Injecting a vaccine into the deltoid muscle may result in local inflammation or soreness at the injection site. Muscle tissue also tends to keep vaccine reactions localized. Spencer Platt/Getty Images Muscles keep the action localized It’s much easier to roll up one’s shirt sleeve than to drop one’s drawers – and it is faster, too. When vaccines are given in the thigh, the lymph vessels don’t have far to travel to reach the cluster of lymph nodes in the groin. For instance, many vaccines are injected in the deltoid because it is close to lymph nodes located just under the armpit. Lymph nodes, key components of our immune system, contain more immune cells that recognize the antigens in vaccines and start the immune process of creating antibodies.Ĭlusters of lymph nodes are located in areas close to vaccine administration sites. Once a vaccine is recognized by the immune cells in the muscle, these cells carry the antigen to lymph vessels, which transport the antigen-carrying immune cells into 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. The immune cells in the muscle tissue pick up these antigens and present them to the lymph nodes. In the case of the COVID-19 vaccine, it is not introducing an antigen but rather administering the blueprint for producing antigens. These immune cells recognize the antigen, a tiny piece of a virus or bacteria introduced by the vaccine that stimulates an immune response. Muscles make an excellent vaccine administration site because muscle tissue contains important immune cells.