A method of preparing an oxidised polysaccharide-protein conjugate by oxidising a polysaccharide with an oxidising agent to form an oxidised polysaccharide and combining such oxidised polysaccharide with a protein. The oxidised polysaccharide is reacted with a protein to form a composition comprising a conjugate wherein the oxidised polysaccharide and the protein are conjugated via one or more imine bonds and wherein the oxidised polysaccharide comprises essentially no alpha-hydroxy aldehyde units. The conjugate may be used to provide sustained or latent activity of the protein.
