via Health Line

The vaccine responsible for the scar you – probably – has on your arm is the vaccine against tuberculosis or smallpox. Tuberculosis is a disease that usually affects the lungs and respiratory tract.

This vaccine must be applied at a very early age, from newborn children up to 5 years old, and most of the time, it is applied in the left arm, although it can be applied in the other arm or the pomp.


But Why Does It Leaves A Scar?

via Health Line

The reason why it leaves a scar is that it has an intradermal application. That is, it is applied in a different way than the others, more superficially, in the first layer of the skin.

The vaccine is applied with the arm stretched at 45 degrees and introducing the needle between 10 and 15 degrees concerning the skin – what we know as the dermis – that is why the person who applies it receives training for this.

The scar eventually occurs as a reaction of the body to the vaccine, although there are also cases of people in whom it leaves no trace because all organisms are different, so it is important to have a vaccination booklet and a record of what is applied.

After being vaccinated, there will be an immediate response of the body to the vaccine. Between 10 and 15 minutes after the vaccine is applied, a papule should form, which is like a little pink welt. That happens because the solution applied is underneath it.

After that, it will take two to three weeks for the papule to go away and instead be a reddish spot with no lump. By the fourth week, it will have a hard consistency when touched.

By the sixth week, it will begin to swell and form a nodule, something like a mosquito bite. It may remain there for one or two weeks, and doctors ask you to keep the area clean but do not scratch, rub, or squeeze it. Finally, by week 12, a scab will form, which will eventually fall off and leave the scar.