Good point! For every value of R_0 there's a minimal efficacy a vaccine has to have for it to be possible to achieve herd immunity (assuming the vaccine actually stops transmission). For R_0=2.8 this means efficacy needs to be at least 1/(1-1/R_0))=0.64 which equates to 64%.
Good point! For every value of R_0 there's a minimal efficacy a vaccine has to have for it to be possible to achieve herd immunity (assuming the vaccine actually stops transmission). For R_0=2.8 this means efficacy needs to be at least 1/(1-1/R_0))=0.64 which equates to 64%.