The Equilis Prequenza equine influenza vaccine has been shown in a study to provide high levels of protection against a new strain that triggered major outbreaks in Europe in 2018.
The outbreaks in Europe occurred despite various equine flu vaccines being widely available, Sylvia Reemers and her colleagues noted in the journal Vaccine.
Influenza viruses are known for their high mutation rate, causing antigenic drift over time.
In 2018 and early 2019, major outbreaks were reported in South America and a great part of Europe. Genetic analysis revealed the virus belonged to the H3N8 Florida Clade 1 strain.
The Florida Clade 1 sub-lineage is present in most commercially available vaccines.
In most countries with a high vaccination rate, the number of outbreaks was low. However, in France, many outbreaks still occurred, despite vaccination coverage greater than 80%, and most horses being vaccinated with a vaccine expressing the FC1 antigen.
Both unvaccinated and vaccinated horses were affected in the outbreak.
This raised the question, investigated by Reemers and her fellow researchers, of whether other attributes of equine flu vaccines, such as whether the antigen is whole virus or a subunit, and the type of adjuvant may be equally important factors affecting the performance of vaccines in the field.
The study involved 10 Shetland ponies.
Five were vaccinated with Equilis Prequenza, a commercially available ISCOMatrix-adjuvanted, whole inactivated virus vaccine. There was an initial dose, followed by a second 28 days later.
Forty-nine days after the first dose, the five animals, together with five unvaccinated controls, were experimentally infected with the new FC1 outbreak strain.
Serology, clinical signs, and virus shedding were evaluated in the vaccinated compared to unvaccinated horses.
The results showed a significant reduction in clinical signs and a lack of virus shedding in the vaccinated horses compared to unvaccinated controls. Sterile immunity had been induced in the vaccinated horses.
“This was an interesting finding, since field experience with this new FC1 strain as reported from France was that several outbreaks had involved correctly vaccinated animals, the vast majority vaccinated with a vaccine fully in line with OIE (World Organisation for Animal Health) recommendations.”
One of the main differences between the predominant vaccine used in France and Equilis Prequenza is that the former expresses only viral hemagglutinin, while the latter contains all viral proteins, as it is a whole inactivated virus vaccine.
“From these results, it can be concluded that Equilis Prequenza provides a high level of protection to challenge with the new FC1 outbreak strain,” the study team concluded.
“This suggests that, apart from antigenic differences between vaccine and field strain, other aspects of the vaccine may also play an important role in determining field efficacy.”
Aside from antigenic differences between the vaccine and field strain, the nature of the antigen and/or adjuvant may likely play an important role in vaccine efficacy against new field strains, they said.
Further research is needed to investigate the role of antigenic differences between vaccines and field strains, and other attributes of flu vaccines, such as vaccine type or adjuvant, in performance in the field, the researchers said.
The study team comprised Reemers, Sander van Bommel, Qi Cao, and Saskia van de Zande, with MSD Animal Health in The Netherlands; and David Sutton, with MSD Animal Health in Britain.
Reemers, S.; Bommel, S.; Cao, Q.; Sutton, D.; Zande, S. Protection against the New Equine Influenza Virus Florida Clade I Outbreak Strain Provided by a Whole Inactivated Virus Vaccine. Vaccines 2020, 8, 784.