Foot-and-mouth disease (FMD) routine and emergency vaccination of small ruminants is mandatory in many endemic countries, yet data on the field effectiveness of the used vaccines is scarcely published. In this study we present an investigation of serotype O FMD outbreak, which took place in a sheep and goat pen and enabled estimating the effectiveness of various routine vaccination statuses as well as assessing protection provided by colostrum administration and emergency vaccination. Animals routinely vaccinated twice were not affected clinically, while disease incidence was observed among animals routinely vaccinated only once (p=0.004 from a two-side Fisher's exact test). In groups vaccinated only once, there was a significant association between the average time that elapsed since last vaccination and the incidence (n=5; Spearman correlation coefficient: rs=1.0, p<0.01). In addition, non vaccinated lambs fed with colostrum from dams vaccinated more than two months before parturition showed mortality of 33%. Administration of emergency vaccination two days after the occurrence of the index case was the probable reason for the rapid blocking of the FMD spread within 6 days from its onset in the pen.