 Vaccine effectiveness is more easily tracked than the vaccine efficacy considering the difference in environment; however, the vaccine efficacy is more expensive and difficult to conduct.Because the trial is based on people who are taking the vaccination and those not vaccinated, there is a risk for disease, and optimal treatment is needed for those who become infected.The advantages of a vaccine efficacy have control for all biases that would be found with randomization, as well as prospective, active monitoring for disease attack rates, and careful tracking of vaccination status for a study population there is normally a subset as well, laboratory confirmation of the infectious outcome of interest and a sampling of vaccine immunogenicity.