Vaccines work by teaching our bodies how to recognize and fight off infections from things like viruses and bacteria. Although vaccines can make you feel a little sick, this is usually a good sign. There are special cells in the body that are trained to fight infection, and the vaccine helps them to train better. Feeling sick after a vaccine shows that your body has learned how to fight infection and is launching an immune response. That’s how your body learns to fight off infection. Vaccines don’t give you the disease. Even the best vaccines are not 100% effective at preventing illness; they can lose effectiveness over time. That’s why it is important to stay up to date with new vaccine updates as they become available.