A method for preserving viral particles comprises: (i) providing an aqueous solution of one or more sugars, a polyethyleneimine and said viral particles wherein the concentration of polyethyleneimine is 15 μM or less based on the number-average molar mass (Mn) of the polyethyleneimine and the sugar concentration or, if more than one sugar is present, total sugar concentration is greater than 0.1 M and (ii) drying the solution to form an amorphous solid matrix comprising said viral particles.
