A model-based approach to evaluate the effect of vaccination of the herd on transmission of equine herpesvirus 1 in naturally occurring outbreaks.

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Equine herpesvirus 1 (EHV-1) infection is the cause of high impact disease syndromes, affecting the global horse industry. The effect of vaccination on transmission dynamics of EHV-1 in naturally occurring outbreaks is not quantified. Our aims were to estimate R0 for EHV-1 in equine populations from outbreak data, and evaluate the effect of vaccination status of the herd on R through a systematic review, model-based estimations and meta-analysis. A literature search for outbreak reports was carried out. Depending on available data, the early epidemic growth rate (GR) or final attack rate (AR) approach was used to estimate the basic reproduction number for that outbreak. Herd vaccination status, as well as virus genotype and use of antivirals were recorded. Only outbreaks in herds where either none or all of the horses had been vaccinated were included. An overall estimate for R0 (non-vaccinated herds) and Rv (vaccinated herds) was computed by meta-analysis and the two groups were compared using a random effects model. Twelve outbreaks, in herds of 16-135 horses, met the inclusion criteria, of which six occurred in non-vaccinated herds and six in vaccinated herds. One R0 calculation from a report describing empirical determination of a herd immunity threshold was also included. We found no evidence for a significant difference between estimates of R0 and RV in outbreaks: Rˆ0=3.3(2.6-4.0) and RˆV=2.7(2.1-3.2), p=0.15. Our main limitations were our inability to investigate the influence of genotype or antivirals on results. Sensitivity analyses gave volatile p-values. In conclusion, we found no robust evidence for a significant reduction on transmission of EHV-1 in herds where all horses were vaccinated vs non-vaccinated herds. Rˆ in herds where all horses were vaccinated was substantially >1 and vaccination as a sole mitigating measure may have limited effect on transmission of EHV-1.