Injection Anatomy & SIRVA: How Can a Deltoid Injection Injure the Shoulder?
A vaccine intended for intramuscular administration in the upper arm has a seemingly straightforward target: the deltoid muscle.
The anatomy beneath that target, however, is complex. Depending on where the needle enters, how deeply it travels, the patient's anatomy, and the position of the arm, a needle may potentially approach the subacromial/subdeltoid bursa, rotator cuff, proximal humerus, axillary nerve, posterior circumflex humeral vessels, or other periarticular structures.
This anatomical relationship forms the basis of the proposed mechanism of Shoulder Injury Related to Vaccine Administration (SIRVA).
The important concept is not simply that a patient received a vaccine in the shoulder. Rather, SIRVA has traditionally been proposed to result when an injection intended for the deltoid is placed outside the intended intramuscular compartment, allowing either direct tissue injury, vaccine deposition into an unintended structure, or both.
The current literature supports several overlapping mechanisms rather than a single universally proven pathway, with SIRVA pathophysiology likely involving several overlapping processes.
Key Takeaways
The site of needle entry and depth of penetration are separate variables. A vaccine can be placed too high, too deep, or both.
The subacromial/subdeltoid bursa may extend several centimeters below the acromion and, in some individuals, lie only approximately 0.8–1.6 cm beneath the skin.
Published cases demonstrate that rotator cuff tendon insertions and the proximal humerus can also be anatomically accessible to a standard vaccination needle.
Needle length must be considered relative to subcutaneous thickness plus deltoid thickness, not in isolation.
The axillary nerve and posterior circumflex humeral vessels represent additional structures potentially at risk from improper placement.
A needle entering an unintended structure may cause direct trauma, but deposition of vaccine material into bursal or synovial tissue may produce a secondary inflammatory response considerably greater than the mechanical puncture itself.
No single surface landmark or imaging finding can retrospectively prove where a needle traveled in an individual case.
The Intended Target: Deltoid Muscle
The purpose of a deltoid intramuscular vaccination is to deposit vaccine within the muscle rather than within subcutaneous fat or deeper shoulder structures.
That means the ideal needle trajectory has a relatively narrow anatomical objective:
skin
↓
subcutaneous tissue
↓
deltoid muscle
↓
needle terminates within muscle
The problem arises when the needle continues beyond that compartment. The amount of available deltoid muscle varies among patients, and accurate intramuscular placement depends partly on the thickness of the overlying adipose tissue and underlying muscle. Jenkins and Duckworth emphasize that although much attention has been given to injection location, relatively less is known about ideal depth and that appropriate placement necessarily depends on needle length and adipose thickness.
Thus, two patients receiving an injection with the same needle length may not have the same anatomical result.
What Does an Injection Being “Too High” Actually Mean?
“Too high” is one of the most common descriptions in SIRVA reports.
It generally refers to a vaccine being administered too close to the acromion rather than into the thicker central portion of the deltoid. This matters because structures immediately beneath the upper deltoid differ from those beneath its central muscular portion.
In the Australian literature, patient reports of excessively high administration have been strikingly common. Jenkins and Duckworth noted 75.5% of affected individuals self-reported that the vaccine had been administered too high in the arm.
Self-report cannot establish exact needle location, but the frequency of this observation is consistent with the anatomical hypothesis underlying SIRVA.
The Subacromial/Subdeltoid Bursa Is Particularly Important
The subacromial/subdeltoid bursa is probably the structure most frequently discussed in the SIRVA mechanism literature.
The reason is anatomical. Bodor and Montalvo's early work demonstrated that the subdeltoid bursa can extend 3–6 cm below the lateral edge of the acromion and may lie only approximately 0.8–1.6 cm beneath the skin in some patients. These measurements have subsequently been repeatedly cited in the SIRVA literature.
Nakajima and colleagues likewise emphasized that a bursa at this depth may be reachable with a 2.5-cm needle and, depending on anatomy, potentially even a shorter one.
That creates a relatively simple anatomical problem:
If the injection is placed high enough that the bursa lies beneath the skin entry point, and the needle travels deeper than the overlying soft tissues and deltoid muscle, the needle may enter the bursa.
Why Does Entering a Bursa Matter?
A brief needle puncture of a bursa may cause pain. But SIRVA theory goes further.
The concern is that vaccine material itself may be deposited into a synovial structure.
Bodor and Montalvo originally proposed that vaccine deposited into the subdeltoid bursa produced a periarticular inflammatory response associated with subacromial bursitis, bicipital tendinitis, and adhesive capsulitis.
Cantarelli Rodrigues and colleagues similarly described the possibility that antigen inadvertently introduced into shoulder synovial tissue can generate a more prolonged inflammatory response than the transient reaction expected after proper intramuscular vaccination.
Thus, from an anatomical standpoint, the bursa matters not simply because it can be punctured, but because it represents a different biological compartment into which the vaccine was not intended to be injected.
The Bursa Is Not Confined to a Tiny Area
One reason simple landmark rules can be imperfect is that the bursa extends inferiorly from the acromion.
Nakajima and colleagues specifically questioned reliance on a fixed “two to three finger widths below the acromion” rule because the bursa itself may extend 3–6 cm caudal to the acromion.
Cook's work reached a related conclusion. In an anthropometric study of 536 older adults, structures potentially injured during deltoid injection were mapped relative to surface landmarks. Cook reported that commonly recommended site-selection methods could potentially place the subacromial/subdeltoid bursa or anterior branch of the axillary nerve at risk when the arm was in a neutral position.
These studies illustrate why anatomical relationships may be more informative than a fixed number of fingerbreadths.
What Lies Beneath the Bursa?
If a needle travels even deeper, the rotator cuff and proximal humerus become relevant.
The rotator cuff consists of the supraspinatus, infraspinatus, subscapularis, and teres minor tendons, which insert around the proximal humerus.
Case reports provide examples of abnormalities in locations that could be reached by a deeply placed needle.
The Natanzi series is particularly informative because the investigators correlated MRI findings with measured anatomy. Two women developed immediate shoulder pain after vaccination and subsequently demonstrated MRI abnormalities involving the teres minor tendon insertion and adjacent humerus. The authors measured skin-to-lesion distances of approximately 2.1 and 2.2 cm. Because a standard 1-inch needle is approximately 2.54 cm long, they considered direct overpenetration anatomically plausible.
The Teres Minor Cases Are Anatomically Important
Teres minor pathology would not necessarily be the first structure considered in a typical shoulder injection injury. Natanzi and colleagues demonstrated, however, that the insertion lies directly beneath a portion of the upper deltoid.
They proposed that vaccine injection into the teres minor tendon insertion and periosteum accounted for the focal tendon and humeral edema in their cases.
They also emphasized that suboptimal positioning during vaccination may have contributed to excessive penetration.
This illustrates an important principle:
The structure potentially injured depends on the actual needle trajectory—not merely on the generic anatomy of the shoulder.
Can the Needle Reach the Proximal Humerus?
Yes, under some anatomical circumstances. The Natanzi cases demonstrated humeral marrow edema directly adjacent to the involved tendon insertion.
Barnes, Ledford, and Hogan reported another highly illustrative case. A 22-year-old woman developed pain within approximately two hours of influenza vaccination. Subsequent ultrasound and MRI demonstrated a partial supraspinatus tear, cortical irregularity, bony contusion, and subacromial bursal effusion.
These reports do not mean that every marrow or cuff abnormality after vaccination represents needle injury.
They demonstrate that the proximal humerus and cuff can be anatomically reachable, making direct trauma plausible in selected cases.
What About the Glenohumeral Joint?
The glenohumeral joint is deeper and is not ordinarily directly beneath most correctly performed deltoid injections.
However, vaccine material may potentially reach synovial structures through an abnormal trajectory or altered anatomy.
Natanzi and colleagues noted that in the setting of a full-thickness rotator cuff tear, material introduced into the subdeltoid/subacromial space could potentially communicate with the glenohumeral joint.
The classic case reported by McColgan and Borschke of pseudoseptic arthritis following accidental intra-articular deposition of pneumococcal vaccine provides clinical evidence that vaccine placed directly into a joint can produce a marked inflammatory response. The case has subsequently been repeatedly cited in the SIRVA literature.
The Axillary Nerve Is Another Anatomical Concern
Not every post-vaccination shoulder injury is necessarily classic inflammatory SIRVA.
An improperly positioned needle can also produce peripheral nerve injury.
The axillary nerve, particularly its anterior branch, travels deep to the deltoid and represents a potential risk depending on injection location. Jenkins and Duckworth specifically identify the axillary nerve and posterior humeral circumflex artery as neurovascular structures that landmark-based injection techniques attempt to avoid.
They distinguish isolated axillary or radial neuropathy potentially related to needle placement from broader conditions such as Parsonage-Turner syndrome, which has a different proposed mechanism.
This distinction matters diagnostically. Direct nerve trauma is not synonymous with SIRVA, even though both can occur after a deltoid injection.
How Close Is the Axillary Nerve to Common Injection Landmarks?
Nakajima and colleagues cited cadaveric measurements placing the nerve approximately 53–62 mm from the acromion and argued that this proximity is another reason to be cautious about relying simply on a fixed fingerbreadth landmark.
The challenge is therefore to avoid being:
too proximal, where the bursa and upper cuff may be at risk, while also avoiding an unnecessarily inferior trajectory that might approach neurovascular structures.
That is why the concept of the thicker central deltoid has become important in anatomy-based recommendations.
Why Does Patient Body Habitus Matter?
Depth is patient-specific. A vaccination needle must travel through:
skin + subcutaneous fat before reaching muscle.
Then it must enter enough muscle to achieve intramuscular deposition without continuing into deeper structures.
In a person with substantial subcutaneous tissue, an excessively short needle may fail to reach the muscle.
In a patient with relatively little subcutaneous tissue and a thin deltoid, a comparatively long needle inserted completely may travel through muscle into deeper structures. Chow and colleagues therefore emphasized selecting needle length based on characteristics including age, sex, BMI, and subcutaneous thickness.
This is why the statement:
“A one-inch needle was used”
does not, by itself, establish whether the injection was too deep.
The relevant question is:
How did that needle length compare with the patient's tissue depth at the actual injection site?
Needle Length and Needle Depth Are Not Exactly the Same Thing
A needle may be 25 mm long, but the effective penetration depends on how it is used.
Relevant factors include:
whether it was inserted fully to the hub,
whether the surrounding soft tissues were compressed,
angle of entry,
injection site,
and patient anatomy.
Natanzi and colleagues specifically noted that compression of the soft tissues could permit even greater effective penetration relative to the underlying anatomy. Thus, retrospective analysis based solely on documented needle length may be incomplete.
Why Does Injection Angle Matter?
Intramuscular vaccination is generally performed with the needle oriented approximately perpendicular to the skin.
An oblique needle changes the relationship between the skin entry point and the deeper structure ultimately encountered.
Nakajima and colleagues noted that even when an appropriate surface landmark is selected, oblique insertion can potentially reach the bursa.
Cook's later protocol likewise advocated a 90-degree approach to the deltoid.
For a medical-legal analysis, this means that the reported surface location of an injection does not necessarily reveal the complete needle trajectory.
Does Arm Position Change the Anatomy?
Yes. Changing shoulder position can alter the relationship between the deltoid and structures beneath it.
Cook's anthropometric work proposed placing the hand on the ipsilateral hip, producing approximately 60 degrees of shoulder abduction, while targeting the midpoint between the acromion and deltoid tuberosity. In that study, this maneuver reduced exposure of the bursa and axillary nerve relative to the proposed injection point.
Behrens and Patel similarly described approximately 60 degrees of abduction as a strategy proposed to reduce exposure of the subacromial/subdeltoid bursa.
Other vaccination guidance uses a relaxed arm at the side, so the literature is not completely uniform regarding preferred positioning. Chow and colleagues, for example, discuss recommendations with the recipient seated, deltoid fully exposed, and arm resting at the side, while also noting the alternative hand-on-hip method.
That variation is worth acknowledging rather than presenting one technique as universally established.
Why Does Injector and Patient Position Matter?
Several SIRVA publications recommend having the vaccinator and recipient at comparable heights.
The rationale is straightforward. If a standing vaccinator injects a seated recipient, the needle trajectory may be more likely to approach the upper portion of the deltoid from above.
Natanzi's cases specifically discuss positioning and note suboptimal positioning as a possible contributor to overpenetration.
Likewise, Atanasoff's early work led to recommendations that both parties be seated as a possible strategy to reduce injections placed excessively high.
Again, this is supportive mechanism evidence, not proof of what occurred in any individual undocumented injection.
Are “Two to Three Finger Widths Below the Acromion” Always Safe?
Not necessarily. This is one of the more interesting disagreements in the anatomical literature.
Some guidance has used two to three fingerbreadths below the acromion as a practical landmark.
But Nakajima and colleagues point out that the subdeltoid bursa itself may extend into this region, while the axillary nerve also lies relatively nearby. They therefore favored the central deltoid over reliance on a fixed fingerbreadth method.
Cook similarly concluded from anthropometric mapping that some commonly recommended landmark methods could potentially place deeper structures at risk.
The broader lesson is that surface landmarks are approximations applied to variable anatomy.
What Does the Literature Suggest About a Safer Region?
Several anatomy-focused publications emphasize the middle or central portion of the deltoid, often conceptualized as the midpoint between the acromion and deltoid tuberosity.
Behrens and Patel summarize anthropometric work suggesting that an anatomically central region approximately 7–13 cm below the mid-acromion, depending on patient size and sex, may avoid both the subacromial/subdeltoid bursa and anterior branch of the axillary nerve.
Nakajima cites sex-specific anthropometric ranges of approximately 6.8–8.5 cm below the acromion in men and 5.5–7.3 cm in women for the midpoint between the acromion and deltoid tuberosity.
These differences underscore that no single distance applies universally to every adult.
Anatomical Misplacement Is Only the First Part of SIRVA
Even when a needle enters the wrong structure, anatomy alone does not explain why some patients experience prolonged symptoms.
Cantarelli Rodrigues and colleagues specifically observed that the magnitude of reported SIRVA injury may exceed what would ordinarily be expected from a simple needle puncture.
That leads to the second component of the proposed SIRVA mechanism:
incorrect anatomical placement, followed by
inappropriate vaccine deposition, followed by
inflammation or immune activation.
Macomb and colleagues describe the widely accepted theory as vaccine intended for muscle being inadvertently deposited in the subdeltoid bursa, producing a robust inflammatory and immune response.
Thus, anatomy determines where the vaccine travels, while local biology may determine what happens afterward.
Why Can Inflammation Spread Beyond the Initial Injection Point?
The shoulder's synovial and bursal anatomy allows inflammatory processes to involve more than one nearby region.
Nakajima's case is particularly illustrative. MRI demonstrated high signal extending continuously from the subdeltoid bursa into the subacromial bursa, and the authors proposed that inflammatory material extended toward the supraspinatus region.
This provides one possible explanation for why the eventual symptomatic or imaging abnormality may not correspond exactly to a single needle-sized puncture site.
Anatomy May Also Explain Different SIRVA Phenotypes
Different needle trajectories could plausibly produce different clinical presentations.
A high and relatively shallow misplacement might predominantly involve the bursa.
A high and deep injection might involve the cuff or proximal humerus.
An inferior or poorly positioned injection might involve a nerve.
A trajectory entering or communicating with synovial tissues could produce a broader inflammatory response.
This may help explain why the SIRVA literature contains such heterogeneous diagnoses:
bursitis, tendinitis, rotator cuff pathology, synovitis, capsulitis, bone abnormalities, and occasionally neuropathic presentations.
Jenkins and Duckworth emphasize this heterogeneity and caution against treating every structural abnormality identified after vaccination as causally related.
Can Imaging Reconstruct the Needle Path?
Usually not with certainty. MRI may show:
bursitis,
tendon edema,
tendon tearing,
marrow edema,
cortical abnormality,
synovitis,
or capsular inflammation.
Ultrasound may identify focal tendon, bursal, or cortical abnormalities. But most of these findings are not specific for needle injury. The strongest cases are those in which several elements converge:
known or reported high/deep injection
immediate symptoms
focal pathology in an anatomically plausible location
little evidence of prior disease
no better alternative explanation.
Even then, the precise trajectory may remain inferential.
Rotator Cuff Findings Require Particular Caution
Rotator cuff abnormalities are common with age. Therefore, a supraspinatus tear discovered after vaccination cannot automatically be interpreted as evidence that the needle penetrated the tendon.
Obeidat and colleagues reported a high frequency of rotator cuff abnormalities in their COVID-19 SIRVA MRI cohort but specifically acknowledged the difficulty of separating possible vaccine-related pathology from age-associated cuff disease.
The anatomy may make direct cuff involvement possible. Imaging alone does not necessarily make it probable.
What Evidence Supports the Anatomical Mechanism Beyond Case Reports?
The evidence includes several different types of studies.
Anatomical and anthropometric studies demonstrate that structures implicated in SIRVA can lie within the potential penetration distance of commonly used needles.
Case reports provide examples of focal pathology corresponding to plausible trajectories.
Pharmacovigilance and clinical cohorts repeatedly document reports of injections perceived as excessively high.
Population-level work has also demonstrated an increased risk of subdeltoid bursitis following influenza vaccination. Hesse and colleagues' population-based cohort study is frequently cited in this context.
No single evidence type establishes the complete SIRVA mechanism, but together they provide biological and anatomical plausibility.
A Practical Framework for Evaluating Injection Anatomy
When reviewing an individual suspected SIRVA case, anatomy can be approached through several linked questions:
Question | Why it matters |
Where was the injection reportedly placed? | Helps assess proximity to the acromion, bursa, cuff, and nerve |
Was it described as unusually high or low? | May suggest a different structure at risk |
What needle length was used? | Establishes maximum potential penetration |
Was the needle inserted fully? | Influences actual depth |
What was the patient's body habitus? | Changes skin-to-muscle and muscle-to-bone distances |
What was the arm position? | Alters the relationship of deeper structures to the injection site |
Was pain immediate? | May support direct tissue contact, although it is nonspecific |
What did early imaging show? | Acute edema or bursal inflammation may help localize pathology |
Does the pathology match the proposed trajectory? | Tests anatomical plausibility |
Is there another explanation? | Plausibility alone does not establish causation |
This is a clinical reasoning framework, not a validated diagnostic scoring system.
Anatomy Can Support Causation—but Cannot Establish It Alone
A common error is to reason:
“A one-inch needle can reach the bursa, therefore the vaccine entered the bursa.”
That conclusion does not follow automatically.
The correct interpretation is:
If the documented injection location, patient anatomy, needle length, depth, and trajectory would permit entry into a particular structure, then that mechanism is anatomically plausible.
Determining whether it actually occurred requires additional evidence.
The same caution applies in reverse.
An injection record describing the vaccine as “intramuscular deltoid” does not necessarily establish the precise anatomical location of the needle tip unless the injection itself was directly imaged—which routine vaccinations are not.
The Bottom Line
Injection anatomy is central to understanding SIRVA because the intended target, the deltoid muscle, sits immediately superficial to multiple pain-sensitive and immunologically active shoulder structures.
The literature demonstrates that:
the bursa can lie within the penetration distance of standard vaccination needles;
rotator cuff insertions and the proximal humerus may be reached by sufficiently deep or proximal trajectories;
the axillary nerve and posterior circumflex humeral vessels are additional structures requiring consideration; and
patient anatomy, needle length, injection angle, site selection, and arm position all influence where the needle ultimately terminates.
But anatomy provides only one part of the SIRVA mechanism. The most useful model is:
Injection location and depth determine whether an unintended shoulder structure can be reached. Mechanical contact may produce immediate injury, while vaccine deposition outside the intended muscle compartment may trigger a subsequent inflammatory or immune-mediated response.
For an individual case, the relevant question is therefore not merely whether the injection was described as “too high.” It is:
Does the reported injection technique create an anatomically plausible pathway to the structure implicated by the patient's symptoms and objective findings—and does the rest of the clinical evidence support that mechanism over reasonable alternatives?
References
Bodor M, Montalvo E. Vaccination-related shoulder dysfunction. Vaccine. 2007;25(4):585-587. doi:10.1016/j.vaccine.2006.08.034. T
Atanasoff S, Ryan T, Lightfoot R, Johann-Liang R. Shoulder injury related to vaccine administration (SIRVA). Vaccine. 2010;28(51):8049-8052.
Cook IF. An evidence-based protocol for the prevention of upper arm injury related to vaccine administration (UAIRVA). Hum Vaccin. 2011;7(8):845-848.
Barnes MG, Ledford C, Hogan K. A “needling” problem: shoulder injury related to vaccine administration. J Am Board Fam Med. 2012;25(6):919-922. d
Uchida S, Sakai A, Nakamura T. Subacromial bursitis following human papilloma virus vaccine misinjection. Vaccine. 2012;31(1):27-30.
Cook IF. Subdeltoid/subacromial bursitis associated with influenza vaccination. Hum Vaccin Immunother. 2014;10(3):605-606. doi:10.4161/hv.27232.
Cross GB, Moghaddas J, Buttery J, Ayoub S, Korman TM. Don't aim too high: avoiding shoulder injury related to vaccine administration. Aust Fam Physician. 2016;45(5):303-306.
Martín Arias LH, Sanz Fadrique R, Sáinz Gil M, Salgueiro-Vazquez ME. Risk of bursitis and other injuries and dysfunctions of the shoulder following vaccinations. Vaccine. 2017;35(37):4870-4876.
Bancsi A, Houle SKD, Grindrod KA. Getting it in the right spot: shoulder injury related to vaccine administration (SIRVA) and other injection site events. Can Pharm J (Ott). 2018;151(5):295-299.
Natanzi N, Hebroni F, Bodor M. Teres minor injury related to vaccine administration. Radiol Case Rep. 2020;15(5):552-555.
Hesse EM, Navarro RA, Daley MF, et al. Risk for subdeltoid bursitis after influenza vaccination: a population-based cohort study. Ann Intern Med. 2020;173(4):253-261.
Cantarelli Rodrigues T, Hidalgo PF, Skaf AY, Serfaty A. Subacromial-subdeltoid bursitis following COVID-19 vaccination: a case of shoulder injury related to vaccine administration (SIRVA). Skeletal Radiol. 2021;50:2293-2297.
Behrens RH, Patel V. Avoiding shoulder injury from intramuscular vaccines. Lancet. 2021. Feb 6;397(10273):471.
Chow JCK, Koles SL, Bois AJ. Shoulder injury related to SARS-CoV-2 vaccine administration. CMAJ. 2022 Jan 17;194(2):E46-E49.
Nakajima K, Miyata A, Kato S, Oshima Y, Tanaka S. Calcific tendinitis of the shoulder induced by an mRNA vaccine for COVID-19: a case report. Mod Rheumatol Case Rep. 2023;7(1):211-214.
Jenkins PJ, Duckworth AD. SIRVA: Shoulder injury related to vaccine administration. Bone Joint J. 2023 Aug 1;105-B(8):839-842.
McColgan BP, Borschke FA. Pseudoseptic arthritis after accidental intra-articular deposition of the pneumococcal polyvalent vaccine: a case report. Am J Emerg Med. 2007 Sep;25(7):864.e1-3.
Macomb CV, Evans MO, Dockstader JE, Montgomery JR, Beakes DE. Treating SIRVA Early With Corticosteroid Injections: A Case Series. Mil Med. 2020 Feb 12;185(1-2):e298-e300.
Obeidat N, Khasawneh R, Alrawashdeh A, Abdel Kareem AM, Al-Na'asan MK, Alkhatatba M, Bani Essa S. Shoulder Injury Related to Vaccine Administration (SIRVA) Following COVID-19 Vaccination: Correlating MRI Findings with Patient Demographics. Tomography. 2025 May 2;11(5):53.
About the Author
Walter I. Sussman, DO, FAAPM&R is a board-certified Sports Medicine and Physical Medicine & Rehabilitation physician.
This article is intended for educational purposes and does not constitute a medical opinion regarding any individual case. Assessment of diagnosis and causation requires review of the specific medical history, records, imaging, and circumstances involved.
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