Vaccines are one of the most effective tools we have in preventing and reducing the burden of infectious diseases. In the midst of the COVID-19 pandemic, vaccines are once again poised to change the tide in our favor in the fight against a deadly virus. But how exactly do vaccines work? And are they safe? “You can think of your body’s immune system like an orchestra,” says Yale immunobiologist Akiko Iwasaki, PhD. “The different functions of the immune response are like different instruments. And vaccines work like sheet music for the orchestra, telling the immune system what to do and how to do it.” Different viruses require different types of immune responses in order to confer protection, and some of them can be complex. But with SARS-CoV-2, the virus that causes COVID-19, a simple type of response is all that’s needed to prevent infection. “You just need to trigger an antibody response where the antibodies bind to the surface of the virus and prevent it from entering our cells,” says Ruslan Medzhitov, PhD, professor of immunobiology. “And these types of vaccines tend to be extremely safe.” In addition to the inherent safety of this kind of “training” for the immune system, experts emphasize that the expedited timeline of COVID-19 vaccine development is not a reflection of lax safety standards. “Before a vaccine is approved, it goes through a rigorous amount of testing for safety and efficacy,” says Iwasaki. “So, once a vaccine is made to be publicly available, we should be lining up.” Watch this video to learn more about the fundamentals of how vaccines work, how they are developed, and the importance of vaccination for public health.