What type of reaction does the term "myalgia" refer to in the context of vaccines?

Myalgia refers to muscle pain, which is primarily considered a systemic reaction. In the context of vaccines, systemic reactions usually involve the body responding more broadly to the vaccination, which can include symptoms such as fever, fatigue, and muscle aches. Myalgia as a reaction is indicative of the immune system's response to the vaccine as it stimulates an immune reaction throughout the body, rather than just at the site of injection.

Local reactions typically involve symptoms confined to the area where the vaccine was administered, such as soreness, redness, or swelling. Allergic reactions are specific immune responses caused by allergens, which can vary significantly from vaccine to vaccine, but myalgia is not synonymous with an allergic response. Adverse reactions cover a wide range of potential reactions, but myalgia specifically aligns with systemic reactions, particularly because it reflects an inflammatory response that can occur beyond just localized symptoms. This understanding helps to clarify why myalgia is categorized as a systemic reaction in the context of vaccination.