Regulating Select Agent Chimeras: Defining the Problem(s) Through the Lens of SARS-CoV-1/SARS-CoV-2 Chimeric Viruses

In late 2021, the US Centers for Disease Control and Prevention (CDC) posted an interim final rule (86 FR 64075) to the federal register regulating the possession, use, and transfer of SARS-CoV-1/SARS-CoV-2 chimeric viruses. In doing so, the CDC provided the reasoning that viral chimeras combining the transmissibility of SARS-CoV-2 with the pathogenicity and lethality of SARS-CoV-1 pose a significant risk to public health and should thus be placed on the select agents and toxins list. However, 86 FR 64075 lacked clarity in its definitions and scope, some of which the CDC addressed in response to public comments in the final rule, 88 FR 13322, in early 2023. To evaluate these regulatory actions, we reviewed the existing select agent regulations to understand the landscape of chimeric virus regulation. Based on our findings, we first present clear definitions for the terms "chimeric virus," "viral chimera," and "virulence factor" and provide a list of SARS-CoV-1 virulence factors in an effort to aid researchers and federal rulemaking for these agents moving forward. We then provide suggestions for a combination of similarity and functional characteristic cutoffs that the government could use to enable researchers to distinguish between regulated and nonregulated chimeras. Finally, we discuss current select agent regulations and their overlaps with 86 FR 64075 and 88 FR 13322 and make suggestions for how to address chimera concerns within and/or without these regulations. Collectively, we believe that our findings fill important gaps in current federal regulations and provide forward-looking philosophical and practical analysis that can guide future decisionmaking.