Avoiding shoulder injury from intramuscular vaccines

With the roll-out of COVID-19 vaccination programmes to tens of millions of people, some individuals might receive vaccines, which have received rigorous safety checks and approval from regulatory bodies, via intramuscular injection. However, the safety around the technique used and the site of injection, in particular, has received little attention. As recommended by the Joint Committee on Vaccination and Immunisation (JCVI), adults aged 16 years or older will be the main population receiving the intramuscular vaccine. The JCVI recommends the deltoid muscle as the optimal injection site, shown graphically as a triangle with the base starting around 1–3 cm below the acromion (appendix).¹ However, this site is not universally accepted as the most appropriate; other organisations advocate alternative sites, such as a triangular region with the base around 5 cm below the acromion and the apex at the level of the axilla apex (approximating the middle third of the deltoid muscle),² or midway between the acromion and the deltoid tubercle.3, 4

The site closest to the acromion and origin of the deltoid has several anatomical structures within its vicinity, including the posterior circumflex humeral artery, the anterior branch of the axillary nerve (located 5 cm below the acromion lateral border), and the subacromial–subdeltoid bursa.² The subdeltoid bursa can extend to 4·0 cm below the acromion and 1·3 cm below the skin.

A range of injuries have been reported to the Vaccine Adverse Event Reporting System database in the USA following vaccination (mostly for influenza). Injuries were predominantly shoulder pain and dysfunction (due to pain, joint-range restriction, bursitis, and stiff shoulder), and patients reported that the vaccines were administered “too high” on the arm.⁵ Spanish pharmacovigilance organisations have similarly reported bursitis and other shoulder injuries following intramuscular vaccination administered in the deltoid.

Anthropometric studies of the optimal site of vaccination have identified that the safest anatomical site in adults of both sexes would be approximately (varying by size and sex) 7–13 cm below the mid-acromion, anatomically midway between the acromion and the deltoid tuberosity (appendix). This region avoids the anterior branch of the axillary nerve or the subacromial–subdeltoid bursa.3, 4 The risk of injury can be further reduced by the recipient placing their hand on the ipsilateral hip (ie, abducting the shoulder to 60°) when receiving the injection. This manoeuvre reduces exposure of the subacromial–subdeltoid bursa to injury. An injection administered at 90° to the skin's surface with a 25 mm needle routinely penetrates at least 5 mm of muscle in men and women.

Updating policy and training vaccinators to safely administer the vaccine in the appropriate intramuscular site will be essential for ensuring efficacy of the vaccine, as placement in a bursa or joint will prevent immune system exposure, and for increasing comfort and reducing pain in vaccine recipients.