To maintain high efficacy and cost-effectiveness of health interventions, such as immunization programs and health information, systematic and careful evaluation is necessary. This is particularly relevant when health resources are scarce, as in most developing countries. Whereas vaccines used in immunization programs are generally evaluated for efficacy and safety prior to their application in routine health service, very little attention is paid to continuous or operational evaluation. Because of practical reasons (cost, time, feasibility, etc.) or ethical reasons (randomization), the most reliable method for evaluation, the classical RCT, cannot be applied for the operational evaluation of vaccine efficacy, and alternative methods such as the case control method are needed. This method is relatively cheap, quicker, poses fewer ethical problems, and can also provide continuous evaluation.