Method for generating in vitro an attenuated single stranded positive RNA virus of human or animal recipient comprising the following steps: step I) re-encoding the viral genome of an infectious RNA virus by randomly substituting a part of the nucleotide codons of the entire viral genome of said infectious RNA virus by another nucleotide codon encoding for the same amino acid, with the proviso that: i) the number and position of rare nucleotide codons present in said viral genome are not modified, said rare nucleotide codons being CGU, CGC, CGA, CGG, UCG, CCG, GCG and ACG; and ii) the regions of said viral genome which are involved with RNA secondary structure are not modified; and, step II) generating an attenuated RNA virus by : sub-step II.a) introduction of a promoter of DNA-dependent RNA polymerase in position 5' and optionally a terminator and a RNA polyadenylation sequence in position 3' of the re-encoded viral genome as obtained in step I); sub-step II.b) amplification of the re-encoded viral genome as prepared in sub-step a) including said promoter and optionally said terminator and RNA polyadenylation sequence, in at least 2, preferably at least 3, 4, 5 or 6 overlapping cDNA fragments; sub-step II.c) transfection of said cDNA fragments into a host cell; sub-step II.d) incubation of said host cell of sub-step c); and sub-step II.e) recovery of the attenuated RNA virus from said incubated host cell. step I) re-encoding the viral genome of an infectious RNA virus by randomly substituting a part of the nucleotide codons of the entire viral genome of said infectious RNA virus by another nucleotide codon encoding for the same amino acid, with the proviso that: i) the number and position of rare nucleotide codons present in said viral genome are not modified, said rare nucleotide codons being CGU, CGC, CGA, CGG, UCG, CCG, GCG and ACG; and ii) the regions of said viral genome which are involved with RNA secondary structure are not modified; and, i) the number and position of rare nucleotide codons present in said viral genome are not modified, said rare nucleotide codons being CGU, CGC, CGA, CGG, UCG, CCG, GCG and ACG; and ii) the regions of said viral genome which are involved with RNA secondary structure are not modified; and, step II) generating an attenuated RNA virus by : sub-step II.a) introduction of a promoter of DNA-dependent RNA polymerase in position 5' and optionally a terminator and a RNA polyadenylation sequence in position 3' of the re-encoded viral genome as obtained in step I); sub-step II.b) amplification of the re-encoded viral genome as prepared in sub-step a) including said promoter and optionally said terminator and RNA polyadenylation sequence, in at least 2, preferably at least 3, 4, 5 or 6 overlapping cDNA fragments; sub-step II.c) transfection of said cDNA fragments into a host cell; sub-step II.d) incubation of said host cell of sub-step c); and sub-step II.e) recovery of the attenuated RNA virus from said incubated host cell. sub-step II.a) introduction of a promoter of DNA-dependent RNA polymerase in position 5' and optionally a terminator and a RNA polyadenylation sequence in position 3' of the re-encoded viral genome as obtained in step I); sub-step II.b) amplification of the re-encoded viral genome as prepared in sub-step a) including said promoter and optionally said terminator and RNA polyadenylation sequence, in at least 2, preferably at least 3, 4, 5 or 6 overlapping cDNA fragments; sub-step II.c) transfection of said cDNA fragments into a host cell; sub-step II.d) incubation of said host cell of sub-step c); and sub-step II.e) recovery of the attenuated RNA virus from said incubated host cell. <EOS>