It’s worth noting that most, but not all, vaccines are given in the muscle – this is known as an intramuscular injection. Some vaccines, like the rotavirus vaccine, are given orally. Others are given just beneath the skin, or subcutaneously – think of the measles, mumps and rubella vaccine. However, many others are given in the muscle. But why is the muscle so important, and does location matter? And why the arm muscle – called the deltoid – in the top of the shoulder? Muscles have immune cells Muscles make an excellent vaccine administration site because muscle tissue contains important immune cells. These immune cells recognize the antigen, a tiny piece of a virus or bacteria introduced by the vaccine that stimulates an immune response. In the case of the COVID-19 vaccine, it is not introducing an antigen but rather administering the blueprint for producing antigens. The immune cells in the muscle tissue pick up these antigens and present them to the lymph nodes. Injecting the vaccine into muscle tissue keeps the vaccine localized, allowing immune cells to sound the alarm to other immune cells and get to work.