Individualized Deltoid Landmark and Needle Length for Safe Intramuscular Vaccination in Southeast Asian Adults: An...

Background/Objectives: An incorrect intradeltoid injection technique can cause shoulder injury related to vaccine administration, including bursitis, septic arthritis, and axillary nerve injury, particularly when Western-derived landmarks and needle-length tables are applied to smaller-framed Southeast Asian adults. We aimed to define an individualized deltoid injection landmark and needle length that avoid the axillary nerve while ensuring reliable intramuscular delivery in Southeast Asian adults. Methods: In this cross-sectional ultrasound study of adults aged ≥18 years, four vertical landmarks below the acromion (individual contralateral 2-fingerbreadth (FB), individual contralateral 3-FB, average 2-FB, average 3-FB) were assessed in two arm positions (adduction and approximately 30° abduction with the hand on the waist). For each combination, we recorded the presence of the axillary nerve and measured skin-to-subcutaneous and deltoid muscle thickness to estimate whether 0.5-, 1-, or 1.5-inch needles would terminate within muscle or penetrate the subdeltoid bursa. Results: Eighty-two participants (39 males, 43 females) were included. The axillary nerve was not visualized at the individualized contralateral 2-FB landmark in adduction but was present at 31.7-50.0% of other landmark-position combinations. At the individualized 2-FB site in adduction, mean skin-to-subcutaneous thickness was <12.7 mm and mean skin-to-subdeltoid fascia distance exceeded 12.7 mm in all strata, implying that a 0.5-inch needle would consistently terminate within the deltoid muscle. Conclusions: In Southeast Asian adults, the contralateral individualized 2-FB landmark in arm adduction provides a neurovascularly safe window for intradeltoid vaccination, and a 0.5-inch needle offers reliable intramuscular delivery while minimizing the risk of bursal penetration.