Millions of people have rolled up their sleeves for the COVID-19 vaccine, but why haven’t they rolled up their pants legs? Why do we get the most shots in our arms?
As a associate professor of nursing with a background in public health and, as a mother of two curious children, I ask this question quite frequently. So here’s the science behind why we have most arm vaccines.
It is worth noting most, but not all, of the vaccines are given to the muscle, this is known as intramuscular injection. Some vaccines, such as rotavirus, are given orally. Others are administered just under the skin or subcutaneously. Think about it vaccine against measles, mumps and rubella. However, many others are given to the muscle.
But why is muscle so important and location important? And why the arm muscle – called deltoid– on the top of the shoulder?
Muscles have immune cells
Muscles are an excellent vaccine delivery site because muscle tissue contains important immune cells. These immune cells recognize the antigen, a small piece of virus or bacteria introduced by the vaccine that stimulates an immune response. In the case of Covid vaccine19, it is not about introducing an antigen but about administering the plan to produce antigens. Immune cells in muscle tissue collect these antigens and present them to the lymph nodes. Injection of the vaccine into muscle tissue keeps the vaccine localized, allowing immune cells to sound the alarm to other immune cells and get to work.
Once vaccines are recognized by muscle immune cells, these cells transport the antigen to the lymph vessels, which transport the antigen-carrying immune cells to the lymph nodes. Lymph nodes, key components of our immune system, contain more immune cells that recognize vaccine antigens and begin treatment immune process of antibody creation.
Lymph node groups are found in areas close to vaccine administration sites. For example, many vaccines are injected into the deltoid because it is close to the lymph nodes located just below the armpit. When vaccines are given to the thigh, the lymph vessels do not have much to travel to reach the cluster of lymph nodes in the groin.
The muscles maintain the localized action
Muscle tissue also tends to keep vaccine reactions localized. If a vaccine is injected into the deltoid muscle it can occur local inflammation or pain at the injection site. If certain vaccines are injected into the fatty tissue, increases the possibility of irritating and inflammatory reaction because fatty tissue has a poor blood supply, which causes poor absorption of some components of the vaccine.
Vaccines that include the use of adjuvants – or components that enhance the immune response to the antigen – should be given to a muscle to prevent widespread irritation and inflammation. Adjuvants act in various ways to stimulate a stronger immune response.
Another decisive factor in vaccine administration location is the size of the muscle. Adults and children three years of age and older tend to receive vaccines in the upper arm to the deltoid. Younger children receive half-thigh vaccines because the arm muscles are smaller and less developed.
Another consideration during vaccine administration is patient comfort and acceptability. Can you imagine taking off your pants at a mass vaccination clinic? Dragging the sleeve is much easier and preferred. Outbreaks of infectious diseases, such as during the flu season or in the midst of epidemics such as COVID-19, require our public health system to vaccinate as many people as possible in a short time. For these reasons, a shot in the arm is preferred simply because the upper arm is easily accessible.
However, for the influenza vaccine and the COVID-19 vaccine, for most adults and children, the arm is the preferred route of vaccination.