Typically, vaccines are injected into the deltoid muscle or the outer thigh. So the immune system works better, and there will be fewer side effects.
If the needle does not reach the muscle, but stops in the subcutaneous fat, then there will be fewer blood vessels and the vaccine will work differently. Usually worse.
Now about the soft spot
In the same place, as in any adipose tissue, there are fewer blood vessels, but in addition there are also features of immunity.
When the effectiveness of vaccination against hepatitis B was tested, the vaccine worked clearly worse in the soft spot than in the shoulder. There is an opinion that the matter is not only in fat, but also there may be fewer suitable immune cells in a soft spot.
Side effects
In muscle, the vaccine rarely causes serious side effects. But in the subcutaneous fat, an abscess can easily mature. It's all about the blood supply. The vaccine will quickly dissolve from the muscle, and it will remain in the subcutaneous tissue for a long time.
If the vaccine contains a traditional adjuvant like aluminum salts, then a piece of subcutaneous fat can literally die from it.
Needle
A nurse with a large needle may look intimidating, but it’s better so. A long needle can be guaranteed to reach the meat through the fat.
The wide lumen of the needle allows the vaccine to be dispensed over a large volume of muscle. It turns out that the concentration of the vaccine per unit of muscle mass will be lower. The lower the concentration, the less it stings and hurts. So the big needle hurts less.
For some people, the shoulder in terms of the amount of fat will give odds to that very soft spot. Therefore, the needles can be large.
If there is no muscle in the shoulder
The vaccine manufacturer usually suggests injecting the vaccine into the deltoid muscle or the outer thigh. If there is no meat in the shoulder, then the thigh will help out. Even a very skinny person will have room there.
What kind of needle did they prick you?
