Why Are Vaccines Given in the Shoulder?
- Fysiobasen
- Oct 13
- 2 min read
When you receive a vaccine, it is usually injected into your upper arm—specifically into the large muscle called the deltoid. But why there? The choice of the shoulder is not random. It comes down to efficiency, safety, and the body’s immune response. Here’s why healthcare professionals almost always choose the shoulder as the injection site for vaccines.
The Deltoid – A Perfect Vaccine Muscle
The deltoid is a thick, well-developed muscle that covers the shoulder joint. It has several features that make it particularly well suited for vaccine injections:
Good blood supply, helping the vaccine spread quickly through the body
Sufficient muscle mass to absorb the fluid without damaging underlying structures
Easy access, allowing healthcare staff to administer the vaccine quickly without the patient needing to undress¹
This makes the shoulder both a safe and practical placement—for patients and healthcare professionals alike.
Why Intramuscular Injection Matters
Most vaccines are given intramuscularly—into the muscle tissue, not just under the skin. This is important because:
Muscle tissue contains immune cells that help the body recognize and respond to the vaccine
Muscles can hold larger fluid volumes than fatty tissue without causing irritation
Muscle injection ensures steady release of vaccine antigens²
If a vaccine were injected into fatty tissue, absorption could be irregular, reducing effectiveness and increasing the risk of side effects.
Why Not Other Muscles?
Other muscles—such as the gluteus maximus in the buttocks—can also be used but have important disadvantages:
Higher risk of hitting major nerves or blood vessels, such as the sciatic nerve
Greater variability in fat layer thickness, which can make injections less accurate³
Less practical placement, often requiring patients to lie down or remove more clothing
For these reasons, the deltoid muscle in the shoulder is considered a safer and more standardized option.
Are There Exceptions?
For most people, the shoulder is the best vaccination site. Exceptions include:
Infants, who typically receive vaccines in the thigh (vastus lateralis muscle), as it is more developed in small children
Individuals with severe muscle injury in the shoulder, who may require an alternative
injection site after medical assessment
Summary – The Shoulder Is the Best Choice
Shoulder vaccination provides the ideal balance of safety, immune response, and practical application. It is a carefully chosen method, based on anatomy, physiology, and medical experience, ensuring that vaccines provide the best possible protection with minimal risk.
Sources
Cook IF. “An evidence based protocol for the prevention of upper arm injury related to vaccine administration (UAIRVA).” Human Vaccines & Immunotherapeutics, 2011.
Zuckerman JN. “The importance of injecting vaccines into muscle.” BMJ, 2000.
Poland GA, et al. “Vaccine administration: technique matters.” Mayo Clinic Proceedings, 2010.
