Almost everyone who has been vaccinated for Covid-19 in the last 16 months recalls a rapid prick in the upper arm. This is because most vaccinations, including those for Covid-19, are most effective when delivered intramuscularly into the deltoid muscle in the upper arm, according to specialists.
Why are vaccines usually given muscle?
There are various reasons for this, but the most essential is that muscles have a well-developed blood supply system. When a vaccination containing an antigen is injected into the muscle, the antigen is released, and the antigen is disseminated via the muscular vasculature, which is the arrangement of blood vessels in the muscle. The antigen is subsequently picked up by dendritic cells, a type of immune cell that functions by displaying antigens on its surface to other immune system cells. The antigen is carried to the lymph node by dendritic cells through the lymphatic fluid.
“Through years of research, we’ve learned that lymph nodes include T cells and B cells, the body’s primary protector cells. “Once this antigen is flagged and given to T cells and B cells, we can start developing an immune response against a specific 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 for which we have been running vaccination programmes,” said Dr Rahul Pandit, director of critical care at Fortis Hospital in Mumbai and a member of the national Covid-19 task force.
The rich blood supply network also acts as a protective mechanism for the muscles. This means that vaccine ingredients like aluminium salts do not cause significant local responses. “However, 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 you may have a failed immune response; similarly, the additives, which may be toxic, may cause a local reaction,” Dr. Pandit explained.
When the vaccination is given intradermally, the same thing could happen (just below the outermost skin layer, the epidermis). As a result, most immunizations are now given intramuscularly.
In addition, muscles contain less pain receptors than the epidermis or subcutaneous tissue, therefore an intramuscular injection does not hurt as much as a subcutaneous or intradermal injection.