Barcoded influenza viruses and deep mutational scanning libraries including the same

A method for barcoding an influenza virus genome segment that comprises:

• a 3′ viral RNA genome packaging signal;
• an open reading frame;
• and
• a 5′ viral RNA genome packaging signal, comprising:
• inserting a nucleic acid comprising a barcode and a copy of the 5′ viral RNA genome packaging signal between the end of the open reading frame and the non-coding portion of the 5′ viral RNA genome packaging signal,
• wherein the method has minimal to no effects on viral fitness,
• wherein the barcode comprises 5-100 nucleotides in length, and
• wherein the open reading frame encodes hemagglutinin (HA), neuraminidase (NA), M1 matrix protein (M1), M2 ion channel protein (M2), nuclear protein (NP), nonstructural protein 1 (NS1), nonstructural protein 1 (NS2), or a subunit of an RNA-dependent RNA polymerase complex selected from PB1, PB2, and PA.