What are the side effects of COVID-19 vaccines? Many people do not experience any side effects while others experience mild side effects including pain or swelling at the injection site, headache, mild nausea, muscle aches or chills, or even a mild fever. Usually these side effects last just one to three days. Those who get vaccinated can sign up for v-safe, a smartphone-based tool that uses text messaging and web surveys to provide personalized health check-ins after you receive a COVID-19 vacc