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

Overview

• This research investigates how vaccinating all horses in a herd affects the spread of equine herpesvirus 1 (EHV-1) during natural outbreaks by estimating the virus’s reproduction number (R) and comparing vaccinated and non-vaccinated herds using meta-analysis methods.

Background

• Equine herpesvirus 1 (EHV-1) causes significant disease syndromes that impact the global horse industry.
• The effect of vaccinating horses on how the virus spreads within herds during actual outbreaks has not been clearly quantified prior to this study.

Objectives

• Estimate the basic reproduction number (R) — a measure indicating how many other horses one infected horse will spread the virus to on average — in outbreaks of EHV-1 from the available outbreak data.
• Evaluate how the vaccination status of a herd (fully vaccinated vs. non-vaccinated) influences R, and therefore disease transmission dynamics.

Methods

• Conducted a systematic literature review to collect reports of naturally occurring EHV-1 outbreaks in horse herds.
• Included only outbreaks where either all horses were vaccinated or none were, to clearly define herd vaccination status.
• Recorded additional factors such as the virus genotype and whether antivirals were used, although these were not analyzed further due to data limitations.
• Calculated R for outbreaks using two approaches:
  • Growth Rate (GR) method: Based on the early epidemic growth within the herd.
  • Final Attack Rate (AR) method: Based on the proportion of horses infected by the end of the outbreak.
• Performed a meta-analysis combining R values from 12 outbreaks (6 vaccinated herds, 6 non-vaccinated herds) plus one additional empirical study on herd immunity thresholds.
• Compared R values between vaccinated and non-vaccinated groups using a random effects statistical model.

Results

• The pooled estimate of R for non-vaccinated herds was approximately 3.3 (confidence interval 2.6 – 4.0).
• The pooled estimate of R for vaccinated herds was roughly 2.7 (confidence interval 2.1 – 3.2).
• No statistically significant difference was found between R values of vaccinated and non-vaccinated herds (p = 0.15), suggesting vaccination did not strongly reduce virus transmission in these outbreaks.
• R remained greater than 1 in vaccinated herds, indicating that despite vaccination, EHV-1 could still spread and cause outbreaks.
• Sensitivity analyses yielded unstable p-values, indicating some variability and uncertainty in the results.

Limitations

• Could not analyze the impact of different EHV-1 genotypes or antiviral drug use on the transmission dynamics due to insufficient data.
• Relied on published outbreak reports, which may differ in quality, methods, and data completeness.
• Sample size was relatively small, with only 12 qualifying outbreaks included.

Conclusions and Implications

• The study found no conclusive evidence that full herd vaccination alone significantly reduces the transmission of EHV-1 in natural outbreak settings.
• The reproduction number remaining above 1 in vaccinated herds indicates that vaccination may limit but does not halt epidemic spread.
• This suggests vaccination should not be the sole method relied upon to control EHV-1 outbreaks and that complementary biosecurity and management strategies are needed.
• Further research is necessary to explore how vaccination combined with other measures can reduce transmission and the influence of viral genotype and antiviral drugs.