Why do babies get vaccines in the thigh and adults in the arm?

Babies receive vaccines in the thigh, specifically the vastus lateralis muscle, because it is a larger muscle that can accommodate intramuscular injections safely and effectively. The thigh muscle is also less likely to be affected by muscle tension or movement in infants. In contrast, adults typically receive vaccines in the deltoid muscle of the arm, which is more accessible and provides sufficient muscle mass for effective absorption of the vaccine. This practice also minimizes discomfort and maximizes the immune response in both age groups.