Why are vaccines administered into the upper arm?

Subject: Science & Tech

Section: Biotechnology

Context- Almost everyone vaccinated for Covid-19 over the last 16 months will remember that he or she received a quick prick in the upper arm.

Concept-

Why are vaccines generally administered into muscle?

This is because most vaccines, including those for Covid-19, are most effective when administered through the intramuscular route into the upper arm muscle, known as the deltoid.
There are several reasons, but the most important one is that the muscles have a rich blood supply network.
This means whenever a vaccine carrying an antigen is injected into it, the muscle releases the antigen, which gets dispersed by the muscular vasculature, or the arrangement of blood vessels in the muscle.
The antigen then gets picked up by a type of immune cells called dendritic cells, which function by showing antigens on their surface to other cells of the immune system.
The dendritic cells carry the antigen through the lymphatic fluid to the lymph node.

Role of T Cells

T Cells also called T lymphocyte, type of leukocyte (white blood cell) that is an essential part of the immune system.
T cells are one of two primary types of lymphocytes—B cells being the second type—that determine the specificity of the immune response to antigens (foreign substances) in the body.
Once the antigen gets flagged and is given to the T cells and B cells that is how we start developing an immune response against a particular virus.

Other options for vaccination

Conversely, if the vaccine is administered into the subcutaneous fat tissue [between the skin and the muscle], which has a poor blood supply, absorption of the antigen vaccine is poor and therefore one may have failed immune response.
The same thing could happen when the vaccine is administered intradermally (just below the outermost skin layer, the epidermis).
Hence, the route chosen now for most vaccines is intramuscular.
Also, compared to the skin or subcutaneous tissue, the muscles have fewer pain receptors, and so an intramuscular injection does not hurt as much as a subcutaneous or an intradermal injection.

But why the upper arm muscle in particular?

In some vaccines, such as that for rabies, the immunogenicity — the ability of any cell or tissue to provoke an immune response — increases when it is administered in the arm.
If administered in subcutaneous fat tissues located at the thigh or hips, these vaccines show a lower immunogenicity and thus there is a chance of vaccine failure.

Why not administer the vaccine directly into the vein?

This is to ensure the ‘depot effect’, or release of medication slowly over time to enable longer effectiveness.
When given intravenously, the vaccine is quickly absorbed into the circulation. The intramuscular method takes some time to absorb the vaccine.

Which vaccines are administered through other routes?

One of the oldest vaccines that for smallpox, was given by scarification of the skin.
However, with time, doctors realised there are better ways to vaccinate beneficiaries.
These included
- the intradermal route,
- the subcutaneous route,
- the intramuscular route,
- oral, and
- nasal routes.
There are only two exceptions that continue to be administered through the intradermal route.
These are the vaccines for BCG (Bacillus Calmette–Guérin) and for tuberculosis because these two vaccines continue to work empirically well when administered through the intradermal route.