Methods of making and using live attenuated viruses

1. A method of delivering a live, attenuated viral vaccine to a subject, comprising:

providing a genetically-engineered virus, wherein the virus has been genetically-engineered to comprise a miRNA-21-recognition nucleic acid sequence to which miRNA-21 binds;

culturing the genetically-engineered virus in a cell line that has been genetically-engineered to knock-out expression of the endogenous miRNA-21, thereby producing a live, attenuated viral vaccine; and

delivering the live, attenuated viral vaccine to a subject, wherein the subject comprises cells that endogenously express miRNA-21.

2. The method of claim 1 , wherein the genetically-engineered virus comprises one miRNA-recognition nucleic acid sequence.

3. The method of claim 1 , wherein the genetically-engineered virus comprises a plurality of miRNA-recognition nucleic acid sequences.

4. The method of claim 1 , wherein the virus is an Influenza B virus, respiratory syncytial virus (RSV), polio virus, West Nile virus, Chikungunya virus, Ebola virus, Lassa virus, Dengue virus, SARS coronavirus, and Middle East Respiratory Syndrome (MERS) coronavirus.

5. The method of claim 1 , wherein the cell line that has been genetically-engineered to knock-out expression of the endogenous miRNA-21 comprises a mutation.

6. The method of claim 1 , wherein the cell line that has been genetically-engineered to knock-out expression of the endogenous miRNA-21 comprises a transgene.

7. The method of claim 6 , wherein the transgene encodes at least one inhibitory nucleic acid.

8. The method of claim 7 , wherein the at least one inhibitory nucleic acid is an antisense RNA, a RNAi, or a siRNA.

9. The method of claim 1 , wherein the subject is a human, a bird, a cow, a pig, a ferret, a dog, or a cat.

10. The method of claim 5 , wherein the mutation is an insertion, a deletion, a substitution, or a point mutation.