There was a piece about expelling the air from the flu vaccine syringe in the November/December 2014 Vaccine Update and the answer given was as follows: You shouldn’t get rid of the air bubble. To try to expel it risks accidently expelling some of the vaccine and therefore not giving the patient the full dose. The air bubble is also there for a reason – the air injected into the muscle forms an airlock preventing the medication seeping out along the needle tract into subcutaneous tissue and onto the skin. The small bolus of air injected following administration of medication clears the needle and prevents a localised reaction from the vaccination. The link to the reference is here.
Should this apply to all vaccines including travel? The Australian Immunisation Handbook of 2013 says for all injectable vaccine as follows: Do not extrude small air bubbles through the needle for injection. However, in the rare instance of a large air bubble in a pre-filled syringe, first draw back on the needle to ensure no vaccine is expelled along with the air, and then expel the air through the needle, taking care not to prime the needle with any of the vaccine, as this can lead to increased local reaction. See here for the link to this statement in this excellent publication.