Why is the intradermal (in the skin) flu shot recommended?

Intradermal Flu Vaccination: Rationale and Use

The intradermal flu vaccine is administered to adults aged 18-64 years as an alternative delivery method that targets the dermis, which is rich in immune cells, potentially enhancing immune response while using less antigen than traditional intramuscular vaccines. 1

Why Intradermal Administration?

Immunologic Advantage

• The dermis contains abundant dendritic cells and other antigen-presenting cells, making it an immunologically favorable site for vaccine delivery that can generate robust immune responses with lower antigen doses 2, 3
• Studies demonstrate that intradermal vaccines containing 15 μg hemagglutinin per strain show superior immunogenicity compared to standard intramuscular vaccines in adults, particularly in elderly populations aged 65+ years 4
• In younger adults (18-59 years), intradermal formulations with only 9 μg per strain achieve non-inferior immune responses compared to 15 μg intramuscular doses, representing a dose-sparing benefit 3, 5

Clinical Evidence

• Seven out of eight studies in adults 18-60 years showed comparable efficacy between intradermal and intramuscular administration 6
• In adults over 60 years, two out of six studies demonstrated superiority of intradermal over intramuscular delivery 6
• The intradermal vaccine induces superior responses against both influenza A strains (H1N1, H3N2) compared to intramuscular controls 5

Current Guidelines and Restrictions

Age Limitations

• Intradermal influenza vaccines are approved ONLY for adults aged 18-64 years 1
• The vaccine must be administered preferably over the deltoid muscle using only the device included in the vaccine package 1
• Children cannot receive intradermal flu vaccine - they must receive intramuscular formulations 1

Administration Requirements

• Intradermal vaccines are shipped and stored at 2°C to 8°C (36°F-46°F) 1
• The vaccine is supplied in a single-dose, prefilled microinjection system (0.1 mL) for adults 1
• Only the proprietary microinjection device (Soluvia™) should be used - standard needles are not appropriate 2, 3

Safety Profile

Expected Reactions

• Local injection-site reactions are MORE frequent with intradermal administration, specifically erythema, swelling, induration, and pruritus, particularly when assessed over 7 days 6
• However, systemic adverse events and severity of reactions are comparable between intradermal and intramuscular routes 6, 4
• The vaccine has an excellent acceptability, tolerability, and safety profile based on clinical trials and post-marketing surveillance 2

Practical Advantages

Patient Acceptance

• Intradermal delivery may increase vaccination coverage rates, especially in people with fear of needles or high apprehension about annual vaccination 2
• The microinjection system is less invasive than traditional intramuscular injection 5

Dose-Sparing Potential

• The ability to use lower antigen doses (9 μg vs 15 μg per strain) while maintaining efficacy represents a significant advantage during vaccine shortages 3, 5

Important Caveats

The intradermal route is NOT recommended for children or adults outside the 18-64 year age range - these populations should receive standard intramuscular vaccines 1. For adults ≥65 years, preferentially use high-dose inactivated, recombinant, or adjuvanted intramuscular vaccines rather than intradermal formulations 7. The historical guidelines from the 1990s emphasized that intramuscular administration was the only adequately evaluated route at that time, which is why it became the standard 1.