Recent advances in immunology, molecular biology, and peptide biochemistry have allowed the construction of subunit vaccines based on viral or bacterial recombinants, peptides or plasmid vectors. Although none of these approaches is currently being used for mass vaccination (with the exception or vaccinia-rabies G protein recombinant virus for wildlife immunization); several of them are undergoing clinical trials. None of these different vaccine constructs is likely to be totally effective in either the prevention of infectious diseases or immunotherapy of cancer. Recombinant viral vaccines such as those based on vaccinia or adenovirus as a rule induce potent immune responses. Vaccinia viruses have the added advantage of being heat stable and immunogenic after oral application, making them good candidates for wildlife immunization. Recombinants based on replication-defective adenoviruses are safer compared with vaccinia virus recombinants and, as far as our data indicate, have superior efficacy. In addition, they induce excellent immunity upon application to mucosal membranes, suggesting their usefulness as vaccines for infectious agents that enter through the airways or the genital tract. Peptides are of limited benefit in infectious disease prevention but might provide custom-made vaccines for cancer therapy. Genetic vaccines that were first described less than 5 years ago have already progressed to phase I clinical trials in healthy human adults. Provided that their safety can be confirmed, they might be suited to induce immunity to numerous agents.

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