Vaccine side effects happen because the vaccine activates your immune system. Some side effects are a physical response to the injection (such as soreness at the injection site). Either way, there are many ways to relieve side effects after a vaccine to make the experience more comfortable.
Centers for Disease Control and Prevention. (2020). Possible side effects from vaccines.
Centers for Disease Control and Prevention. (2023). Possible side effects.
U.S. Department of Health and Human Services. (2022). Vaccine side effects.