Stochastic modeling of the horse population and vaccination data indicates low annual vaccination coverage for West Nile virus in Canada (2016-2020).

Vaccination is the most efficient tool for preventing West Nile virus (WNV) disease in horses. The objectives of this retrospective study were to: i) estimate the provincial and national horse population in Canada; ii) estimate the annual vaccination coverage for WNV in each province; and iii) compare estimates of provincial annual vaccination coverage. Horse population data (2006-2023) and the number of vaccine doses sold in Canada (2016-2020) were obtained by province. Based on these data and expert consultations, the annual number of vaccine doses administered, and provincial horse populations were modeled at the provincial level using Program Evaluation and Review Technique (PERT) distributions, whereas the probability of primary vaccination in vaccinated horses was modeled with a uniform distribution. The annual number of horses vaccinated was estimated using a binomial distribution. Monte Carlo simulations were used to estimate provincial annual vaccination coverage with 95% credible intervals (CI). National horse population and vaccination coverage were estimated by summing the provincial estimates. The national horse population in Canada (2016-2020) was estimated at 795 420 horses, with a vaccination coverage of 16% (95% CI: 13 to 20%). The vaccination coverage estimates for WNV ranged from 10% (95% CI: 6 to 20%) in the Atlantic provinces to 35% (95% CI: 22 to 53%) in Manitoba. The coverage was significantly higher (P ≤ 0.01) in Manitoba than in the Atlantic provinces, Saskatchewan, Alberta, and British Columbia. The study revealed low annual WNV vaccination coverage across Canada, emphasizing the need to promote vaccination and collect more comprehensive data on horse population, as uncertainties in these data drive the uncertainty in vaccination coverage based on model sensitivity analysis.