Final answer:
The rare reaction following the MMR vaccine is anaphylaxis.
Step-by-step explanation:
The rare reaction following the MMR vaccine is Anaphylaxis. Anaphylaxis is a severe and potentially life-threatening allergic reaction that can occur shortly after receiving a vaccine. It is characterized by symptoms such as difficulty breathing, swelling, and a drop in blood pressure. To counteract the effects of anaphylactic shock, epinephrine is used as the first-line treatment.