James J. Bull, Mark W. Smithson, Scott L. Nuismer

Genetic engineering now enables the design of live viral vaccines that are potentially transmissible. Some designs merely modify a single viral genome to improve on the age-old method of attenuation whereas other designs create chimeras of viral genomes. Transmission has the benefit of increasing herd immunity above that achieved by direct vaccination alone but also increases the opportunity for vaccine evolution, which typically undermines vaccine utility. Different designs have different epidemiological consequences but also experience different evolution. Approaches that integrate vaccine engineering with an understanding of evolution and epidemiology will reap the greatest benefit from vaccine transmission.

Trends:

• Two types of live viral vaccine are amenable to transmission: attenuated and recombinant vector vaccines.
• The epidemiological consequences of vaccine transmission vary with vaccine design and are often case-specific.
• Recombinant vector vaccines offer the greatest and least appreciated potential for transmission.
• Vaccine evolution is a major issue that stems from transmission and can undermine vaccine utility.
• Attenuated vaccines can now be designed that largely suppress evolution; recombinant vectors are prone to evolution.