Simple SummaryParatuberculosis is an economically important disease in ruminants, and control in affected herds/flocks primarily relies on test and cull strategies and good hygiene and management practices. Vaccination against the causative agentMycobacterium aviumsubsp.paratuberculosisis also used as a tool in some paratuberculosis control programs. The effects of the whole-cell heat-killed Silirum®vaccine on bacterial shedding in the feces and serological response were studied in a controlled field study involving seven French dairy herds, where calves were vaccinated at various ages. A statistically significant reduction of the probability of fecal shedding was observed for cows vaccinated before 4 to 5 months of age compared to non-vaccinated controls. A strong effect of age at vaccination on the serological status investigated in adulthood was also demonstrated, which was also associated with a difference in the protective effect of vaccination.Abstract(1) Background: paratuberculosis is an important disease in ruminants, causing worldwide economic losses to the livestock industry. Although vaccination is known not to prevent transmission of the causative agentMycobacterium aviumsubsp.paratuberculosis(Map), it is considered an effective tool for paratuberculosis in infected herds. The objectives of this controlled field study were to evaluate the effects of the whole-cell heat-killed Silirum®vaccine on Map fecal shedding and serological status in dairy herds infected with paratuberculosis. (2) Methods: The serological status (ELISA) and fecal shedding (qPCR) of 358 vaccinated cows were assessed over 3 years in 7 infected dairy herds in the Meuse department, France. Within each herd, cows from the last non-vaccinated birth cohort (n= 265) were used as controls. The probability and level of Map fecal shedding and the serological status were modeled using multivariable mixed general linear regression models. (3) Results: Overall, 34.7% of cows tested positive at least once on fecal qPCR, with significant differences between herds, but high shedding levels were observed in only 5.5% of cows. Compared to non-vaccinated seronegative cows, a statistically significant reduction in the probability of Map shedding was found only in cows vaccinated before 4 to 5 months of age that tested negative for Map antibodies throughout the study period (odds ratio = 0.5, 95% confidence interval: 0.3–0.9,p =0.008), but no significant effect of vaccination on the amount of Map shedding could be evidenced. Finally, the younger the cows were when vaccinated, the less they tested positive on the serum ELISA. (4) Conclusions: a beneficial effect of vaccination on Map fecal shedding may exist in cows vaccinated before 4 to 5 months of age. The variability of the serum ELISA response in vaccinated cows remains to be investigated.