A randomized experimental study was conducted to evaluate the effectiveness of a new pertussis vaccine. One thousand healthy children were randomized to receive either the new vaccine (500 children) or the old vaccine (500 children). The children were followed for two years to monitor the incidence of pertussis. At the end of the study, the risk ratio for developing pertussis was 0.5 among the children who received the new vaccine compared to children who received the old vaccine. The 95% confidence interval for this relative risk was 0.2-0.8 and the p value was 0.01. State in words your interpretation of the risk ratio.