It takes about two weeks after receiving a flu shot for the body to build up enough antibodies to provide protection against the flu. During this two-week period, a person is not yet fully protected and could still contract the flu. The flu shot is designed to protect against multiple strains of the flu virus and aims to reduce the risk and severity of flu illness once effective immunity is developed. This is why it is recommended to get the flu shot before flu season peaks, ideally by the end of October, since flu season typically intensifies between December and February. The protection provided by the flu vaccine generally lasts for the duration of the flu season, which can be several months.