A booster shot is an additional dose of vaccine given after the initial vaccination series to strengthen or extend immune protection. Boosters remind the immune system about a pathogen it was previously trained to fight, improving the body's ability to respond quickly if exposed. The need for boosters varies by vaccine—some (like tetanus) require boosters every ten years, while others (like measles) typically provide lifetime protection from childhood doses. COVID-19 vaccines have required boosters to maintain protection, particularly against new variants. Public health authorities provide guidance on recommended booster schedules based on evolving evidence about immunity duration.