Inefficient splicing of segment 7 and 8 mRNAs is an inherent property of influenza virus A/Brevig Mission/1918/1 (H1N1) that causes elevated expression of NS1 protein

► Natural sequence polymorphism in influenza A virus segment 7 and 8 mRNAs affects viral splicing efficiency. ► Influenza A virus A/Brevig Mission/1918/1 (H1N1) derived segment 7 and 8 mRNAs are very poorly spliced. ► Poor splicing of influenza A virus A/Brevig Mission/1918/1 (H1N1) segment 8 mRNAs yields higher NS1 production.