Update Influenza Vaccines

Veterinarians and horse owners, as caretakers of the health and wellbeing of our animals, tend to be vigilant about ensuring the safety and efficacy of vaccines. But do we ever step back to examine if these vaccines, which include multiple antigens and adjuvants, contain unnecessary components? For example, by continuing to include the A1 strain in flu vaccines, we may actually be providing more than our horses need.

Human influenza vaccines never would contain a flu strain that in recent years has not caused disease outbreaks. In equine flu vaccines, however, the A1 strain is still used in many vaccines, even though there has not been a confirmed case of equine influenza caused by an A1 strain in more than 20 years.

In fact, in 1999, members of the Office International des Epizooties (OIE) also known as the World Organization for Animal Health (an international group that collects and analyzes the latest scientific information on animal disease control) recommended omitting the A1 strain from equine influenza vaccines on the basis that it was "epidemiologically irrelevant."

A1 Background

Influenza viruses are classified into different groups called subtypes. Two subtypes have been recognized in horses, commonly called A1 and A2. The original isolate, or prototype, strain of the A1 flu subtype was first identified in 1956 as the Prague/56 strain. In 1963, the prototype of the A2 subtype was identified as the Miami/63 strain. Equine vaccines typically have included strains of both these flu subtypes because a vaccine against A1 flu does not protect against A2 flu, and vice versa. From 1963 to 1978, both subtypes were known to circulate in the horse population; however, the last confirmed isolation of A1 in the world was in 1980.

Although there have been reports from India, Egypt, and other places around the world of A1 outbreaks, insufficient evidence exists for OIE laboratories to conclude that the cause of the outbreaks was the A1 strain. In addition, there have been reports of horses with no vaccine history seroconverting to the A1 strain (indicating exposure), but it is unknown how and why this seroconversion happens without the virus present in the environment. Some of these could be false positive testing artifacts.

OIE Recommendation

Members of OIE Reference Laboratories help gather and analyze animal disease information, which in turn is made available to countries worldwide to help improve methods of controlling and eradicating diseases. In addition, the role of a reference laboratory is to function as a center of expertise and standardization of diagnostic techniques for its designated disease.

Because the last confirmed isolation of A1 in the world was in 1980, the 1992 World Health Organization (WHO)/OIE Consultation on equine influenza virus emphasized the need to collect additional information to determine whether or not the A1 strain was still circulating among horses.

In 1995, OIE and WHO experts on equine influenza reviewed the status of A1 flu and decided that, if there were no A1 cases reported in five years, they would recommend the omission of the A1 subtype from equine influenza vaccines. No subsequent cases emerged, and in 1999, at the Fourth International Meeting of OIE and WHO experts on equine influenza, this recommendation was made. Despite the recommendation, OIE and WHO experts on equine influenza will continue to watch for A1 strains in case they return.

The Future of Equine Vaccines

Veterinarians expect OIE's recommendations affecting vaccines to reflect their belief that vaccines should protect horses from diseases they are at risk of encountering.

Human influenza vaccines are annually reviewed and updated with virus strains believed to best represent flu strains currently circulating around the world. This should be the case with equine influenza as well; there is no point in vaccinating against flu strains that no longer cause problems in horses.

By removing the A1 strain from equine influenza vaccines, animal health companies can rebalance flu vaccine formulations to create vaccines that are more effective and safer for horses. Some companies are now taking this step.

About the Author

Thomas M. Chambers, PhD

Thomas M. Chambers, PhD, is Head of the Office International des Epizooties International Reference Laboratory for Equine Influenza at the University of Kentucky's Gluck Equine Research Center.

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