The effect of vaccination of the herd on transmission of equine herpesvirus 1

We know the equine herpes virus vaccines are not great, but the results of this study surprised me. Comparing the disease spread within herds that were not vaccinated and ones, where 100% of the individuals were vaccinated, did not find a significant difference in the rate of infection of the two groups. The numbers show a slight improvement in the vaccinated herds, but it may be a coincidence when analyzed. We continue to get conflicting information as to whether the vaccines, including the MLV vaccine, are effective.
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A model-based approach to evaluate the effect of vaccination of the herd on transmission of equine herpesvirus 1 in naturally occurring outbreaks

Prev Vet Med. 2025 Jan 4:236:106418. doi: 10.1016/j.prevetmed.2025.106418. Online ahead of print.
Authors
R M A C Houben 1 , C van Maanen 2 , J R Newton 3 , J van den Broek 4 , M M Sloet van Oldruitenborgh-Oosterbaan 5 , J A P Heesterbeek 4
Affiliations

1 Department of Clinical Sciences, faculty of Veterinary medicine, Utrecht University, the Netherlands. Electronic address: r.m.a.c.houben@uu.nl.
2 Royal GD, Deventer, the Netherlands.
3 Equine Infectious Disease Surveillance (EIDS), Department of Veterinary Medicine, Cambridge, UK.
4 Department of Population Health Sciences, faculty of Veterinary Medicine, Utrecht University, the Netherlands.
5 Department of Clinical Sciences, faculty of Veterinary medicine, Utrecht University, the Netherlands.

PMID: 39798166
DOI: 10.1016/j.prevetmed.2025.106418

Abstract

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.

Keywords: EHV-1; Epidemiology; Equine herpesvirus; Meta-analysis; R(0); Vaccination.

Copyright © 2025 The Authors. Published by Elsevier B.V. All rights reserved.
Conflict of interest statement

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.