Healthcare providers do not use a hypodermic needle, usually used for vaccinations, to give the smallpox vaccine. Instead, they use a tiny, two-pronged needle that is dipped into the vaccine solution. When removed from the solution, the needle keeps a droplet of the vaccine. The needle is used to prick the skin, usually in the upper arm, a number of times within a few seconds. The pricking is not deep, but it will cause a sore spot and one or two droplets of blood.
If the vaccination is successful, a red and itchy bump develops at the vaccine site in three or four days.
In the first week, the bump becomes a large blister, fills with pus, and begins to drain. During the second week, the blister begins to dry up and a scab forms. In the third week, the scab falls off, leaving a small scar. People who get the vaccine for the first time have a stronger reaction than those who are revaccinated.