Equine influenza last made headlines in 2007 with the Australia epizootic that affected approximately 50,000 horses. Since eradicated from Australia, equine flu viruses still circulate in much of the world, including the United States. Antigenic drift, which produces new virus strains and gradually undermines existing vaccines’ effectiveness, necessitates periodic vaccine updating to combat the new strains.

The vaccine manufacturers look to scientists to advise them on which vaccine virus strains need to be replaced and with what. In 1995 an ad hoc working group of equine flu scientists was founded for just this purpose. Called the Expert Surveillance Panel, this group includes scientists from the OIE (World Organization for Animal Health) reference laboratories for equine influenza in England, Ireland, Germany, and the United States; other labs specializing in equine flu virus; and the World Health Organization.

Each year the panel assembles and reviews evidence of equine flu activity worldwide, looking especially for cases of infection in vaccinated horses. It also reviews data comparing flu strains isolated from the past year’s outbreaks with flu strains used in vaccines. The critical piece of evidence is how well the antibodies stimulated by vaccination will react with the circulating flu strain in the exposed horse. Researchers at Cambridge University have developed a new technique called "antigenic cartography" that makes these analyses easier. Fortunately, in most years the Expert Surveillance Panel reports that the equine flu vaccines are still working effectively. However, constant surveillance is critical.

Scientists now recognize three surviving branches of the equine flu "family tree," one of which currently circulates in the United States: the Florida clade 1 branch typified by strains such as Ohio/03. The Florida clade 2 branch constitutes the majority of recent isolates from Europe and is typified by strains such as Richmond/07. Some older American strains like Kentucky/97 are antigenically similar to Richmond/07. The branch called the "Eurasian lineage" circulated mainly from 1990 to 2005