There is a potential for injection site reactions (redness, swelling, and pain) as well as fever, fatigue, headache, chills, vomiting, diarrhea, muscle pain, and/or joint pain. Similar adverse reactions are commonly seen with other vaccines as well. To ensure the vaccine is effective, it is important that you receive the second dose (for a two-dose vaccine series such as Pfizer & Moderna) even if you experience side effects after the first dose.