Maximizing the Benefits of Influenza Vaccination

"Beware the vaccination paradox!" began J. Richard Newton, BVSc, MSc, PhD, DLSHTM, DipECVPH FRCVS, of the Animal Health Trust in Suffolk, United Kingdom, during his presentation on influenza vaccination at the 43rd annual British Equine Veterinary Association Congress, held Sept. 15-18 in Birmingham.

He described the vaccination paradox as follows:

  • A disease is highly prevalent.
  • People vaccinate against it.
  • The disease incidence is notably reduced.
  • People stop vaccinating because it doesn't appear to be a common threat.
  • The number of susceptible individuals increases.
  • The disease reappears!

As an example of this process, Newton recounted events of late 2003, in which there was a large outbreak of equine influenza in South Africa not long after the South African Jockey Club made flu vaccinations optional instead of mandatory as they had been. He quoted a news report from that time stating that more than 1,000 horses were affected with the flu, one was dead, and racing was cancelled in several areas.

Most outbreaks occur in non-vaccinated animals, he said, although vaccine 'breakdown' does occur periodically, mostly in young Thoroughbreds (usually because of their frequent exposure to other horses, but also possibly through travel and exercise stress). This 'breakdown' in vaccination protection (i.e. appearance of infection and clinical signs in properly vaccinated animals) happens when the virus that causes the disease mutates, thereby becoming less susceptible to the immune response produced by vaccines containing older strains of virus. In other words, the viruses in the vaccine and those that a horse encounters in the field are sufficiently different that the vaccine is no longer effective in conferring complete protection.

"This confirms the need for a potent vaccine with epidemiologically relevant strains," Newton stated. He added that researchers need a better understanding of "the real-life factors" that affect an animal's ability to respond to vaccination and therefore facilitate the best use of current vaccines.

He discussed several studies on flu vaccine effectiveness, reporting that protective antibody levels in one study of yearling Thoroughbreds:

  • Increased with the number of vaccine doses;
  • Varied with the vaccine type given;
  • Were improved with vaccination after six months of age; and
  • Decreased with time since the last vaccination.

Based on this and other studies, Newton made several recommendations for influenza vaccination as follows:

  • Use a potent vaccine containing epidemiologically relevant strains of virus. (i.e., the viruses in the vaccine should be closely related to those strains circulating in different parts of the world at the present time)
  • Give the first vaccination to foals at six months of age or older.
  • Vaccinate before high-risk periods (sales, races, start of training, etc.).
  • Extend the primary course interval in primed horses (the time between the first two doses of the year in previously vaccinated horses). He noted that the Jockey Club allows up to 92 days between primary course doses.
  • Decrease booster intervals to around six months in young horses.
  • Decrease intervals between primary and other courses to around three to four months.

He added that the data sheets (i.e., product inserts containing the manufacturer's information) for current flu vaccines recommend the first booster six months after the second dose of the primary course.

"Some data suggest that a three- to four-month booster interval might be beneficial in providing more complete protection by decreasing the period at which the horse is at most risk (when his antibody titer is low)," he commented.

"In conclusion, flu is a serious disease, vaccines prevent it, the virus evolves over time, and the vaccines get out of date, so surveillance is necessary and is being conducted. Consequently, vaccines and protocols are having to be continually refined," he summarized.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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