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The changing landscape of the vaccine industry and our preventative strategies (Proceedings)
The deliberate induction of active immunity to an agent by exposure to the agent or to non-replicating components, with the intent of inducing protective immunity to challenge with a virulent infectious agent, is termed "vaccination". Actively acquired immunity is that provided by an antigen specific response of the challenged host's own immune system in response to materials recognized as non-self.
1. Generalities of Vaccination and Vaccine Formulations
The deliberate induction of active immunity to an agent by exposure to the agent or to non-replicating components, with the intent of inducing protective immunity to challenge with a virulent infectious agent, is termed "vaccination". Actively acquired immunity is that provided by an antigen specific response of the challenged host's own immune system in response to materials recognized as non-self. Active immunity is usually long-lived with generation of memory cells. While we, as veterinarians, will never be privy to all of the information due to proprietary protections, we can nonetheless develop rational strategies for vaccine use based on the data we have. This data should be used to achieve the goals of maximizing the effective immune response. By maximizing the immune response, the secondary goals of safety and cost can be met. Expectations a practitioner should have in mind when choosing a vaccine formulation: First and foremost a vaccine should provide protection against virulent challenge. Whether or not a vaccine actually does this, is at the heart of most of our vaccine conundrums and reflects the degree to which a currently marketed vaccine has undergone efficacy testing (addressed in the next section). Second, a vaccine must induce a protective immune response among essentially all members of a population. Premarketing efficacy testing should demonstrate an efficacy level of at least 90-95% preventative activity. Many currently marketed vaccines do not provide this level of efficacy overall or within target populations. Knowing these deviations from this goal is important for effective vaccination. Third, a vaccine should induce memory for long-lasting protection. Most animal vaccines do have to show induction of memory. Many are simple formulations that do not induce long-lasting protection. Fourth, a vaccine formulation should not be susceptible to rapid evasion by strains of an infectious agent that vary antigenically from the type strain used in development of the vaccine. We are fortunate in the equine industry to not have rapid expansion of new strains in many of our diseases, even influenza. However, because of the regulatory barriers to frequent updating of vaccines, the industry is susceptible to development of new strains of many diseases which could result in reemergence of disease. Fifth, it must be safe and without serious side-effects. In the case of attenuated live vaccines, this includes no reversion to virulence. Sixth, vaccine use should not cause confusion in the diagnosis of the active infection. Unfortunately vaccines are developed with little regard to this expectation and frequently it is the diagnostic community that comes up with new testing strategies in response to a vaccine. Seventh, vaccine should be stable and easy to transport. Unfortunately, for most veterinarians, this requirement often leads to biasing of vaccine use only toward killed vaccines. Many killed vaccines are excellent, however, there are other more recently developed vaccines that a far superior and the main barrier to their widespread use is the actual product formulation. If this is the only barrier to use of a superior vaccine, then clients are ill-served. Eighth, the vaccine should be affordable. The definition of affordability is ill-defined. If one chooses to use a shorter acting vaccine because it is less expensive than a longer acting vaccine, then this is not a less expensive strategy for the client due to lack of protection or the need for more frequent veterinary contact.
What a vaccine MAY NOT do is eradicate a disease. Eradication of disease through vaccination is a rare event. These diseases generally have little subclinical or chronic states so that infected animals can be easily identified and separated from susceptible populations: the best example is small pox vaccination. These diseases usually have a short incubation period and organism spread is usually confined to the period of clinical signs. Eradicable diseases usually do not have wildlife reservoirs. Thus, a serious discussion with clients on the expectation that vaccination will minimize disease but not erase the disease or totally mitigate the risk of disease is required. An example of these are equine respiratory viruses. Vaccination against many of the respiratory diseases, decrease severity of disease and decrease viral, but these common viruses will not eradicated through vaccination.
In general a vaccine should NOT be used for treatment! There are many times during the incubation and onset of disease that active priming of the immune response at that time is futile and may even be deleterious. For example, we have recently published evidence that horses vaccinated against WNV within two weeks of exposure actually had a higher chance of exhibiting clinical signs if they developed WNV within this time period (Rios LV. Et al. 2009)
Vaccine quality can vary due to the enormous number of variables which ultimately define an effective immune response. The type of antigen is the most important of these; type-proteins > carbohydrates > fat, complexity, size, dose. Thus the industry often expects very basic preparations composed of compounds that may not be of the best composition because of the variability of the immunogenisic properties of the organism itself. For example, rabies and tetanus, a viral immunogen and basically a protein are very good immunogens while others, may require adjuvants or modifications of the immunogen itself to enhance its basic response when inactivated.
Vaccine response overall and the type of response varies according to the route of inoculation. Most equine vaccines are deep intramuscular and have historically been developed as such because of the possible cosmetic outcomes associated with reactions. Whether or not this is true, intramuscular administration is inherently less immunostimulatory than subcutaneous. This is one of main reasons why many killed and live vaccines have adjuvants. Oral and nasal vaccines are an excellent strategy for stimulation mucosal immunity throughout the body. However, an oral or nasal vaccine may not produce high levels of IgG systemically. For example, if the goal in vaccinating broodmares were to produce colostral IgG, then an intranasal vaccine would be counter indicated because these vaccines induce primarily local musosal immunity.
Adjuvants. Adjuvants are materials which, when administered with an antigen, greatly enhance the immune response to that antigen. There are two major avenues by which this is achieved in the industry, by depot effect and by immunomodulation. In the first, most adjuvants are immiscible with aqueous solutions (such as the tissue environment), or are very low in solubility. Because of this, material incorporated into an adjuvant is released to the immune system slowly and in low concentrations (referred to as a "depot effect"). This provides prolonged stimulation and tends to favor selection of lymphocyte clones producing high affinity antigen-binding receptors. Also, the depot may provide a protective and masking environment so the innate immune system does not destroy the particles so rapidly that antigen-specific priming is not achieved. Second, many adjuvants cause a non-specific stimulation of cells which must present antigens for a primed immune response (mainly macrophages, Dendritic cells and B cells). Preparations may be specifically formulated to bias the immune response. There are many new types of adjuvants under development that offer promise which may enhance particular responses. However, currently the four components found in vaccines are aluminum compounds including aluminum compounds (aluminum hydroxide, aluminum phosphate, potassium aluminum sulfate), calcium phosphate (saponin), oil emulsions (paraffin, mineral oil, lanoline, squalene, ISA-70, and Montanide) and/or glycerin (to act as a pseudo-ISCOM). Drawbacks to the use of adjuvants include formation of granulomas and even sterile abscesses at the site of injection. In fact these non specific responses are the cause of many of the reports of localized pain and local tissue reactions. Some animals (and owners) are completely intolerant of adjuvants in vaccines. Several cytokine adjuvants are currently in phase I and I human clinical trials. Biodegradable polymers are on the horizon for use in livestock and horses. In these the vaccine is packaged into a slow release polymer which allows slow release and thus long term antigen stimulation for long term protection (maybe even of years duration). These can packaged for several routes of administration.
2. Testing of Vaccine Prior to and After Marketing and What This Means For Your Strategy
For particulars in licensing please refer to: http://www.aphis.usda.gov/animal_health/. Prior to marketing, vaccine testing in animals usually is composed of a safety testing which employs giving very high doses of the product to limited numbers of animals and testing for reversion to virulence (modified live) or reactions (generally killed, adjuvanted) in those animals depending of formulation. In all less than 20 or 30 animals are involved in this phase. In addition there is a field testing phase which usually takes the final product and filed practitioners inject 500-1000 animals for observation of untoward reactions and events. This phase has very limited observational categories, for instances, rarely is rectal temperature obtained in these field studies, especially in large animals. So "safety" testing is a relatively restricted set of parameters prior to marketing an animal vaccine. Efficacy testing usually involves the lowest dose of vaccine that is expected to mitigate infection tested in a challenge model. There frequently is NO gold standard challenge model. Endpoints can also vary. In addition, there is no power analysis requirement.
3. What Vaccine Labels Indicate in Terms of Actual Protection
Labeling. Label claims are written to reflect the degree of efficacy and safety testing performed. Based on this the USDA will grant up to 5 levels of protection for a vaccine and these include in order of degree of known protection: 1) prevention of infection, 2) prevention of disease, 3) aid in disease prevention, 4) aid in disease control, and 5) other claims. The label claims must be accurate and reflect the actual expectations of product performance. Thus "prevention of infection" can only be made if the product is able to prevent ALL colonization or replication of the challenge organism. For "prevention of disease", the entire 95% confidence interval of efficacy must achieved a minimum of 80% prevention of disease due to infection. The most common label level consists of an "aid in disease prevention" where a significant reduction in disease is obtained. The label "aid in disease control" is made for products which alleviate disease severity, reduce disease duration or delay disease onset. "Other" claims include reduction of pathogen shedding. The degree of efficacy reflects the model under which testing was performed. After Market Testing. Aftermarket testing is commonly pursued by manufacturers and by independent investigators. The most common after market testing for a vaccine is performed in order to address issues protective immunity in comparative efficacy trials where one vaccine is compared against another. Aftermarket testing would be required when a new strain of an organism emerges and the industry needs updated information without new labeling. A third common after market form of research asks questions regarding specific populations of animals with examples including studies of protective immunity in young or old animals, pregnant animals, animals with maternal antibody, etc.
4. Strengths/limitations of Vaccine Formulations
Inactivated vaccines. Inactivated vaccines consist of whole infectious agents may be treated chemically to kill the agent prior to incorporation into a vaccine. Live replicating vaccines. Most commonly, live vaccines are "attenuated" whole organisms. Subunit vaccines. Subunit vaccines derived of non-replicating components of the infectious agent may take several forms. Live recombinant vector vaccines. Live recombinant vector vaccine is different from live replicating vaccine in that the live or replicating organism is the virus vector and not the organism that one is trying to make the vaccine against. DNA Vaccine. DNA vaccines are constructed in an E. coli expression plasmid which in transfected cells secrets the main proteins of the pathogen.
Table 1. Vaccine Formulations-Strengths and Weaknesses
New Uses for Vaccines-What to Look for on the Horizon
Improvements in killed vaccine formulations. Much of the up and coming improvement in production animals is through adjuvant development. In this area swine formulations are currently being developed to overcome the affects of maternal antibody immunity. This is an important focus in equine medicine; our recent studies using killed vaccines in foals have shown exceptionally limited antibody in foals from multiparous mares in the production to antibody to tetanus, EEEV, and WNV. A second area of focus is the development of a faster regulatory pipeline for addition of new strains of viruses into licensed vaccines. This is especially important for EHV1, Influenza and West Nile virus.
Improvements in live vaccine formulations. Live vaccines administered systemically cause mild infection. In certain instances, this is considered to increase risk associated with live vaccines. One advantage underutilized until recently, is the fact that live vaccines offer more opportunity to enhance protection by administration mucosally through nasal, oral, vaginal and ocular routes. Development of vaccines that do not replicate systemically and only locally offer opportunities to produce extremely safe, targeted protective strategies.
Development of DIVA Vaccines
The single most important contribution that DIVA vaccines will have is that countries will be able to introduce vaccine strategies that protect against foreign animal diseases. DIVA vaccines are engineered vaccines that leave a signature that can be detected by serology. This can protect a population highly at risk of introduction of a new agent while still being able to differential vaccine responses from disease. Although these vaccines are uncommon, several cattle vaccines have been developed which include foot and mouth disease (FMD), infectious bovine rhinotracheitis (IBR), and pseudorabies virus.
Molecularly Engineered Vaccines
There are many, many new avenues which combine the safety of a "subunit" vaccine with that of a highly replicating viral vector to develop various combinations of enhanced vaccines that specifically target ONLY the proteins of the pathogen. In addition these viral vectors demonstrate enhanced safety because they are of alternate host origin. One of the most common of these platforms is the Canarypox vector. This vector has been used quite safely in the horse to induce protection against influenza and West Nile virus. Continued mining of stable replicating vectors for use in the horse is an important area for development of vaccines that can be used in the face of outbreaks and in the pregnant animal. In addition, these allow the development of DIVA platforms by their very nature.
Non-Infectious Replicating Particles
Although none are currently licensed for equine use, non-infectious viral replicating particles hold much promise for the future of animal and human vaccinology. Essentially both cell culture and viruses are engineered to create an empty shell of virus with important pathogen proteins that stimulate the immune response. These particles can be designed to have almost any protein from any pathogen on its surface. Thus both antibody responses and cell mediated responses can be used to stimulate an immune response.
DNA Vaccines and Needleless Inoculation
The influenza field in humans is clearly leading the way here. Naked DNA containing the particles of a vaccine has long been viewed as having high potential for success against pathogens. One drawback to their use has been limited antigenicity by needle injection. Current engineering of needleless devices offer new opportunities to pursue these vaccines in animals.