Why is the vaccine administered in the arm? The reason isn’t as simple as you might think!

Have you ever wondered why vaccines are typically administered in the arm rather than the buttocks or thigh? Although rolling up your sleeve is certainly much more convenient, the choice of injection site isn’t as simple as it seems.

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Hundreds of millions of people have already been vaccinated against COVID‑19, so why do we roll up our sleeves rather than our pant legs when getting vaccinated? And why are most vaccines administered in the arm? What scientific principles lie behind these practices?

It is worth noting that, for most people (though not all), vaccinations are administered in the arm, a procedure known as an “intramuscular injection.”

So why does intramuscular injection account for such a large share, and are there specific considerations regarding the muscle site chosen for injection?

Muscles contain immune cells.

Muscle is an excellent site for vaccine administration because muscle tissue contains crucial immune cells. These immune cells can recognize the antigens in the vaccine that trigger an immune response.

Take the COVID-19 vaccine as an example: immune cells in muscle tissue recognize these antigens and then transport them to the lymph nodes.

Injecting the vaccine into muscle tissue allows it to remain localized, where immune cells in the muscle can alert other immune cells and signal them to initiate their response.

Intramuscular injection can confine vaccine reactions to the local site.

In addition, intramuscular injection often confines vaccine‑related reactions to the injection site. Injecting the vaccine into the deltoid muscle may cause local redness, swelling, or tenderness at the injection site.

If a vaccine is administered into adipose tissue, the likelihood of local irritation and erythema increases, as adipose tissue has an inadequate blood supply, leading to poor absorption of certain vaccine components.

Vaccines containing adjuvants must be administered intramuscularly to prevent widespread irritation and erythematous reactions. The use of adjuvants can enhance the immune response to the antigen.

(This article is reprinted from China Daily Online.)