Serological studies allow us to study diseases distribution by an indirect way, trough serological detection of infection and immunity markers. These studies are applied in public health research, in studies to infections, to evaluate control and immunization programs and to evaluate historical background when a new microorganism is found. In the vaccines field, epidemiological studies present two main objectives: The study of the epidemiology of the infection and the study of the protective immunity in vaccinable or immuno preventable diseases, and planning and evaluation of vaccine programs. For the good quality of epidemiological studies is important to use serum samples which are representative of the total population under study, and to take into account that the serological tests used have to show a high sensitivity.