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VE is calculated with this equation. Where ARU is the AR in unvaccinated and ARV is the AR in the vaccinated. Thus vaccine efficacy is the percent reduction in disease incidence attributable to vaccination.

 

If you divide the equation  throughout by ARU, the result is 1 minus the relative risk, where RR is equal to ARV divided by ARU.

 

Thus VE is also the preventive fraction.

 

AS long as the AR in the vaccinated population is lower than the ARU - which you would expect - then RR will be less than one