Intramuscular injection is associated with improved immune response when compared with subcutaneous injection of various vaccines. Groswasser and colleagues measured tissue thickness at two sites recommended for vaccine injection to compare injection techniques and to determine optimal needle length for vaccination of pediatric patients.

Eighteen toddlers and 40 infants were included in the study. Tissue thickness was measured with ultrasonography at the anterolateral aspect of the quadriceps muscle or at the deltoid muscle. The first injection technique, widely used in the United States, involves bunching the thigh muscle at the injection site to increase muscle mass and minimize the chance of striking bone. The other injection technique, recommended by the World Health Organization (WHO), involves stretching the skin flat between the finger and thumb, and pushing the needle down at a 90 degree angle through the skin. A ⅞-in (22-mm) needle is widely recommended. However, unit-dose vaccines are often supplied with a ⅝-in (16-mm) needle.

The skin-to-bone depth of the thigh in infants and toddlers was about 17 mm. The subcutaneous thickness in infants was 8 mm, regardless of the gender of the baby. Toddlers had somewhat thicker quadriceps muscle tissue, with a median of 9.3 mm.

The authors conclude that the injection technique chosen should determine the appropriate needle size. The WHO technique was ideally suited for use of the ⅝-in (16-mm) needles that are supplied with many vaccines; use of the longer needle could cause neurovascular or bone damage. However, the “bunching” injection technique that is more widely used in the United States is more suited to the use of a ⅞-in (22-mm) needle, thus minimizing the risk of subcutaneous injection of the vaccine.