The following vaccine trial was performed: 1000 randomly selected children two years of age were given a vaccine against a certain disease and followed for 10 years. Of these, 80% were never afflicted with the illness. Which is the most correct conclusion regarding the efficacy of the vaccine?
A. The vaccine is an excellent one because of the high rate of immunization
B. TNo conclsion is possible, since no follow-up was made of non-vaccinated children
C. The vaccine is not very effective because it should have produced a higher immunization rate
D. 80% is not a reliable figure and must, therefore, be discounted.