Comparative Efficacy of Vaccines

Several researchers have been working the past few years to determine if any U.S.-registered equine vaccine had advantages over others. It turns out that the serologic responses to--and protection given by--these vaccines varies significantly. Hugh G. G. Townsend, DVM, MSc, of the Department of Large Animal Clinical Sciences at the Western College of Veterinary Medicine in Saskatoon, Saskatchewan, presented results of the comparative study.

The scientists looked at seven killed vaccines and one modified-live vaccine produced by major U.S. manufacturers. Fifty naìve nine-month-old horses were used in the study. Blood samples were taken every four weeks to measure serologic response to influenza, tetanus, and equine herpesvirus (EHV) vaccines until they were challenged with equine influenza virus (EIV).

The following vaccines were used: Intervet's Encevac T (Eastern and Western equine encephalitis, or EEE and WEE, and tetanus) and Prestige II (EHV types-1 and -4, and EIV); Fort Dodge Animal Health's (FDAH) Equiloid (EEE, WEE, tetanus) and Fluvac EHV-4/1 Plus (EIV, EHV-1 and -4); Boehringer Ingelheim's (B-I) Cephalovac EWT (EEE, WEE, and tetanus), Calvenza EHV (EHV-1), and Calvenza EIV; and Intervet's Flu Avert (EIV modified-live vaccine) in combination with Encevac T and Prestige II.

"We were interested in what would happen if we primed with modified-live Flu Avert, then came back and boosted with Prestige and Encevac," said Townsend.

Each group was vaccinated at 0, 4, and 16 weeks after the initiation of the study. A fifth group of unvaccinated control horses was included in the study.

Fourteen weeks after the horses received their final vaccinations, all of the animals (including the controls) were transported to a challenge facility, where they were kept in one large pen. Two weeks later, the horses were challenged with live EIV.

According to the authors, "This study provides the first published evidence that killed influenza vaccines marketed in North America are capable of reducing clinical signs and viral shedding in experimentally challenged horses."

All horses receiving vaccines experienced a significant increase in serum antibody concentration against EIV, although there was significant variation with response (denoting better protection), especially after the third vaccination. The best response occurred after vaccination with Calvenza, with horses experiencing less fever, nasal discharge, and viral shedding after virus challenge than animals in the Prestige II and Fluvac groups. However, Calvenza horses did not show significantly less clinical signs or viral shedding than the horses primed with Flu Avert and boosted with Prestige II. Horses receiving Prestige II alone had the second-highest serum antibody concentrations behind Calvenza.

All vaccinations protected against weight loss typically caused by EIV. Over the 10 days following challenge, control horses lost an average of 40 pounds while vaccinates gained an average of 20 pounds.

All vaccine groups responded to the tetanus vaccine, and the antibody response improved after each vaccination. Encevac T achieved significantly greater concentrations of antibody than Equiloid, but not significantly greater than Cephalovac.

Over the 16 weeks following the final vaccination, serum tetanus antibody concentrations declined steeply, suggesting that duration of immunity with tetanus vaccines should be revisited. (It previously was suggested that duration of immunity might extend beyond a year.)

In summary, Townsend said, "All of the vaccines were efficacious. Calvenza and Flu Avert were the best.

"A model is a model," he added, reminding attendees that the horses in this study were maintained in one group with control horses shedding high amounts of virus. As a result, the viral exposure of vaccinated horses in this was more intense and prolonged than would be expected during a natural challenge. Under natural field conditions, the vaccines used in the study might yield even better protection than the scientists observed.

He said that the antibody response to EHV vaccines in this study was significant, but of a low magnitude. Other points were that, 1) a relationship hasn't been demonstrated between serum antibody concentration and EHV-related respiratory disease; and 2) that results of comparative respiratory challenges are needed to make a complete assessment of EHV vaccines. (See article #4980 online.)

Immune Responses to Vaccines

"Infectious disease is common in horses; for this reason we use vaccination as an important component of our preventive strategy. Nevertheless, we're often left with a lack of studies and information that could be important (in decision-making regarding vaccination)," said Paul Lunn, BVSc, MS, PhD, head of the Department of Clinical Sciences at Colorado State University's College of Veterinary Medicine. Lunn presented studies that he and six other scientists completed comparing immune responses between commonly used, commercially available vaccines.

The scientists first did a seroconversion study of EIV, EHV-1, and EEE vaccines. Second, they looked at the immune responses to two EHV-1 anti-abortion vaccines.

The horses used in both studies were aged, non-pregnant mares whose full vaccination histories were not available, although it was thought that the majority would have received annual spring vaccinations in the past. The mares were all maintained in a single barn in one large group and were run through a chute system for sample collection and vaccination. Each had an electronic identification chip.

For the first study, a total of 40 mares were used, divided into the following groups of 10:

  • Group A: Intervet Vaccines: Encevac T, Prestige II;
  • Group B: Fort Dodge Animal Health vaccines: Equiloid and Fluvac EHV-4/1 Plus;
  • Group C: Boehringer Ingelheim vaccines: Cephalovac EWT and Calvenza EIV/EHV; and
  • Group D: The control group received a placebo.

Two shots were given one month apart, and blood samples were taken at the time of each vaccination and at monthly intervals for six months after the second vaccine. "None of the vaccines in either of the two studies in this relatively small number of horses demonstrated any adverse effects of vaccination," said Lunn.


Influenza--"In this experiment, all of the horses had a background level of antibody to influenza, indicating prior exposure through exposure or vaccination," said Lunn. "Against this background antibody level, the responses to vaccination were generally low, although Boehringer Ingelheim's Calvenza vaccine (Group C) did produce a statistically significant response compared to the control horses. Nevertheless, the responses were all relatively low and the statistically the three vaccines could not be distinguished from each other in any other comparison."

EHV-1--Responses to EHV-1 vaccines were even less impressive, said Lunn. Over the course of the experiment, EHV-1 antibody responses were not elevated in any vaccine group when compared to the control levels.

EEE--Antibody responses to vaccination in the Intervet (Group A) and FDAH (Group B) groups were both significantly greater than the control group over the time course of the experiment.

Tetanus--Antibody responses to tetanus demonstrated the greatest difference between the vaccines. All the vaccines generated antibody responses greater than the controls, but the Intervet vaccine group (A) had higher tetanus antibody levels than either of the other two vaccine groups.

"There were certainly significant differences between different products, and no single manufacturer had a product that excelled in terms of antibody response to all of these antigens. Each had some strengths and weaknesses," he concluded about the first study.

"The responses to respiratory products were generally lower," Lunn said. "In defense of the B-I vaccine, it is labeled for a three-dose regimen, but the authors used it extra-label (only two doses). We would recommend all of these vaccines be administered three times for the primary course, as is stated in current AAEP recommendations."

EHV-1 Abortion Vaccines

In the EHV-1 anti-abortion vaccine study, the scientists compared Intervet's Prodigy (Group E) to the FDAH Pneumabort K vaccine (Group F). Three groups of five horses each received either the Intervet or the FDAH vaccine or a placebo (control group) three times at two-month intervals. Blood was drawn before administration of the first vaccine, one month after the second vaccine, and one month after the third vaccine. Antibody responses were measured, as was interferon-gamma production (INF-g, which fights viral infection by preventing virus multiplication and might be critical for preventing EHV-1 abortions).

Both vaccination groups demonstrated a strong antibody response to the first vaccine, with no further increase after the second dose. In the Intervet group, there was a response after a third dose, and antibody levels in this group were significantly different from the controls over the course of the study. The Fort Dodge vaccine responses did not quite achieve statistical significance compared to control horses, which might have been a result of the relatively small number of horses in this study.

"Generally, a INF-g response is something you want to see," said Lunn. "The only responses were after the third vaccination" in both vaccination groups, although this increase was statistically different from the controls only in the Intervet group (E). This is the first demonstration of a INF-g response to EHV-1 in horses. The Intervet and FDAH vaccines were not found to be statistically different from one another, and it might need a bigger study to establish whether one of these vaccines is clearly superior to the other or not.

The virus neutralization titers of these vaccines were much higher than the responses seen in the respiratory products in the conventional seroconversion study. "Given the greater immune responses to vaccination, it may be wise to consider using the anti-abortion EHV-1 formulations for control of other forms of EHV-1 infection as well as abortion," the researchers suggested. (See article #4981 online.)


 Group 1  X  X            
 Group 2      X  X        
 Group 3          X  X    
 Group 4  X  X          "Prime"  
 Group 5                x


These studies were generously supported by Intervet.

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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