by Boehringer Ingelheim / July 13, 2026

Equine influenza virus (EIV) or flu remains one of the most contagious respiratory diseases affecting horses worldwide. Outbreaks can spread rapidly through barns, showgrounds and racetracks, disrupting training schedules, competitions and even basic care routines. One of the most effective tools in protecting against these outbreaks is vaccination, specifically with vaccines that include the current circulating strains of the Florida sublineage clade 1 and clade 2 equine influenza viruses.

Working with your veterinarian helps ensure your horse receives the most appropriate vaccine for its individual needs and that proper handling and administration practices are followed. Your veterinarian can also help you consider factors that vary between vaccines, such as which clades they cover and their duration of immunity (DOI).
 

What do clade 1 and clade 2 flu strains mean for me and my horse?

Equine influenza viruses evolve and change, with the virus mutating within the Florida sublineage of the H3N8 subtype. The two major circulating branches today are clade 1 and clade 2¹:

• Clade 1 strains are what are identified in the U.S.
• Clade 2 strains are found in certain parts of Europe and Asia.

Because horses and people travel internationally for breeding, training and competition, both clades can pose a risk.

If clade 2 flu strains are only in Europe, why should I vaccinate against both?

1. If your horse travels back and forth to Europe, it is wise to have your horse protected with a vaccine that contains both the clade 1 and clade 2 flu strains.
2. If your horse is exposed either in your barn or at shows to horses that travel to Europe, it would be wise to have your horse administered a vaccine that contains both a clade 1 and a clade 2 flu strain.
    

The risks of not keeping up with current flu strains

Outdated vaccines that don’t include representative strains of current circulating flu may offer limited protection. This leaves horses more vulnerable to infection, even if they are “up to date” on vaccinations by a manufacturer and/or veterinarian-recommended schedule. Infected horses may suffer from respiratory disease, fever, cough, nasal discharge, lethargy and loss of performance. Outbreaks can also cause barns or entire events to close.
 

What equine governing bodies recommend

The World Organization for Animal Health (WOAH) recommends that equine influenza vaccines include representative strains from both clade 1 and clade 2. This dual-clade inclusion helps ensure broader immunity. 

The American Association of Equine Practitioners (AAEP) recommends following WOAH vaccine recommendations.

Entities that govern competitions also have guidelines:

• Flu vaccination within six months prior is required at United States Equestrian Federation (USEF) licensed competitions.
• Federation Equestre Internationale (FEI) requires horses to be vaccinated against equine influenza in order to compete at FEI events. 
• The FEI supports the WOAH recommendation that equine influenza vaccines should contain both clade 1 and clade 2 strains of the Florida sublineage.
   

How updated vaccines protect horses

Modern equine influenza vaccines should be designed to include protection against both clade 1 and clade 2 strains. Vaccinating with updated vaccines helps: 

• Protect individual horses from severe illness and downtime.
• Protect the herd by creating a population-level barrier against infection.
• Prevent outbreaks by reducing viral shedding and spread.
• Support competition and travel requirements, as many organizations mandate proof of influenza vaccination.
   

Who should be vaccinated?

Every horse should be vaccinated against equine influenza, regardless of activity level. Horses that travel, compete or live in large barns are at especially high risk. Young horses, whose immune systems are less mature, also benefit greatly from up-to-date protection. Older horses, or those that are immunosuppressed due to other conditions, such as pituitary pars intermedia dysfunction (PPID), should also be on a regular vaccination program.
 

Partnering with your veterinarian

Your veterinarian is the best resource to ensure your horse receives the most appropriate vaccine protocol. In the U.S., the flu vaccine for horses at a minimum should contain a clade 1 flu strain. Your veterinarian can recommend the optimum schedule for your horse and ensure proper vaccine handling and administration, maximizing the vaccine’s effectiveness. When working with your veterinarian to choose what’s right for your horse, consider duration of immunity (DOI) for influenza, as not all vaccines are the same. For example, the Boehringer Ingelheim VETERA^® vaccines have demonstrated a six-month DOI against Florida sublineage clade 1 flu, the current circulating strain in the U.S.¹ These vaccines have also been proven safe in pregnant mares and can be administered in foals as early as 4 months of age.
 

A Smart Step for Every Owner

By vaccinating your horse against both Florida sublineage clade 1 and clade 2 flu strains, you are not only protecting your own horse, you are also helping to protect the broader equine community. Just as important, you are safeguarding your horse’s ability to stay healthy, active and competition ready. 

About Boehringer Ingelheim – Animal Health business  

Boehringer Ingelheim provides innovation for preventing and treating diseases in animals. The company offers a wide range of vaccines, parasite-control products and medicines for pets, horses and livestock to veterinarians, animal owners, farmers and governments. As a leader in animal health, Boehringer Ingelheim values that the health of humans and animals is deeply connected, and strives to make a difference for people, animals and society. Learn more at https://www.boehringer-ingelheim.com/us/animal-health.  

About Boehringer Ingelheim

Boehringer Ingelheim is a biopharmaceutical company active in both human and animal health. As one of the industry’s top investors in research and development, the company focuses on developing innovative therapies that can improve and extend lives in areas of high unmet medical need. Independent since its foundation in 1885, Boehringer takes a long-term perspective, embedding sustainability along the entire value chain. Our approximately 54,500 employees serve over 130 markets to build a healthier and more sustainable tomorrow. Learn more at https://www.boehringer-ingelheim.com/.

VETERA^® is a registered trademark of Boehringer Ingelheim Vetmedica GmbH, used under license. ©2026 Boehringer Ingelheim Animal Health USA Inc., Johns Creek, GA. All rights reserved. US-EQU-0195-2026

¹Expert surveillance panel on equine influenza vaccine composition. World Organisation for Animal Health. 2023-2024 Recommendations.