Flewett did much collaborative work on rotaviruses with others to establish the varieties of rotavirus which infect the young of virtually every species of animal. His research group were the first to describe the different serotypes of rotavirus. This work was important to the development of a rotavirus vaccine. He also identified two new species of adenoviruses (later called types 40 and 41), as well as confirming the presence of caliciviruses, astroviruses, and faecal coronaviruses. He, along with H.G. Pereira also discovered picobirnaviruses, and with other colleagues first described human torovirus.