Another influenza pandemic is inevitable, and new measures to combat this and seasonal\ud influenza are urgently needed. Here we describe a new concept in antivirals based on a\ud defined, naturally occurring defective influenza RNA that has the potential to protect against\ud any influenza A virus in any animal host. This protecting RNA (244 RNA) is incorporated\ud into virions which although non-infectious, deliver the RNA to those cells of the respiratory\ud tract that are naturally targeted by infectious influenza virus. A small intranasal dose of this\ud 244 protecting virus (120 ng) completely protected mice against a simultaneous lethal (10\ud LD50) challenge with influenza A/WSN (H1N1) virus. 244 virus also protected mice against\ud a strong challenge dose of all other subtypes tested (H2N2, H3N2, H3N8). This prophylactic\ud activity was maintained in the animal for at least 1 week prior to challenge. 244 virus was 10\ud to 100-fold more active than previously characterised influenza A defective viruses, and the\ud protecting activity was confirmed to reside in the 244 RNA molecule by recovering a\ud protecting virus entirely from cloned cDNA. There was clear therapeutic benefit when\ud protecting 244 virus was administered 24-48 h after lethal challenge, an effect which has not\ud been previously observed with any defective virus. Protecting virus reduced, but did not\ud abolish, replication of challenge virus in mouse lungs during both prophylactic and\ud therapeutic treatments. Protecting virus is a novel antiviral which has the potential to combat\ud influenza infections in humans, particularly when the infecting strain is not known, or is\ud resistant to antiviral drugs
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