Vaccines protect you from infection by essentially training your immune system how to effectively respond to a specific microbe (usually a bacterial, viral or fungal strain). Over time, this immune memory may diminish and reintroduce the risk for acquiring the infection. In these circumstances, repeating vaccination will "boost" your immune system memory to improve detection and elimination of the infectious threat. Various factors assist in determining if a booster vaccine is necessary, which may include the infection targeted by the vaccine, expert recommendations and use of blood samples to test for immune system antibodies. – Jace Knutson, Clinical Pharmacist