Live attenuated viruses for protection against the novel coronavirus which emerged in Wuhan, Hubei Province of China, designated as Sars-CoV-2 by the World Health Organization (WHO) are provided. The live attenuated chimeric virus strains are based on a live attenuated influenza virus (LAIV), used a master backbone, which includes deletion of the viral virulence element, the NS1 (non-structural protein 1) (DeLNS1), engineered to express one or more antigens of the Sars-CoV-2 (herein, CoV2Ag). The chimeric virus strain is referred to generally herein, as DelNS1-Sars-CoV-2-CoV2Ag. The DelNS1-Sars-CoV-2-CoV2Ag strain preferably shows spontaneous cold adaption with preference to grow at 30-33° C. The DelNS1-Sars-CoV-2-CoV2Ag strain can be used to protect a subject in need thereof, against a challenge of Sars-CoV-2. DelNS1-Sars-CoV-2-CoV2Ag is an important strategy for making highly attenuated and immunogenic live attenuated vaccines with the ability to induce protective immunity against Sars-CoV-2.

The present invention discloses a replication-limited mucosal immune vaccine for influenza virus. The present invention first protects a mutant protein, which is obtained by mutating two amino acid residues of NS1 protein of influenza virus as follows: the amino acid residue at position 38 is mutated from arginine to alanine, and the amino acid residue at position 41 is mutated from lysine to alanine. The present invention also protects a recombinant virus, which is obtained by mutating the codons encoding two amino acid residues of NS1 protein in the influenza virus genome as follows: the codon for the amino acid residue at position 38 from the N-terminus is mutated from the arginine codon to an alanine codon, and the codon for the amino acid residue at position 41 from the N-terminus is mutated from the lysine codon to an alanine codon. The present invention also protects use of any one of the above-mentioned recombinant viruses for preparing a vaccine for influenza virus. The present invention has great application value for the prevention and treatment of influenza virus.

The invention relates to a method for producing influenza infectious viruses wherein CHO cells are infected with a seed of infectious influenza virus which has been generated by transfecting cells with an appropriate set of expression vectors. The invention also relates to a recombination cassette, and to a vector comprising said recombination cassette, that may be used in methods for producing infectious viruses, and particularly in the method according to the invention.