Monitoring the immune landscape against rabies in domestic dogs, Japan

BACKGROUND: To assess the rabies vaccination program in dogs in Japan, a system to monitor the proportion of domestic dogs in Japan with immunity at population level is critical but has yet to be established. METHODS: We developed a mathematical model to capture the history of vaccination over the course of a dog's life. To quantify the system, an epidemiological survey of dog owners was carried out (n = 534 owners), allowing us to reconstruct the age-specific history of vaccination as a function of calendar time and age and to estimate conditional probabilities of vaccination at age a given the dog's vaccination status at the previous age a-1. RESULTS: The age-specific vaccination coverage was estimated to be 83.3% at age 0 years, 71.7% at age 1 year, and approximately 50% at older ages. Using the vaccination coverage at age 0 years in the latest year of 78.8% (95% confidence interval: 58.7, 97.7), we reconstructed the vaccination history of the entire population; the overall immune proportion in 2021 was estimated to be 64.3%. If vaccine uptake were three times greater than the actual rate among previously unvaccinated dogs, the immune fraction of the population would exceed 80%. By extending the annual vaccination cycle to every 2 years, the immune fraction would fall short of the herd immunity threshold. CONCLUSION: Although not substantially high, the domestic dog population in Japan is considered to have continued to achieve sufficient herd immunity to prevent major epidemics, i.e., the critical immune fraction at 58.7% as calculated from the basic reproduction number of 2.42. Dogs aged 0 years were shown to be the age group most vulnerable to rabies infection.