The reason for avoiding antivirals such as amantadine around the time one gets "vaccinated", especially via the nasal route, is that the flu vaccine is a live, attenuated (weakened, non-infectious) virus. Amantadine will diminish the absorption of the virus across the nasal mucosa because its mechanism of action is to prevent viruses from crossing cell membranes.

If one receives the vaccine parenterally (injection) the weakened viruses get access to the cells of your body more quickly and efficiently than by the nasal route, and the presence of the amantadine has less effect.

Robert