Which of the following adverse events is considered a systemic reaction to a vaccine?

A fever is considered a systemic reaction to a vaccine because it results from the body's overall immune response to the vaccine. When a vaccine is administered, it stimulates the immune system to produce a response, which can include the release of pyrogens that elevate body temperature. This is a common and expected reaction indicating that the immune system is working to develop immunity against the pathogen contained in the vaccine.

In contrast, tenderness primarily reflects a localized reaction at the injection site, not a systemic response affecting the entire body. Wheezing can indicate a respiratory reaction that may signify an allergic response but isn't classified as a systemic reaction in the broader sense. Angioedema, while serious, is also related to an allergic reaction that is typically localized and involves swelling of deeper layers of the skin, not indicative of a generalized immune response like fever. Thus, fever aligns with the characteristic of a systemic reaction to vaccination.