The article, Recombinant Vaccine Technology in Veterinary Medicine, by Kent R. Van Kampen, DVM, PhD (p 535-538 in Vol 31, No 3, Vet. Cl. of NA: Small Animal Practice, May 2001) was interesting and difficult. It's been over 20 years since I did anything serious with DNA and gene-splicing, so I had to dredge up a lot of information from the corners of my mind to understand this article. The exciting thing is this form of vaccine seems to hold great promise -- not only to develop new vaccines for different diseases, but also because the vaccine avoids some of the adverse reactions.
The standard vaccine types have problems. The killed vaccines often provide only short-lived immunity. The modified live vaccines have the real risk of causing the disease. So, researchers developed recombinant vaccines, or, genetically engineered vaccines, which are focuses on specific antigens (reactive agents). With these vaccines, there is little chance of a cat developing the disease.
The general method (and this is a huge simplification) is to chop up the DNA of the pathogen, such as rabies. Part of the DNA or RNA is then inserted into a recipient cell, such as a yeast, bacterium or virus. The vaccine is then derived. This manipulation of genes can create vaccines in three general ways:
Type I -- Subunit Vaccines
For this type, the gene responsible for producing the antigen is snipped from the pathogen and spliced into a host. The host is then cultured, which allows it to produce the antigen. The antigen is then purified and becomes the vaccine.
Type II -- Gene-deleted Vaccines
For this type, one or several genes are removed from the DNA of the pathogen to decrease its pathogenicity, or rather, it's tendency to produce the disease. The pathogen is cultured and used as the vaccine.
Type III -- Vectored Vaccines
This is the most complicated type. The genes from the pathogen that are associated with protection from the disease are isolated and then inserted into a gene-deleted host. This host replicates, is purified, and then used as the vaccine. The host organism enters various cells in the cat and 'infects' the cat with the protective genes. Canarypox is often used as the vector (host) and was licensed for use in rabies vaccines.
This article is the tip of the iceberg when it comes to recombinant vaccines. It was written as the concept was just beginning to be widely studied, in 2001. From my limited reading of newer articles, the recombinant rabies vaccine is considered safe, very effective, and without a risk of sarcoma. I hope to have time this summer to do more reading on this topic.