A 5/8 inch needle is NOT recommended for deltoid flu shot administration in most adults
The Advisory Committee on Immunization Practices (ACIP) explicitly recommends a needle length of ≥1 inch (≥25 mm) for deltoid intramuscular flu vaccination in adults and older children, as needles <1 inch may be insufficient to penetrate muscle tissue in many patients. 1
Why a 5/8 inch needle is inadequate
Evidence from ultrasound studies demonstrates poor muscle penetration:
A 5/8 inch (16 mm) needle failed to achieve adequate muscle penetration (≥5 mm into muscle) in 17% of men and 48.4% of women in a prospective ultrasound study of healthy adults. 2
Even in a Thai population study with lower average BMI (24.54 kg/m²), a 12.7 mm (0.5 inch) penetration depth was the minimum required to achieve 100% deltoid muscle injection—this represents the full depth of needle insertion, not accounting for the hub or any technique variation. 3
In adults with obesity, 45% had skin-to-deltoid-muscle distances >20 mm at the New Zealand recommended site, meaning a standard 25 mm (1 inch) needle would not ensure intramuscular delivery, let alone a shorter 5/8 inch needle. 4
Clinical implications of inadequate penetration:
Subcutaneous rather than intramuscular deposition may reduce vaccine efficacy and increase local adverse reactions. 5
The CDC explicitly states that deviation from the recommended intramuscular route might reduce vaccine efficacy or increase local adverse reactions. 5
Recommended needle selection algorithm
For adults and older children receiving deltoid injections:
Use a 1 inch (25 mm) needle as the standard for most adults. 1, 5
For men weighing 59-118 kg: A 1 inch (25 mm) needle ensures at least 5 mm muscle penetration in all subjects. 2
For women <60 kg: A 1 inch needle is still preferred, though a 5/8 inch may be adequate in this specific subgroup only. 2
For women 60-90 kg: A 1 inch (25 mm) needle is required. 2
For women >90 kg or men >118 kg: A 1.5 inch (38 mm) needle should be considered. 2, 6
Practical anthropometric thresholds for needle selection:
Consider a longer needle when arm circumference exceeds 35 cm in men or 30 cm in women. 7
Consider a longer needle when BMI exceeds 24.6 kg/m² in men or 23.7 kg/m² in women for ensuring minimal 5 mm muscle penetration. 7
Proper injection technique
Inject at the deltoid muscle, 2 fingerbreadths (approximately 4 cm) below the acromion process. 3, 6
Insert the needle at a 90-degree angle perpendicular to the skin to ensure proper intramuscular placement. 5
Advance the entire length of the needle to maximize muscle penetration. 3
Critical pitfall to avoid
Do not use a 5/8 inch needle for routine adult deltoid flu vaccination. The overwhelming evidence from multiple ACIP guidelines and ultrasound studies demonstrates that this needle length is inadequate for achieving reliable intramuscular injection in the majority of adults, particularly women and those with higher BMI. 1, 2