Epidemiology of infectious diseases. Evaluation of vaccine effectiveness and efficacy in the field
Vaccines are widely used in veterinary medicine to protect animals from infectious diseases. A good vaccine should provide protection from clinical disease (and possibly from infection) for a significant portion of animals and for a long duration. It should also be safe to use and inexpensive. The protection provided by veterinary vaccines is mostly evaluated by challenge studies, during which animals are artificially infected by the relevant pathogen and followed for a certain time. Though the results of these studies correlate with protection, they encompass several weaknesses, e.g. lack of statistical power due to small sample size, failure to represent protection from natural infection and lack of assessment of protection provided in sub-optimal conditions. More-over, they provide no information on the complexities which are inherent in the use of vaccines in the field (e.g. protection among different population of animals, comparison of different vaccination schedules, effect of time elapsed from vaccination to infection).
For these reasons, the best method to evaluate vaccines is by measuring their protective efficacy or effectiveness, which represent the percentage reduction in disease incidence attributable to vaccination under field conditions. Field randomized controlled clinical trials are used for evaluating vaccine efficacy. In these studies animals are allocated randomly by the investigator to vaccinated and non-vaccinated groups. Vaccine effectiveness, however is mostly evaluated after vaccine distribution and its routine use in the field. Though such studies may suffer from inherent biases, they provide important insight into the complexity of vaccine use in the field. Performance of both efficacy and effectiveness studies depends on the existence of natural infection by the relevant pathogens. In addition, field evaluation of vaccines is hampered by the fact that in some countries, use of vaccines for some of these pathogens is not authorized. It stems that vaccine efficacy and effectiveness can be evaluated in vaccinating countries in which the relevant pathogens are either hyper-endemic or frequently epidemic. Proper exploitation of such conditions depends on building mutual interests and overcoming barriers for collaboration, establishing efficient and accurate surveillance systems and using existing surveillance methods. In this talk I will present several examples for exploitations of such systems for evaluating vaccine efficacy and effectiveness. Specifically, I will elaborate on the experience gained on evaluation of vaccines against lumpy skin disease, foot and mouth disease and brucellosis in cattle.