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Explained: Why are Covid-19 vaccine shots administered into the upper arm?

Most vaccines, including Covid-19, are most effective when administered through the intramuscular route into the upper arm muscle. But why the upper arm muscle in particular?

Written by Rupsa Chakraborty | Mumbai |
Updated: April 19, 2022 11:23:31 am
Express Explained, Express exclusive, Covid, Covid-19, Covid vaccine, Covid-19 vaccine, vaccination, jabs, shots, coronavirus, coronavirus vaccine, Explained, Indian Express Explained, Opinion, Current AffairsMost vaccines are most effective when given via intramuscular route into the upper arm. Express Archive

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. 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, experts say.

Why are vaccines generally administered into muscle?

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.

“Through the course of research over the years, we have 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, which in this case 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,” said Dr Rahul Pandit, director of critical care, Fortis Hospital, Mumbai, and a member of the national Covid-19 task force.

The rich blood supply network also gives the muscles a protective mechanism. This means that additives to the vaccine, such as aluminium salts, do not lead to severe local reactions. “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 you may have failed immune response; similarly, the additives which could be toxic, could cause a local reaction,” said Dr Pandit.

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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 buttocks, these vaccines show a lower immunogenicity and thus there is a chance of vaccine failure,” said Dr Harshad Limaye, senior consultant, internal medicine, Nanavati Max Hospital, Mumbai.

“Since the dosage of the vaccine is small (0.5mm) and there is little possibility of severe swelling of the injection site, resulting in inconvenience, its administered in the arm,” said Dr Limaye.

Numerous studies on the fat layer between the skin and muscle have found that in most adults (both men and women), the layers seem to be the thinnest around the deltoid muscle. “Although the muscle mass in the gluteal area (the buttocks) is much more than in the deltoid, the fat layer is also larger, so to deposit the vaccine inside the muscle may need a longer needle. However, men tend to have a lesser fat layer near the upper arm muscle as compared to women, so women sometimes may need a longer needle to deposit the vaccine into the deltoid muscle,” said Dr Pandit.

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,” said Dr Pandit.

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, 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 and for tuberculosis, because these two vaccines continue to work empirically well when administered through the intradermal route,” said Dr Pandit.

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