Everyone that got vaccinated for Covid-19 in the last 16 months must be remembering the prick they got in their upper arm. According to experts, this is because most vaccines including the Covid-19 vaccines are most effective when injected via an intramuscular route into the upper arm muscle called the deltoid.
There are many reasons why vaccines are generally administered in the muscle, but the most important reason is that muscles have a rich blood supply network, which means that a vaccine that carries an antigen is injected into it and the muscle releases the antigen that gets dispersed by the muscular vasculature or the blood vessels in the muscle. These antigens are then picked by a type of immune cell called dendritic cells that function by showing antigens on their surface to other cells of the immune system. The antigen is carried by the dendritic cells through the lymphatic fluid to the lymph node.
The muscles get a protective mechanism through the rich blood supply, which means that the additives to the vaccine like aluminium salts do not lead to severe local reactions.
Identical things happen, when a vaccine is injected below the outermost skin layer, the epidermis. Hence, the vaccine would use the intramuscular route.
Similar to subcutaneous tissue or skin, the muscles have fewer pain receptors by which the intramuscular injection does not hurt as much as a subcutaneous or intradermal injection.
In some vaccines like in the case of rabies, the immunogenicity-the tissue or cell’s ability to provoke the immune response increases when administered in the arm.
Many studies have found that the layer between the muscle and skin in most adults (men and women) are the thinnest around the deltoid muscle.
Why can’t the vaccines be administered directly into the vein?
The reason why this is not done is to ensure the ‘depot effect’, or release the medication slowly over time for long effectiveness. When injected intravenously, the vaccine is quickly absorbed into the circulation whereas the intramuscular method takes some time to absorb the vaccine.
What vaccines are injected through the other routes?
The oldest vaccine for smallpox was given by scarification of the skin, however, with time doctors found better ways that include the subcutaneous route, intradermal route, oral, nasal routes and intramuscular route.
