Reevaluating the Risk of Smallpox Reemergence

Smallpox, caused by variola virus, was eradicated in 1980, but remains a category A bioterrorism agent. A decade ago, smallpox ranked second after anthrax in a multifactorial risk priority scoring analysis of category A bioterrorism agents. However, advances in genetic engineering and synthetic biology, including published methods for synthesizing an Orthopoxvirus, require the assumptions of this scoring for smallpox and other category A agents to be reviewed.

The risk priority framework was reviewed and revised to account for the capability for creation of synthetic or engineered smallpox and other category A agents.

The absolute score for all agents increased because of gene editing and synthetic biology capability, which was not present when the framework was developed more than a decade ago, although new treatments revised scores downward for smallpox, Ebola, and botulism. In the original framework, smallpox scored 0 for global availability, given the high security around known seed stocks of variola in two laboratories in the United States and Russia. Now, smallpox can be created using synthetic biology, raising the score for this criterion to 2. Other agents too, such as Ebola, score higher for availability, based on synthetic biology capability. When advances in synthetic biology and genetic engineering are considered, smallpox and anthrax are now equally ranked the highest category A bioterrorism agents for planning and preparedness.

Revision of a risk priority framework for category A bioterrorism agents shows that smallpox should be elevated in priority for preparedness planning, and that gene editing and synthetic biology raises the overall risk for all agents. The ranking of categories A, B, and C agents should also be revisited, as there is an endless possibility of engineered threats that may be more severe than any agent on the category A list.