Where should an intramuscular vaccine be delivered?

• A. In the muscle
• B. In the bloodstream
• C. Under the skin
• D. In the stomach

Answer: (A)

Vaccines work best when they are placed into tissue with good blood flow and a rich presence of immune cells. Injecting into the muscle puts the vaccine into muscle tissue, which can hold the liquid well and allows the antigen to be exposed to immune cells efficiently, promoting a strong and appropriate immune response with steady absorption.

Putting a vaccine into the bloodstream would bypass this local immune activation and isn’t how vaccines are designed to work. Under the skin is a possible route for some vaccines, but many vaccines are optimized for muscle delivery because of the amount of tissue and immune activity there. The stomach is not a viable site because the vaccine components would be degraded by stomach acids and absorption would be poor.

So, the correct approach is injecting into the muscle.