Abstract: Provided is a process for preparing attenuated viral strains, comprising to contact, at least one sulphated polymer and a virus susceptible to the inhibition of the polymer, via successive passages of the virus with increasing polymeric concentrations, where the amenable virus is characterized by the method of reducing viral plates and where the strain resulting from the attenuated virus has stable phenotypic and genotypic characteristics, different from that of the virus strain in wild state that generated thereto. The process comprises to contact the sulphated polymer(s) with the virus susceptible to the inhibition of the polymer via about 15 or more successive passages with increasing concentrations of the sulphated polymer(s). According to the inventive process, the concentration of the at least one sulphated polymer in the first passage should be less than the IC50 of the polymer for the amenable virus when it is found in wild state.