The droplet size of emulsion adjuvants has significant impact on their potency, due to differences in immune cell-recruitment and -activation.

Self-emulsification is routinely used for oral delivery of lipophilic drugs in vivo, with the emulsion forming in vivo. We modified this technique to prepare novel oil-in-water emulsions of varying droplet size and composition on bench to enable adjuvanted vaccine delivery. We used these formulations to show that smaller droplets (20 nm) were much less effective as adjuvants for an influenza vaccine in mice than the emulsion droplet size of commercial influenza vaccine adjuvants (~160 nm). This was unexpected, given the many claims in the literature of the advantages of smaller particulates. We also undertook cell-recruitment mechanistic studies at site of injection and draining lymph nodes to directly address the question of why the smaller droplets were less effective. We discovered that emulsion droplet size and composition have a considerable impact on the ability to recruit immune cells to the injection site. We believe that further work is warranted to more extensively explore the question of whether, the smaller is not 'better', is a more common observation for particulate adjuvants.