Most people do not have serious problems after being vaccinated, according to the CDC. We will understand more about mild side effects of the COVID-19 vaccine before we start to use it. Note that your arm may be sore, red, or warm to the touch. These symptoms usually go away on their own. Some people report getting a headache or fever when getting a vaccine. These side effects are a sign that your immune system is doing exactly what it is supposed to do. It’s working and building up protection against disease.