It’s five years in, and the first results are in. Find out what they are telling us about rabies vaccines and the duration of immunity.
As any animal lover knows, rabies vaccine is the one immunization required by law. All dogs and cats must receive it at least once every three years, after the initial two-dose series, even though researchers believe this vaccine causes among the most and worst adverse reactions.
To address this situation, we established the Rabies Challenge Fund Charitable Trust (rabieschallengefund.org) in 2005. The mission is to better define the duration of immunity (DOI) conveyed by rabies vaccine in dogs. The eventual goal is to extend the legally required interval for rabies boosters to five and then seven years. The Rabies Challenge Fund also intends to establish the world’s first canine rabies titer standard with research backed by the same federal challenge criteria required to license rabies vaccines.
This vaccine research project is being performed by Dr. Ronald Schultz at the University of Wisconsin School of Veterinary Medicine. The study follows the strictly defined federal standards for licensing rabies vaccines and involves concurrent five- and seven-year challenge trials. The funds are collected by Dr. Dodds at Hemopet, and distributed annually to the University of Wisconsin Foundation.
To date, funds have been sent to the University of Wisconsin Foundation for five years of the study – $175,000 for each of the first two years, $150,000 for year three, and $145,000 for years four and five. Hemopet provides the financial accounting and tax filings for the Trust.
Summary of five-year DOI study
The first rabies vaccine studied was selected based on the superior response it provided in the USDA challenge trials for licensing; it is no longer available now as the company merged with another. Then, a second licensed rabies vaccine was administered to a separate group of dogs two years after the first study began, so that two licensed rabies vaccines would be tested. The second vaccine selected is the one currently administered to a very high percentage of dogs.
Both vaccines demonstrated excellent response based on rabies antibody testing for each of the first three study years. However, fewer than 30% of dogs in the first vaccine group, now six years since vaccination, had serum rabies antibody titer levels considered adequate on the Rapid Fluorescent Focus Inhibition Test (RFFIT) (above the 0.1 IU/mL level stated by the Centers for Disease Control to be an adequate response to rabies vaccination in people).
Some of the dogs with low or no detected RFFIT antibody were further tested to determine if they had “immunologic memory”. This in vitro test shows whether memory is present or not, even in cases when serum antibody cannot be detected at a level considered to be adequate. The results of this further testing indicated that most of the dogs vaccinated five years ago, even without an adequate RFFIT, do have “immunologic memory”.
Our conclusion from studies with the initial rabies vaccine is that the immunity conferred by that product, and assessed by the RFFIT, was excellent for the first three years, then declined during the fourth year, and continued to drop during the fifth. Nevertheless, the dogs that completed the five-year study who were given a second rabies vaccine (even those with rabies antibody titers that dropped below the RFFIT adequate level), demonstrated boosted rabies titers, indicating the presence of an active immune memory cell response. The second vaccine group, which is now four-and-a-half years from vaccination, will remain on study for two more years.
We are continuing to raise funds to complete years six and seven of the Rabies Challenge Fund study and look forward to the results, which should benefit all dogs everywhere
3 Important Issues About Rabies Vaccine
1. Not all vets comply with new mandates – All 50 states now recognize the three-year rabies vaccine, although some county ordinances still require annual boosters. Thus, the vast majority of dogs in the United States can follow a three-year rabies booster protocol after the initial two-dose series is given within 12 months. In some states, however, veterinarians are given the discretion whether to administer more frequent boosters. If veterinarians believe that rabies boosters should be given annually or biennially (every two years), they can usually talk their clients into doing so. In Minnesota, this issue has created a longstanding controversy and public campaign to overturn the practice by which veterinarians date rabies vaccine certificates for only two years even though they administered a three-year vaccine. North Dakota has similar issues.
2. Several factors affect immune response to vaccines – An animal’s size, age and breed, as well as the sampling time and vaccine used, have significant effects on the rabies titers measured to comply with the internationally accepted antibody titer of 0.5 IU/mL for entry into rabies-free countries like the UK and Sweden. The data in two separate studies showed that:
a. Smaller dogs elicited higher rabies antibody levels than larger breeds.
b. The duration of rabies immunity also varied between breeds.
c. Young animals, less than one year of age, generated a lower antibody response to rabies vaccination than adults.
d. The number of days between vaccine administration and when the rabies titers were measured was also an important determining factor.
e. Failure to achieve the required rabies antibody level also varied among different vaccines tested: two vaccines performed equally well, and significantly better than the others. These findings were confirmed in a separate French study.
3. Mercury is used in most vaccines – Despite their toxicity, mercury compounds have been widely used in medicine. Called thimerosol, mercury is used in all but two of the rabies vaccines licensed in North America. Mercury and other heavy metals affect the body in two main ways: through toxic and immunological reactions – the latter causing hypersensitivity or autoimmunity. Studies show that metals, including mercury, can be a risk factor for the development of various autoimmune diseases, such as thyroiditis, multiple sclerosis and kidney disease, as well as nonspecific symptoms such as chronic fatigue and myalgia (muscle pain). Studies have shown that mercury can induce reactions, such as skin disease or autoimmunity, depending on the individual animal’s genetic makeup.