It is not wrong to expel the air from syringes filled by manufacturers, but typically it is such a small amount of air (0.2cc–0.3cc) that it is CDC’s opinion that it would not cause a problem. When the syringe is inverted during an injection, that small amount of air would typically just clear the medication from the needle. This is based on the recommendation that when the Z-track method is used for intramuscular injection of irritating medication (e.g., iron preparations), the guidance is to leave 0.2cc–0.3cc in the syringe to be sure that all of the medication leaves the needle and is not tracked back through subcutaneous tissue as the needle is withdrawn. While the Z-track injection technique is not recommended for vaccine administration, the Z-track method demonstrates the acceptability of leaving a very small amount of air in the syringe for intramuscular injections.
CDC does, however, recommend that when drawing vaccine from a vial into a regular syringe, the air be expelled because the amount of air drawn into the syringe may be larger than the amount in a manufacturer-filled syringe. Expelling the air is part of general medication guidelines for drawing medication into a syringe.