Why is it important to get a flu shot if you have heart disease? When is it too late to get it?Flu season can begin as early as October and extend as late as May, but typically peaks from December to February. Antibodies to the flu peak four to six weeks after getting vaccinated and then slowly decline for six months. The Centers for Disease Control & Prevention recommends that everyone over 6 months of age without a specific reason not to get vaccinated, such as a history of allergic reactions to the shot, get vaccinated by the end of October. However, getting vaccinated any time before January can still be beneficial.