Speaker
Description
To better understand SARS-CoV-2 variant succession during the COVD-19 pandemic, we developed multi-variant transmission models, derived conditions for novel variants to invade and coexist with or replace ancestral ones, and explored phenomena that might explain observed patterns. To invade, novel variants require reproduction numbers greater than unity when ancestral ones are at their endemic equilibria. Replacement occurs when one variant can invade at another’s endemic equilibrium, but not vice versa, and coexistence occurs when both variants can invade at the other’s endemic equilibrium. As transitions among successive Omicron variants almost certainly involved immune escape, we explored three hypotheses for the transitions from Alpha to Beta and Beta to Omicron, greater reproduction numbers, shorter generation times, and immune escape. We found that, while greater reproduction numbers always are advantageous, Beta may also have had a shorter generation time than Alpha by virtue of infecting cells in the upper versus lower respiratory tract. But neither was advantageous while the vaccination of healthcare and other essential workers reduced transmission, nor was immune escape advantageous until susceptible hosts were limiting. Consequently, whether replacement of Beta by Omicron was caused or facilitated by immune escape is unclear. Developing, evaluating, and improving these models increased our understanding of phenomena affecting transitions among SARS-CoV-2 variants.
Joint work with Troy Day (Queens University) and John W Glasser (Emory University).
