A large proportion of at-risk groups don't take any available preventative (they wouldn't be at risk otherwise) so it isn't difficult to find enough people. Pe

A large proportion of at-risk groups don't take any available preventative (they wouldn't be at risk otherwise) so it isn't difficult to find enough people. People are also much more willing to take one or a few doses for free vs having to continuously take a preventative forever (even if free there are time and transport costs to getting refills).