The parts are greater than the whole: the role of semi-infectious particles in influenza A virus biology

•The majority of IAV particles fail to express one or more gene segments.•IAV gene segment packaging is highly efficient but not perfect.•Semi-infectious particles (SIPs) can be produced by a variety of mechanisms.•SIPs are made in vivo but roles in IAV infection and evolution remain unclear.•Variation in viral gene content in single cells may influence infection outcome.