Check out this article in the Times about making a better vaccine using attenuated virus where the actual gene is replaced by a "slow codon speed" gene.

"Cells have evolved to take advantage of this by using different codons for different purposes. Genes for proteins that need to be made quickly tend to be composed of “favorite” codons — the ones that the cell has evolved to use frequently. Genes for “slow” proteins tend to be made of disfavored codons — the ones the cell uses rarely. The reason is that if a codon is rare, the cell takes longer to recognize it, so it gets translated more slowly. A protein from a gene made entirely of rare codons, or rare combinations of codons — for the combinations can matter, too — will thus be made with a fraction of the efficiency of the same protein made from favorite codons or codon combinations. (Certain codon combinations can slow down the cell’s reading machinery.)"

This is the general idea. Very cool.
Here is the article:

http://judson.blogs.nytimes.com/2008/09/02/braking-the-virus/index.html

Omai