The right strategy for infectious bursal disease (IBD) control and its success rate under field conditions depends on hygiene management, IBD field pressure, level and variation in maternally derived IBD antibodies, and the IBD vaccine strains to be used. Usually, standard vaccination programmes are used, which are not always adapted to the specific conditions on the farm and to the immune status of chickens. Employing the "Deventer formula" may help to estimate the optimal time for vaccination for a specific flock based on the maternally derived antibody level, its variation, the genetic background of the chicken, and the IBD vaccine strain. Two field studies with 16 or 20 commercial broiler flocks were conducted, applying an intermediate IBD vaccine before, at the best, and after the estimated optimal vaccination time estimated by the "Deventer formula". These studies showed that flocks IBD-vaccinated between 1 day before, at, or up to 3 days after the estimated optimal time point developed detectable humoral immunity up to 14 days post vaccination. If birds had been vaccinated more than 1 day before the calculated optimal vaccination date, the humoral immune response was delayed or non-detectable until slaughter. The induction of humoral immunity correlated with the incidence of bursa lesions and IBDV detection by reverse transcriptase-polymerase chain reaction. As indicated in this study, under field conditions bursa lesions may develop later than predicted based on experimental experiences. The late incidence of bursa lesions after vaccination may be confused with field virus-induced lesions, in which case sequencing may offer a valuable tool for differentiation.