Investigations into a trigger-based approach for initiating emergency vaccination to augment stamping out of foot-and-mouth disease in New Zealand: a simulation study.

AIMS

To investigate an adaptive management approach to the deployment of emergency vaccination as an additional measure to stamping out (SO) during simulated outbreaks of foot-and-mouth disease (FMD) in New Zealand.

METHODS

A simulation modelling (n=6000 simulations) approach was used. The study population comprised all known farms in New Zealand with FMD-susceptible livestock. Each simulation started with infection seeded into a single randomly selected farm. Each outbreak was randomly assigned to one of four control strategies, comprising SO only; trigger-based vaccination (TRV) where SO was augmented with vaccination if an early decision indicator trigger operating between Days 11-35 of the response indicated a large outbreak was developing; SO plus vaccination started randomly on Days 11-35 of the response (VACr); and SO plus vaccination with a fixed start on Day 21 of the response (VACf). Other parameters, such as the number of personnel available were also varied randomly. Generalised additive models (GAM) were used to evaluate variables associated with the number of infected premises (IP) and epidemic duration.

RESULTS

The mean number of IP was 29 (median 9, min 1, max 757), while epidemics lasted on average 26.9 (median 18, min 1, max 220) days. These excluded 303 extreme outbreaks larger than the UK 2001 FMD epidemic (2,030 cases). Univariable analysis of the pooled vaccination results . SO, showed that vaccination significantly reduced the number of IP (p<0.001) and outbreak duration (p<0.001). GAM of large outbreaks revealed that only the TRV strategy was significantly protective compared to SO alone, reducing the odds of a large outbreak by 22% (OR=0.78; 95% CI=0.63-0.96). The number of veterinarians was non-linearly associated with large outbreaks, with low numbers increasing the odds of a large outbreak, but above 200 veterinarians, the odds reduced. Time to first detection was also non-linearly associated with large outbreaks, with detections <13 days protective and longer detection times increasing the odds of a large outbreak. GAM of long outbreaks showed similar findings, except that all three vaccination strategies significantly reduced duration. Overall, the TRV strategy resulted in the smallest and shortest epidemics.

CONCLUSIONS AND CLINICAL RELEVANCE

An adaptive management approach that deployed vaccination in response to a trigger when a large outbreak was developing outperformed SO and reduced the odds of large or long outbreaks more than the other two vaccination strategies, although the differences between the three vaccination strategies were statistically small. This study provides highly relevant insights into the dynamics of disease establishment and spread that will guide New Zealand's readiness for responding to highly infectious disease incursions such as FMD.