When you donate blood the needle goes into a large, visible vein in your arm or hand but when getting a shot the needle just goes somewhere in your arm.
In: Biology
They aren’t trying to get it into your bloodstream, they intentionally inject it into your muscle.
Injecting a vaccine into your bloodstream could be dangerous as the vaccine components would immediately be carried to all the vital organs, possibly causing damage that could make you very sick or kill you without allowing the immune system to develop immunity to the disease.
Injecting into the muscle greatly reduces the spread of the vaccine, keeping it localized to a non-fatal area so the immune system can respond to it and develop immunity to the disease it represents, while also cleaning up and neutralizing the components of the vaccine so they are safer to dispose of via the bloodstream, liver and kidneys.
In animal medicine vaccines are commonly just injected under the skin of the animal, as many animals have much looser skin than people and plenty of subcutaneous space. Human skin hugs our muscles and fat a bit too tight so there’s not really enough space for subcutaneous injection in most cases, so it is much more effective to just inject it into the muscle.
For other types of injections, like medications, intramuscular injections are used to slow the diffusion of the medication into the bloodstream as the effects of the meds can be pretty sudden and severe if injected into a vein. Other types of meds have to be given into a vein because they are too caustic for muscle tissue and could cause severe damage, but vein linings are much tougher and more resilient than muscle tissue.
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