What is an inactivated (killed) vaccine?

Inactivated or "killed" vaccines are made with pieces of a virus that has been killed with heat, chemicals, or radiation. Consequently, inactivated ("killed") vaccines don't have the risk of mutating and reverting back to their virulent form. They work by stimulating B cells to produce antibodies. And they typically don't require refrigeration. But inactivated vaccines also have some drawbacks. The major one is they often are not as potent as live attenuated vaccines because they only stimulate the production of antibodies and don't engage other aspects of the adaptive immune system, such as the memory T cells. So to maintain immunity, you need periodic booster shots.