More than half of shoulder vaccinations may end up in the subacromial space.

Shoulder Injury Related to Vaccine Administration (SIRVA) can occur as subacromial bursitis, frozen shoulder, or tendinopathy following deltoid intramuscular immunization. This study evaluated needle overpenetration into the subacromial space using a standard needle in 177 patients undergoing arthroscopic shoulder surgery from 2022 to 2024, with a median age of 55. Findings showed a mean needle length of 25.36 mm was required for subacromial penetration, occurring in 51.9% of cases. Significant associations with needle overpenetration included Body Mass Index (BMI), weight, deltoid thickness, fat thickness, and combined fat and deltoid thickness, all statistically significant (p < 0.05). These results suggest the standard needle length may be too long for most adults, increasing complication risks. To prevent subacromial injections, the study recommends vaccination guidelines to adopt a shorter 15 mm needle for most adults, ensuring proper vaccine delivery into the deltoid muscle and minimizing SIRVA risk.