Benign hypersensitivity reactions to smallpox vaccine.

Background: With the reinstitution of smallpox vaccinations, physicians are seeing significant numbers of adverse events for the first time since the 1980s. The most common adverse events seen in our large military population are benign. We observed a clinically and histopathologically distinct reaction pattern that has not been fully characterized previously.

Methods: All smallpox-vaccinated patients at Fort Hood, Texas with adverse cutaneous reactions were referred to the dermatology clinic at Darnall Army Community Hospital. Patients were evaluated by a staff dermatologist who performed a skin biopsy and took clinical photographs. If the patients had intact vesicles or pustules, direct fluorescent antibody testing, viral and bacterial cultures, and polymerase chain reaction (PCR) assays were also performed.

Results: Three hypersensitivity reaction patterns were seen: exanthematous, erythema multiforme-like (EM-like), and urticarial. The patterns had distinct clinical and histopathologic findings. Of the 11,058 vaccinees, six had the exanthematous reaction pattern, two had the urticarial reaction pattern, and one had the EM-like pattern.

Conclusions: We describe a new exanthematous type of hypersensitivity reaction to the smallpox vaccine. Hypersensitivity reactions occur at a rate higher than previously reported. In a carefully screened military population, these three hypersensitivity reactions are much more common than life-threatening or serious reactions. Although the reactions have distinct clinical and pathologic features, they are all characterized by mild or absent systemic symptoms and a benign outcome.