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Engeman referred to republicanism as " an intellectual buzzword " that has been applied to a wide range of theories and postulates that have little in common in order to give them a certain cachet.The most important theorists in this movement are Philip Pettit and Cass Sunstein who have each written a number of works defining republicanism and how it differs from liberalism . Another complication is that , because flu viruses evolve from year to year , and different strains become common at different times , vaccine producers must make calculated guesses regarding which strains will become common in order to produce an effective vaccine .