Background: To be maximally effective and to induce less adverse reactions, injection with many vaccine antigens must penetrate muscle rather than subcutaneous tissue.
Aim: To determine the length of needle needed to penetrate muscle at the anterolateral thigh vaccination site in children aged two, four, six and 18 months.
Method: Ultrasound measurements were made of the subcutaneous and muscle layer thickness of children aged two, four, six and 18 months at the junction of the upper and middle thirds of the anterolateral thigh with the probe applied parallel to the long axis of the leg and at 45 degrees to the vertical and at 90 degrees to the skin's plane.
Results: Subcutaneous tissue (SCT) and muscle layer (ML) thickness were measured in 57 children (2 months, n = 14; 4 months, n = 13; 6 months, n = 18; 18 months, n = 12) with mean SCT thickness of: 8.6 +/- 3.0 mm at 2 months; 9.4 +/- 2.0 mm at 4 months; 10.2 +/- 2.1 mm at 6 months; and 8.1 +/- 1.7 mm at 18 months. Muscle layer thickness in these children was: 10.5 +/- 2.4 mm at 2 months; 12.2 +/- 2.0 mm at 4 months; 14.8 +/- 2.0 mm at 6 months, and 16.5 +/- 4.6 mm at 18 months.
Conclusion: The optimal needle length to routinely penetrate muscle of the anterolateral thigh in children aged two, four, six and 18 months depends on the technique employed. A 16 mm long needle is suitable with the WHO technique (injecting at 90 degrees to skin's surface) and 25 mm long needle with the NH&MRC and American techniques (injecting at 45 degrees to skin's surface).