If you don’t experience any side effects, that doesn’t mean that the vaccine didn’t work. Once you’re fully vaccinated, if you were infected by the virus causing COVID-19, your immune system would be ready to launch an even larger and more powerful attack to protect you.
Think of it this way: The body’s response to the vaccine is like a training mission for the real fight. This process teaches your immune cells to recognize and respond to an “invader.” That’s why you might experience some side effects. The immune system automatically launches a small-scale attack against it. When you get vaccinated, your immune system recognizes something as being foreign. If you get side effects, they are a good sign - they indicate that the vaccine is working by triggering the immune system. Back to top What do the side effects mean? Side effects are more common, and may be more pronounced, after the second dose of Pfizer-BioNTech COVID-19 vaccine (Comirnaty®) or Moderna COVID-19 vaccine. Severe side effects are very rare and treatable. The most common side effects include a sore arm, fatigue (feeling tired), headache, aches, and fever. The good news is that a ll three COVID-19 vaccines have similar side effects that are mild and don’t last long - about one to three days. Like with any vaccination, there is potential for side effects after you get your shot. Find a Researcher Programs & Centers Postdoctoral Positions Research Topics.Gerstner Sloan Kettering Graduate School of Biomedical Sciences.Find a Doctor Refer a Patient Newsletter Sign Up Clinical Trials.Anesthesiology & Critical Care Medicine.Find a Doctor Make an Appointment Visitor Information FAQ.Find a Doctor Make an Appointment Visitor Information MyMSK.