What symptom may occur quickly after receiving a vaccine and indicates anaphylaxis?

A weak and rapid pulse is a critical symptom that may indicate anaphylaxis after receiving a vaccine. Anaphylaxis is a severe allergic reaction that can occur swiftly, often within minutes of vaccination. It involves systemic responses that can affect the cardiovascular system, leading to hypotension and a weak, rapid pulse. This symptom signifies that the body is in distress and is responding to an allergen in a life-threatening manner.

In contrast, other symptoms such as a sudden headache, localized swelling, and continuous bleeding may not necessarily indicate anaphylaxis. A sudden headache can arise from various causes, and localized swelling, while it can occur at the injection site, does not reflect the systemic severity that anaphylaxis entails. Continuous bleeding is not a recognized symptom of anaphylaxis and could indicate a different medical issue. Therefore, the weak and rapid pulse stands out as a significant and alarming symptom indicative of a potential anaphylactic reaction.