Efficacy and immunological safety of EHV vaccines in horses

[Statement on the efficacy and immunological safety of EHV vaccines in horses]
Tierarztl Prax Ausg G Grosstiere Nutztiere. 2025 Feb;53(1):39-48. doi: 10.1055/a-2518-1976. Epub 2025 Feb 18.
[Article in German]
Author
Arbeitskreis Pferde der Ständigen Impfkommission Veterinärmedizin (StIKo Vet)

PMID: 39965581
DOI: 10.1055/a-2518-1976

Abstract
in English, German

In February 2021, a serious EHV-1 outbreak occurred at an international jumping competition in Valencia, with several subsequent outbreaks in various European countries. As a consequence, several equestrian associations introduced compulsory vaccination against EHV-1, and in the immediate aftermath of the outbreak, demand for EHV vaccinations increased sharply. The initial concern has now dissipated. One equestrian association has abolished the compulsory vaccination that had only been introduced a year before, and a general debate began questioning the efficacy and safety of EHV-vaccines. This discussion has been fueled by international meta-studies that systematically re-evaluated published efficacy studies on EHV-vaccines. The meta-studies applied evidence criteria that were originally designed for preclinical studies in human medicine. It was concluded that the quality of the published data was not sufficient to prove the efficacy of EHV vaccines. Naturally, lack of evidence does not prove the opposite! Hence, one of the two author teams of these meta-studies upholds the notion that vaccination is a centerpiece of comprehensive EHV management strategies.StIKo Vet has carefully followed the discussions and the meta-studies. It cannot be expected that other infection studies on horses meeting the evidence criteria will be conducted in the near future, as EHV infection studies on horses pose considerable ethical, logistical and economic challenges. Therefore, StIKo Vet has undertaken an own re-evaluation of the existing efficacy studies. In addition, outbreak investigations were reviewed with regard to the role of EHV vaccination as a risk factor for the development of severe clinical courses.StIKo Vet concludes that, even if vaccination against EHV-1 does not confer absolute protection to the vaccinated individual, the re-evaluated studies show highly significant differences between vaccinated and unvaccinated animals: Vaccination reduces the severity of clinical symptoms and the duration and level of virus excretion. Herd vaccination can reduce overall viral loads and thus interrupt infection chains. There is ample reason to assume vaccine efficacy, but no compelling evidence was found for a correlation between EHV vaccination and severe EHV symptoms. By contrast, a comprehensive analysis of several different outbreaks shows that a herd immunity over 40 % is able to significantly reduce the risk for neurological symptoms. In addition, it has to be emphasized that the safety and efficacy of EHV-vaccines is stringently evaluated during the marketing authorization process. Only facts that have appropriately been proven in licensing studies are documented in the summary of product characteristics, e.g. the packaging leaflet. StIKo Vet therefore upholds EHV as a core-vaccination for horses. Every horse should be vaccinated and protected against EHV-1 at all times. It is likewise clear that with high viral loads and additional predisposing factors, EHV outbreaks occur despite vaccination. So, vaccination is only one component: A comprehensive prevention strategy additionally includes transparent outbreak communication, general hygiene measures, avoiding shared housing in insufficiently ventilated stables, especially at competition events, separating animals of different origin wherever possible and, above all, isolating horses that are already sick or at particular risk of infection.