Why Do We Get Shots In The Arm? It's All About The Muscle

Millions have rolled up their sleeves for the COVID-19 vaccine, but why haven’t they rolled up their pant legs instead? Why do we get the most shots in our arms?

As an associate professor of nursing with a background in public health and a mother of two curious children, I ask this question quite a lot. So here’s the science of why we get the most vaccines in our arms.

It’s worth noting that most, but not all, of the vaccines are given into the muscle – this is called an intramuscular injection. Some vaccines, like the rotavirus vaccine, are given orally. Others are given just under the skin or subcutaneously – think of the measles, mumps, and rubella vaccine. However, many others are put into the muscle.

But why is the muscle so important and does the location matter? And why the arm muscle – called the deltoid – at the top of the shoulder?

Muscles have immune cells

Muscles are great for vaccine delivery because muscle tissue contains important immune cells. These immune cells recognize the antigen, a tiny piece of a virus or bacterium brought in by the vaccine that stimulates an immune response. In the case of the COVID-19 vaccine, he does not introduce an antigen, but manages the blueprint for the production of antigens. The immune cells in the muscle tissue take up these antigens and present them to the lymph nodes. By injecting the vaccine into muscle tissue, the vaccine remains localized so the immune cells can sound the alarm to other immune cells and start working.

As soon as a vaccine is recognized by the immune cells in the muscle, these cells transport the antigen to lymphatic vessels, which transport the antigen-bearing immune cells to the lymph nodes. Lymph nodes, key components of our immune system, contain more immune cells that recognize the antigens in vaccines and start the immune process to produce antibodies.

Lymph node collections are found in areas near vaccination sites. For example, many vaccines are injected into the deltoid muscle because it’s located near the lymph nodes just below the armpit. When vaccinating in the thigh, the lymph vessels do not have to travel far to reach the lymph nodes in the groin.

Muscles keep the action localized

Muscle tissue also tends to keep vaccine responses localized. Injecting a vaccine into the deltoid muscle can cause localized inflammation or pain at the injection site. When certain vaccines are injected into the adipose tissue, the likelihood of irritation and inflammation increases because the adipose tissue has poor blood supply, resulting in poor absorption of some vaccine components.

Vaccines that involve the use of adjuvants – or components that enhance the immune response to the antigen – must be given into a muscle to avoid widespread irritation and inflammation. Adjuvants work in several ways to stimulate a stronger immune response.

Another determining factor in the location of the vaccine is the size of the muscle. Adults and children aged three and over tend to be vaccinated on the upper arm in the deltoid. Younger children get their vaccines in the middle of the thigh because their arm muscles are smaller and less developed.

Another consideration in vaccine administration is convenience and patient acceptance. Can you imagine taking off your pants in a mass vaccination clinic? Rolling up your sleeve is much easier and is becoming more preferred. Outbreaks of infectious diseases, such as in flu season or amid epidemics like COVID-19, require our public health system to vaccinate as many people as possible in a short period of time. For these reasons, a shot in the arm is preferred simply because the upper arm is easily accessible.

All in all, the arm is the preferred route of vaccination for most adults and children in the flu shot and the COVID-19 vaccine.

Libby Richards, Associate Professor of Nursing, Purdue University

This article was republished by The Conversation under a Creative Commons license. Read the original article.