A 71-year-old woman underwent a posttherapy radioiodine scan after 131I administration for papillary thyroid carcinoma. On day 1 of the posttreatment hospital isolation, the patient received a Pneumovax injection in the left deltoid muscle. A localized hypersensitivity reaction ensued with development of soft tissue edema and erythema at the Pneumovax injection site. This localized reaction persisted up to the subsequent posttherapy radioiodine scan 5 days after injection. Increased activity at the left deltoid region suggests a false-positive finding related to radiotracer sequestration due to inflammation from a localized postvaccination hypersensitivity reaction.