Table 1.

Differential Diagnosis of Delayed Hypersensitivity Reaction to Vaccines.

Cause	Clues
Delayed hypersensitivity to vaccines	Erythema, induration approximately 7 days after vaccine, does not progress
Cellulitis	History of trauma, irregular borders with progressive erythema and systemic symptoms
Early abscess	May be identical to cellulitis initially, but nodule or area of fluctuance develops
Hypersensitivity to insect bite	History of a bite or puncta observed at site of erythema, no systemic signs
Fixed drug reaction	History of recurrent rash at same site with medication use, does not progress, resolves with hyperpigmentation
Urticaria	Multiple lesions are more likely, lesions typically last less than 24 hours in same location
Erythema migrans	Typical rash of Lyme disease occurs 5 to 10 days after tick bite evolves over several weeks. Blanching, erythematous patches sometimes with a bullseye appearance