Why is the vaccine applied to the arm?

Have you ever wondered why vaccines against the new coronavirus, as well as several vaccines, are applied to the arm and not elsewhere on the body? Libby Richards, associate professor of nursing at Purdue University in Indiana, in the United States, explained the reasons for choosing the member for application in an article published on May 21, on the website The Conversation.

Couldn’t it be a droplet?

According to the professor, who has a degree in Public Health, it is important to note that most, but not all vaccines, are administered in muscle – the so-called intramuscular injection. Only some of them, like the vaccine against rotavirus – which causes acute gastroenteritis – are administered orally.

Some other vaccines are applied just under the skin or subcutaneously – such as immunizations against measles, mumps and rubella. But many more are given directly to the muscle. And why is muscle tissue so important for immunization?

Muscles have immune cells

According to Professor Richards, the muscles are an excellent location for the administration of the vaccine because the muscle tissue contains important immune cells. These immune cells recognize the antigen – a tiny piece of a virus or bacteria introduced by the vaccine, which stimulates a defense response.

“In the case of the vaccine against covid-19, it is not introducing an antigen, but rather a support against the production of these infectious agents”, explains the author. Immune cells in muscle tissue capture these antigens and present them to our lymph nodes – popularly known as lymph nodes.

The arm muscle keeps the immunizer in the right place to access the lymph nodes.Source: Pixabay

Richards explains that injecting the vaccine into muscle tissue keeps it localized, allowing immune cells to alert other cells of the same type and start taking action. As soon as the immunizing agent is recognized by defense cells in the muscle, they carry the antigen into the lymph vessels, which transport the immune cells, which in turn carry the antigen to the lymph nodes.

“These structures are essential for the immune system and contain more defense cells that recognize the antigens in the vaccines and initiate the immune process of creating antibodies”, details the specialist.

Lymph node clusters are located in areas close to the vaccine application sites. She gives more details on the location of the application: “For example, many of them are injected into the deltoid muscle because it is close to the lymph nodes located just below the armpit. When vaccines are administered in the thigh, the lymphatic vessels do not need to travel far get to the lymph node cluster in the groin “.

Muscles maintain localized action

Richards explains that muscle tissue also tends to keep the vaccine’s allergic reactions localized. The injection of a dose into the deltoid muscle can only result in local inflammation or pain at the injection site – a common report among people immunized against covid-19.

If certain vaccines are injected into adipose tissue, the professor says that the chance of irritation and inflammation increases, because the fat storage tissue has insufficient blood supply – leading to the poor absorption of some components of the immunizer.

The professional warns that vaccines that include the use of adjuvants – components that increase the immune response to the antigen – must be administered to the muscle to avoid generalized irritation and inflammation. These substances act in several ways to stimulate stronger immune action.

Vaccine against covid-19 needs to be applied to easily accessible muscle.Vaccine against covid-19 needs to be applied to easily accessible muscle.Source: Pixabay

“Another decisive factor in defining the injection site is the size of the muscle”, he teaches. Adults and children from 3 years old tend to receive vaccines in the upper arm, in the deltoid. Younger children receive doses in the middle of the thigh because their arm muscles are smaller – and less developed.

Another consideration for adults is the convenience and acceptability of the patient. According to the professor, hardly an adult would want to take off his pants in a vaccination clinic. “Rolling up the sleeve is much easier and preferred,” he says. So an injection into the arm is also preferable because the site is easily accessible.

She ends by stating that, considering everything, for most adults and children, when it comes to the flu and covid-19 vaccine, the arm is the preferred vaccination route for health professionals.

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