Method for preparing an oxidized polysaccharide-protein composition, by (a) oxidizing a polysaccharide with an oxidizing agent to form an oxidized polysaccharide where less than 20% of the oxidized units are comprised of alpha-hydroxy aldehyde units, (b) reacting the oxidized polysaccharide with a protein to form a composition comprising an oxidized polysaccharide-protein conjugate, and (c) maintaining the oxidized polysaccharide-protein conjugate composition by placing it in an environment where the temperature is less than 8° C. The oxidized polysaccharide and the protein are conjugated via one or more imine bonds, the oxidized polysaccharide-protein composition is soluble in aqueous solvent, and the composition is capable of releasing the protein.