Do we really need bi-annual flu vaccines for our horses?

Epidemiology experts are advising that now the equine influenza vaccine supply has returned to normal and, based on sound scientific evidence, a bi-annual vaccine programme should be re-implemented by horse owners.

EI is a highly contagious respiratory disease, which remains endemic in the horse populations across numerous countries and infection is characterised by rapid spread and significant morbidity in the immunologically naive. With the introduction of mandatory EI vaccination by most competitive equestrian disciplines after the early 1980s, the scale and number of outbreaks have in most years been relatively small. However, 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 build on the work recently published by Fleur Whitlock and colleagues in; An epidemiological overview of the equine influenza epidemic in Great Britain during 2019. They discuss what has been learnt from previous outbreaks and explain the evidence from mathematical models to show why bi-annual boosters are beneficial.

Epidemiological data from previous natural EI outbreaks have repeatedly demonstrated the impermanent nature of the protection provided by vaccination and observational field studies, highlighting 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 regularly,  to ensure inclusion of the most epidemiologically-relevant strains, however, this is a slow and expensive process for equine vaccine manufacturers. So, 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.

“The equine industry must surely remain resolute and guided by scientific principles,” said the authors. “The clear evidence from experimental, epidemiological and mathematical modelling studies shows why we must encourage clients to revert to a schedule of bi-annual boosters.”

“We must also remind horse owners that animals already on six-monthly vaccination regimes were best positioned for the vaccine shortage with a built-in tolerance in their vaccination schedule; their levels of immunological protection would not be expected to decline to susceptible levels, even with a slight delay before being re-vaccinated.”