vaccine. I know one personally; he jogs by my house every evening, but by his own estimation, a flu vaccine would almost certainly land him in the hospital.

Pregnancy is one. It wasn't tested on them.

Allergies to one of the components? Raw egg allergies can preclude some vaccinations.

Neither vaccine under consideration today uses eggs or triggers the egg allergies that the most common influenza vaccines can. The general population degree of allergic reactions to vaccines is 1-2 people per million, with hives as the most common outcome; or between 1500 and 3000 people across the United States in a 100% vaccination scenario, of which many would get hives and no worse.

Egg allergies trigger careful treatment protocols surrounding many influenza vaccines, but do not in any way preclude vaccination or exempt one from it. Specifically, if your allergic reaction to eggs is severe enough to impair breathing, delivery of the vaccine must be delivered in a monitored medical setting that can treat you if an allergic reaction that impairs your breathing. You are only precluded from receiving an egg-based vaccination once you have experienced a severe reaction from an egg-based vaccine. Egg allergy alone is not sufficient.