evidence: It varies from person to person, but how dramatic the initial response is does not necessarily relate to the long-term response. In the case of the two mRNA COVID-19 vaccines, well over 90% of people immunized developed the protective adaptive immune response while fewer than 50% developed any side effects, and most were mild. You may never know how strongly your body’s adaptive immune response is gearing up. What are possible side effects? Side effects are normal responses to the injection of a foreign substance. They include things like fever, muscle pain and discomfort at the injection site, and are mediated by the innate immune response. Neutrophils or macrophages in your body notice the vaccine molecules and produce cytokines – molecular signals that cause fever, chills, fatigue and muscle pain. Doctors expect this cytokine reaction to happen any time a foreign substance is injected into the body. In studies where neither recipients nor researchers knew which individuals were getting the mRNA vaccine or a placebo, approximately half of people aged 16 to 55 who received a SARS-CoV-2 vaccine developed a headache after the second dose.