Experts say side effects from COVID-19 vaccines won't last long and they are actually proof that your immune system is working the way it's supposed to. "With the first dose, you are having to generate an immune response from the ground up," said Michael Worobey, head of the Department of Ecology and Evolutionary Biology at the University of Arizona. The body produces antibodies, but also starts to generate immune cells called B cells to make targeted antibodies – and that takes time, Worobey said.