What are the advantages, disadvantages, side effects, and cost of the intradermal (ID) flu shot compared to the traditional flu shot?

Intradermal Flu Shot: Overview and Comparison

The intradermal flu shot (Fluzone Intradermal) is a reduced-dose influenza vaccine administered with a 90% shorter needle into the skin rather than muscle, approved only for adults aged 18-64 years, offering comparable protection to traditional intramuscular vaccination but with significantly more local skin reactions. 1, 2

What is the Intradermal Flu Shot?

The intradermal formulation delivers vaccine into the skin layer using a specialized microinjection system with a 30-gauge, 1.5mm needle—dramatically shorter than standard intramuscular needles. 1, 3 The vaccine contains only 9 μg of hemagglutinin per strain (36 μg total for quadrivalent), compared to 15 μg per strain (60 μg total) in standard intramuscular vaccines, representing a 40% dose reduction. 2, 3

The vaccine is administered preferably over the deltoid muscle using a prefilled microinjection delivery system in a single 0.1 mL dose. 2

Age Restrictions and Approved Use

Critical limitation: This vaccine is licensed ONLY for adults aged 18-64 years. 1, 2 It is not approved for:

• Children and adolescents under 18 years 2
• Adults 65 years and older (who should receive standard-dose or high-dose intramuscular formulations instead) 2

The American Academy of Pediatrics states there is no preference between intradermal or intramuscular vaccination for eligible adults 18-64 years, allowing pediatricians to use either formulation for late adolescent and young adult patients. 1

Advantages

Comparable Efficacy

• Immunogenicity is non-inferior to intramuscular vaccination for H1N1 and H3N2 strains, with similar or better geometric mean antibody titers despite the reduced antigen dose. 3, 4
• Seroprotection rates (antibody titers ≥1:40) are equivalent between intradermal and intramuscular routes: 91.1% vs 91.7% for H1N1, 90.7% vs 91.4% for H3N2, and 87.4% vs 89.3% for influenza B. 3
• A systematic review of 13 randomized trials found 7 of 8 studies in adults 18-60 years showed comparable efficacy between intradermal and intramuscular administration. 5

Practical Benefits

• Significantly shorter needle (90% reduction in length) may improve vaccine acceptance among needle-phobic patients. 1, 2
• Dose-sparing potential: delivers only one-fifth the standard antigen dose while maintaining immunogenicity, which could expand vaccine supplies during shortages. 4
• High patient acceptability: 70% of participants in one study preferred intradermal over intramuscular vaccination for future use. 6
• Self-administration feasibility: 93% of participants successfully self-administered the vaccine on first attempt, potentially reducing healthcare costs and improving access. 6

Disadvantages

Increased Local Reactions

The primary disadvantage is significantly higher rates of injection site reactions compared to intramuscular vaccination. 1

• Overall local reactions occur in 89.2% of intradermal recipients vs 60.2% of intramuscular recipients. 3
• Specific reactions more common with intradermal route include:
  • Erythema (redness): consistently higher rates 1, 5
  • Induration (hardness): more frequent 1
  • Swelling: elevated incidence 1
  • Pruritus (itching): notably increased 1, 5

However, these reactions are typically mild (grade 1-2), transient, and resolve within 3-7 days. 1, 3

Limited Efficacy for Influenza B

• Seroconversion rates for influenza B strain were inferior with intradermal vaccination (46.2% vs 54.2% for intramuscular), though seroprotection rates remained comparable. 3

Restricted Population

• Cannot be used in children, adolescents, or older adults ≥65 years—the populations at highest risk for influenza complications. 2

Side Effects Profile

Local Injection Site Reactions (More Common with Intradermal)

• Redness, induration, swelling, pain, and itching occur at slightly higher rates than intramuscular formulation, particularly when assessed over 7 days post-vaccination. 1
• Pain rates at the time of injection are similar between routes (not significantly different). 1, 3
• Larger areas of redness reported with intradermal administration, though pain ratings may be lower. 6

Systemic Reactions (Similar Between Routes)

• Headache, myalgia, and malaise occur at comparable rates between intradermal and intramuscular formulations. 1
• Exception: Myalgia may be slightly more common with intramuscular vaccination. 3
• Fever, nausea, lethargy, and chills occur with similar frequency. 1

Safety Monitoring

• No new safety concerns identified in post-marketing surveillance. 1
• Serious adverse events occur at similar rates to other adult vaccines (14% of reports classified as serious, comparable to intramuscular formulations). 1

Cost Comparison

The provided evidence does not contain specific pricing information comparing intradermal to traditional intramuscular flu vaccines. Cost data would need to be obtained from current pharmaceutical pricing databases or insurance reimbursement schedules, as this information varies by year, location, and payer.

Clinical Decision-Making Algorithm

For adults 18-64 years:

1. Either intradermal or intramuscular formulation is acceptable—no preference exists. 1, 2

2. Consider intradermal if:

   • Patient has needle phobia or anxiety about injections 1
   • Patient expresses preference for smaller needle 6
   • Vaccine supply constraints exist (dose-sparing benefit) 4

3. Consider intramuscular if:

   • Patient has concerns about visible skin reactions 1
   • Patient has history of significant local reactions to intradermal vaccines 5

4. Do not delay vaccination to obtain a specific formulation—use whichever appropriate product is available. 2

Important Caveats

• The intradermal vaccine contains no thimerosal (mercury-free). 1
• Vaccination should not be delayed to obtain intradermal formulation when intramuscular vaccine is available and appropriate. 2
• Local reactions, while more frequent, are self-limited and should not be considered a contraindication to future vaccination. 3
• The vaccine is contraindicated in anyone with severe allergic reaction to any vaccine component, including egg protein, or after previous influenza vaccination. 7