Dermatology MCQ - Viral Infections - COVID-19 vaccination and skin disease

A 42-year-old woman develops a painful, erythematous, indurated plaque at the site of her mRNA COVID-19 vaccine injection 7 days prior. The lesion is warm to touch and has gradually expanded. She has no fever or systemic symptoms. Which of the following is the most likely diagnosis and its recommended management?

A) Delayed cutaneous hypersensitivity reaction; topical corticosteroids
B) COVID arm; observation and symptomatic care
C) Cellulitis; oral antibiotics
D) Contact dermatitis; allergen avoidance
E) Erythema migrans; doxycycline

Correct Answer: B) COVID arm; observation and symptomatic care

Explanation

This presentation is classic for "COVID arm," also known as delayed large local reactions to COVID-19 mRNA vaccines (e.g., Pfizer-BioNTech, Moderna).

Key Clinical Features of COVID Arm:

• Timing: Typically occurs 5-10 days post-vaccination (delayed onset).

• Presentation:

  • Erythematous, indurated, painful plaque at the injection site.

  • The lesion may be pruritic or warm and can expand over several days.

  • No systemic symptoms (e.g., fever, malaise) typically accompany it.

• Resolution: Usually self-resolves within 3-5 days without intervention.

Pathophysiology:

• Type IV delayed hypersensitivity reaction (T-cell-mediated).

• Not related to infection; it is a robust local immune response to vaccine components (e.g., polyethylene glycol in mRNA vaccines).

Why Not the Other Options?

• (A) Delayed cutaneous hypersensitivity reaction: This is a broad term, but "COVID arm" is the specific entity for mRNA vaccines. Topical corticosteroids may be used, but first-line is observation.

• (C) Cellulitis: Would present with fever, spreading erythema, and systemic symptoms. COVID arm lacks fever and responds to observation, not antibiotics.

• (D) Contact dermatitis: Would occur sooner (hours to days) after exposure to an allergen and often itches intensely with vesicles.

• (E) Erythema migrans: The rash of Lyme disease, which is annular with central clearing, not a localized plaque at an injection site.

Management:

• Observation and reassurance: The condition is self-limiting.

• Symptomatic care: Cool compresses, oral antihistamines for pruritus, and NSAIDs for pain.

• Topical corticosteroids (e.g., triamcinolone 0.1% cream) can be used for severe cases.

• Important: This is not a contraindication to future vaccine doses. Patients can receive subsequent doses in the opposite arm.

Prognosis:
Excellent; resolves without sequelae. Does not indicate vaccine failure or increased risk of other adverse events.

Note: "COVID arm" is a benign and self-resolving reaction. It should be distinguished from cellulitis to avoid unnecessary antibiotics. Always consider the timing and absence of systemic symptoms when diagnosing.