Every year, millions of adult Americans receive shots of various types in the shoulder. It’s not actually in the shoulder per se, but in the deltoid muscle, the bulging muscle that covers the shoulder joint and the upper arm. The deltoid is a large muscle that is very receptive to shots because of its length and thickness. During the COVID-19 pandemic, mass vaccination was a global effort that revealed some things about administering vaccines in the shoulder that led to new guidelines for injections in the deltoid muscle.
Occasionally, after a shot in the shoulder we experience soreness, mild swelling, and redness at the injection site. It hurts to move your shoulder, but in a few days the symptoms resolve and you’re fine. During COVID-19, however, shots were given by volunteers with little or no previous experience, through the window of your car, or in the back seat, or another contorted position. As such, the needle, and vaccine, didn’t always go where they should have. In these cases, the needle entered one of several bursa of the shoulder resulting in acute bursitis. Other needles found the joint itself causing joint inflammation and adhesions in the shoulder joint capsule. There resulted, then, a shoulder that was very painful to move and hurt for weeks.
From these incidents, guidelines and recommendations for the length and size (gauge) of the needles emerged. The size of the patient’s arm and the patient’s gender are other important factors. Women, who have smaller deltoid muscles, are affected by this circumstance more often than men, even when proper technique is used.
Most vaccines are given through a 25mm (one-inch long) needle. In the majority of bursitis cases, this was the length needle used. When studied in detail, it was found the “optimal” needle length was 16.7 mm, instead. This length needle avoided the deltoid bursa while still being long enough to inject vaccine into the deltoid muscle. A 15mm needle length seems to be the best.
So, for safe administration of vaccines, et al., in most upper arms, a 15mm needle length is ideal. Of course, if the patient is a large person, or has a large arm, a 25mm needle is needed to ensure the vaccine is injected into the muscle and not the subcutaneous fat. Vaccines injected into subcutaneous fat will produce a weaker immune response.
Dr. G’s Opinion: I’m impressed that somebody paid attention to this detail. It’s a potentially troublesome situation, especially in women, and using the proper length needle to inject the vaccine in its intended place is proper practice. When we decide to take a vaccine, we want it to be given in a way it will provide the immunity we desire. These guidelines, when followed, will accomplish that goal.
Reference: Spiriano P. Vaccinating in the shoulder: Why needle length matters. Medscape 2025 Dec 2.
