given a specific opportunity, a particular pathway in the event tree will allow release. It includes pathways in the event tree in which all mitigation systems are assumed to be fully functional and pathways in which some mitigation systems fail.


Apart from asking subject matter experts for their opinions, no explanation was provided on how the uSSRA selected the 142 events (Appendix Tables A8-1 and A8-2) that were the bases of the event trees. In designing the event trees, several logical circumstances (as indicated by a node) were omitted (i.e., out-of-containment leaks, power systems failures). Thus, the committee is not confident in assuming that all the critical pathways for escape were considered in the uSSRA.


It is impossible in the time provided for this evaluation to review every event tree and accompanying table in Section 4 and to comment on the adequacy of the data and assumptions that are used to support all of the large number of estimates of probabilities and reduction factors associated with each of the many nodes of each event tree. Even with a sound method, the reliability of the assessment remains completely dependent on the scientific reliability of each of the hundreds of inputs used.

The committee selected a sampling of event trees and analyzed some of the assumptions and data used to develop estimates for Q, Ploss, R0, and Floss. For each node of at least three event trees, the committee examined the probabilities of failure and success of the mitigation systems and the resulting MAR reduction factors. Such analyses were conducted for events associated with ATR (transference of virus to the respiratory tracts of workers in biosafety level-3 [BSL-3] animal holding rooms), OTB (transference to the body in non-containment areas), and AA (aerosol release from BSL-3 animal holding rooms), and for selected other events.


Although the uSSRA provided detailed analysis of various risks, the use of questionable assumptions in the data inputs demonstrates that there was insufficient familiarity with the body of scientific literature or with institutional knowledge. The committee believes that this was manifested in citations’ being too limited to constitute compelling evidence of support. The uSSRA failed to validate some assumptions through multiple sources

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