Which of the following is NOT a symptom of a pyrogenic reaction?

A pyrogenic reaction typically arises due to the administration of substances that trigger the body's immune response, often resulting in a range of symptoms. Fever is commonly associated with these reactions, as the body raises its temperature to fight off perceived threats. Chills often accompany fever as the body feels cold despite an increased internal temperature, indicating that the body is attempting to regulate its temperature. Vomiting can also occur as part of a pyrogenic reaction, potentially due to systemic inflammatory responses or discomfort.

Headache, while it can be experienced in various illness contexts, is not a definitive symptom of a pyrogenic reaction itself. While a fever or infection can lead to headaches, it is not a direct result of the pyrogenic activity but rather a secondary effect of the overall illness process or dehydration caused by fever. Therefore, it is reasonable to conclude that headache is less characteristic of the direct physiological response associated with pyrogenic reactions compared to the other listed symptoms.