Evaluating the effect of vaccination on the impact of BTV-3 in Dutch sheep flocks in 2024.

Bluetongue virus serotype 3 (BTV-3) emerged in the Netherlands in 2023 and re-emerged in 2024, causing a substantial impact on the sheep population. In 2024, inactivated BTV-3 vaccines became available, yet clinical cases and excess mortality were still reported, including among vaccinated flocks. This study aimed to quantify the effect of BTV-3 vaccination on excess mortality in sheep and lambs during the 2024 epidemic, using data from twenty veterinary practises, and to evaluate whether a protective effect could be detected at the national level. Vaccination records from participating veterinary practices were linked to national census and mortality data. Multivariable regression models were used to estimate the association between vaccination status and mortality, adjusting for farm type, flock size, region, season, and prior BTV-3 exposure. Results indicated that inactivated BTV-3 vaccination was associated with a significant reduction in excess mortality in both adult sheep and lambs. However, mortality during the epidemic remained significantly higher than pre-epidemic baselines, regardless of vaccination status. Analysis of national data supported the protective effect of vaccination. Nevertheless, over 65,000 excess sheep deaths during the 2024 epidemic were recorded, although the impact per farm was lower in 2024 compared to 2023. Incomplete vaccine registration and potential misclassification of vaccination status, may have led to a slight under-or over estimation of vaccine effectiveness. In conclusion, inactivated BTV-3 vaccines reduced mortality but did not fully prevent disease or halt transmission. For future outbreaks, improved national systems for recording vaccination data are required to accurately assess vaccination effectiveness and support outbreak response. Reliable census and mortality data remain essential for effective outbreak monitoring and evaluation of disease control strategies.