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.

Why vaccines are 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.
• Through the course of research over the years, it is understood that the lymph nodes have T cells and B cells — the body’s primary protector cells.
• Once this 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.
• It could be any of the new viruses like SARS-CoV-2, the virus that causes Covid-19, or the previous viruses which we have been running vaccination programs for.

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.
• Similarly, the additives which could be toxic, could cause a local reaction.
• 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.
• Wherever a vaccination programme is carried out, it is carried out for the masses.
• To deposit the vaccine, the easiest route would be the oral route (like the polio vaccine).
• However, for other vaccines that need to be administered intravenously or intramuscularly (enabling wider field-based administration), the intramuscular route is chosen from a public health perspective over the intravenous route.

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.

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