Evaluating the benefits of vaccination when used in combination with stamping-out measures against hypothetical introductions of foot-and-mouth disease into New Zealand: a simulation study.

Abstract

AIMS: To evaluate the benefits of vaccination against simulated outbreaks of foot-and-mouth disease (FMD) in New Zealand, when applied as an additional measure to stamping-out. METHODS: A simulation modelling approach was used. The study population comprised all known farms in New Zealand with FMD-susceptible livestock. Infection was seeded into three different areas of New Zealand. Transmission mechanisms included direct and indirect contacts, local spread and airborne spread. Efficacies of some of the stamping-out measures were varied. Vaccination strategies involved different start times, size and type of vaccination zone, and species vaccinated. Personnel resources for vaccination were varied as was the herd immunity profile following vaccination. Altogether, 336 models were specified, with 100 iterations conducted for each model. Generalised linear modelling and boosted regression trees were used to evaluate which variables had the biggest effect on the number of infected premises (IP), epidemic duration and area under control. RESULTS: Vaccination, when used as an adjunct to the standard stamping-out programme, significantly reduced the outbreak size. Vaccination reduced the median number of IP by 26 (95% CI=18-35), epidemic duration by 16 (95% CI=13-19) days and area under control by 474 (95% CI=250-699) kmwhen there was no airborne spread; and when there was airborne spread the median reduction was 87 (95% CI=70-105) IP, 32 (95% CI=28-35) days and 898 (95% CI=665-1139) km, respectively. Multivariable analyses showed that starting vaccination 11 days after first detection of FMD produced greater benefits than starting 16 or 21 days after detection. Increasing vaccination zones resulted in increased benefits. Boosted regression tree analyses showed that the most influential variables on the outcome measures were interval to first detection, incursion location, whether there was airborne spread or not and herd immunity profile. CONCLUSIONS AND CLINICAL RELEVANCE: This study showed that there are benefits to the use of vaccination in combination with a stamping-out policy for control of FMD outbreaks under New Zealand conditions. The optimal vaccination strategy was identified as being a 3-5 km radius suppressive vaccination zone deployed between 11-16 days after first detection. Vaccination had a greater benefit during larger outbreaks, such as when there was airborne transmission. The key factors which were identified from this study will help inform New Zealand's competent authority on how best to deploy vaccination to further strengthen its approach to FMD eradication should New Zealand ever experience an outbreak.