Why Do Some People Get a Rash After COVID Vaccine and When Is It Serious?

Most people tolerate COVID-19 vaccines well, but a small number develop skin reactions. Understanding why these rashes occur, what they look like, and when to seek medical care can help you feel prepared—and stay safe.

How Common Are Rashes After COVID Vaccination?

A registry-based study of 414 cases (McMahon & Amerson, 2021) collected reports from dermatologists and patients after Moderna and Pfizer mRNA vaccines. Key findings:

• Injection-site reactions (“COVID arm”) were the most frequent, often appearing 5–10 days after dose 1.
• Urticaria (hives) and morbilliform (measles-like) rashes followed.
• Most rashes resolved within 1–3 weeks without serious consequences.
• Less common reactions included erythromelalgia-like redness and swelling of hands or feet, and pernio-like (chilblain) lesions.

Overall, cutaneous reactions occurred in well under 1% of vaccine recipients. They tended to be mild, temporary, and more frequent after the first dose.

Why Do Vaccine-Related Rashes Happen?

Vaccines work by stimulating your immune system. In rare cases, this immune activation or ingredients in the shot can trigger skin reactions:

1. Immediate Hypersensitivity (Within Minutes to Hours)

   • Mediated by histamine release from mast cells.
   • Causes hives, itching, redness.
   • Often responds to antihistamines.

2. Delayed Hypersensitivity (Days Later)

   • T-cell–mediated immune response.
   • Leads to “COVID arm”: localized redness, swelling, sometimes itching or pain at the injection site.
   • May occur 5–10 days after vaccination.

3. Response to Excipients (Ingredients)

   • Polyethylene glycol (PEG) or lipid nanoparticles may trigger reactions in very sensitive individuals.
   • True PEG allergy remains rare.

4. Viral-Mimicking Immune Activation

   • Some rashes (morbilliform or pernio-like) resemble those seen in natural COVID-19 infection.
   • Reflects cross-reactive immune pathways.

Common Types of Post-Vaccine Rashes

Understanding the usual patterns can help you gauge severity:

• Injection-Site (COVID Arm)

  • Red, swollen patch at injection site.
  • May feel warm or itchy.
  • Onset: 5–10 days after dose 1; faster and milder after dose 2.
  • Treatment: cool compresses, topical steroids, antihistamines.

• Urticaria (Hives)

  • Raised, red welts that come and go.
  • Often itchy.
  • Onset: minutes to days.
  • Treatment: oral antihistamines.

• Morbilliform Eruption

  • Widespread pink-red spots, similar to a mild measles rash.
  • May involve trunk and limbs.
  • Onset: 1–2 weeks after vaccination.
  • Treatment: topical steroids, antihistamines if needed.

• Erythromelalgia-Like Reaction

  • Burning pain, redness, and warmth in hands or feet.
  • Rare and usually self-limiting.
  • Treatment: cool soaks, pain relief measures.

• Pernio (Chilblain)-Like Lesions

  • Small, purplish bumps on toes or fingers.
  • Often painless or mildly itchy.
  • Similar to “COVID toes” seen in infection.
  • Treatment: keep area warm and dry; creams as advised.

When Is a Rash a Sign of Something Serious?

Most vaccine-related rashes are mild. However, certain features warrant prompt medical attention:

• Signs of Anaphylaxis
  (Typically within minutes to hours)

  • Difficulty breathing or wheezing
  • Swelling of lips, tongue, or face
  • Chest tightness, dizziness, fainting
  • Rapid heartbeat, low blood pressure
    ➔ Call emergency services immediately.

• Extensive or Worsening Rash

  • Covers >10% of body surface area
  • Rapid spread over hours
  • Associated fever >38°C (100.4°F)
  • Blisters, skin peeling, or raw areas
    ➔ Contact your doctor urgently.

• Mucosal Involvement

  • Sores or blisters in mouth, eyes, or genitals
  • Painful, bleeding, or crusted lesions
    ➔ May indicate Stevens-Johnson syndrome or toxic epidermal necrolysis (rare but severe).

• Persistent Symptoms

  • Rash lasting more than 2–3 weeks
  • Increasing pain, swelling, or signs of infection (red streaks, warmth)
    ➔ Seek medical advice.

What You Can Do at Home

If your rash is mild and limited, self-care measures often suffice:

• Apply cool compresses or take cool baths.
• Use over-the-counter oral antihistamines (e.g., cetirizine, loratadine).
• Apply topical low-strength corticosteroid creams for itching or redness.
• Keep the area clean and avoid scratching to prevent infection.
• Stay hydrated and rest as needed.

When to Consider a COVID-19 Symptom Check

Sometimes skin changes mimic those caused by the virus itself. If you also have cough, fever, loss of taste or smell, or other concerns, you might consider doing a free, online symptom check for COVID-19 to help decide if testing is needed.

Why Vaccination Still Matters

• Rashes after vaccination are generally mild and self-limiting.
• Serious cutaneous reactions are extremely rare.
• The protection vaccines offer against severe COVID-19 far outweighs the small risk of skin side effects.
• Even after a rash, most people safely receive their second dose with minimal issues.

Key Takeaways

• A small percentage of people develop rashes after mRNA COVID vaccines; most resolve in days to weeks.
• “COVID arm,” hives, and morbilliform eruptions are the most common.
• Immediate or severe reactions (anaphylaxis, extensive blistering) require urgent medical care.
• Mild rashes can be managed at home with antihistamines, cool compresses, and topical steroids.
• If you have other COVID-like symptoms, try a free, online symptom check for COVID-19.
• Always speak to a doctor if you’re concerned about any life-threatening or serious symptoms.

Speak to a healthcare professional about any rash that worries you or if you experience severe symptoms. Your safety and peace of mind are the top priority.