The present invention relates to the use of an excipient which is a compound of formula (I) or a physiologically acceptable salt or ester thereof: wherein: R1 represents C1-6alkyl； R2 represents hydrogen or C1-6alkyl； and R3 represents C1-6alkyl, for increasing the immunogenicity of an influenza antigen, which use comprises (a) freezing, (b) heat-treating, and/or (c) freeze-drying an aqueous composition comprising the influenza antigen and the excipient.
