The Equine Veterinary Journal has published a special early view article reviewing the science behind enhanced equine influenza vaccination schedules, in particular the replacement of annual boosters with a mandatory bi-annual vaccination programme.

The article argues that as the equine influenza (EI) vaccine supply returns to normal, there is sound scientific evidence why bi-annual vaccination schedules should be re-implemented promptly.

Whilst the scale and number of outbreaks has been relatively small since the introduction of mandatory EI vaccination by most competitive equestrian disciplines in the 1980s, disease events such as those experienced in the UK in 1989, 2003 and most recently in 2019 have demonstrated EI’s epidemic potential, even in vaccinated horse populations.

In their article 'Equine influenza bi-annual boosters: what does the evidence tell us?' Victoria Colgate and Richard Newton discuss what has been learnt from previous outbreaks and explain the evidence from mathematical models to show why bi-annual boosters are beneficial.

The authors say that epidemiological data from previous natural EI outbreaks have repeatedly demonstrated the impermanent nature of the protection provided by vaccination and observational field studies have repeatedly highlighted the potential for 12-monthly boosters to leave a vulnerable immunity gap at both the individual animal and population level.

Mathematical models of EI transmission confirm that six-monthly rather than annual EI booster vaccinations are preferable to establish and maintain effective population level immunity to EI.

Ideally vaccine strains should be updated in a timely manner to ensure inclusion of the most epidemiologically-relevant strains, however, this is a slow and expensive process for equine vaccine manufacturers.

In the absence of updated vaccine strains, bi-annual vaccination is strongly recommended to help compensate for antigenic drift between vaccine and circulating EI viral strains

Professor Celia Marr, Editor of the EVJ said: “Although the recent EI vaccine shortage has necessitated a temporary relaxation of competition vaccine schedules, we must now renew the message that six-monthly boosters are optimal and necessary.”

The Editorial can be found at and is free to view.

Two related articles can be found here: and here: 

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