Talk Title: How biases related to contact and testing behaviours can produce observed negative vaccine effectiveness
Abstract: Since the emergence of the SARS-CoV-2 Omicron variant, observational studies measuring the protective effect of vaccines (against infection and against symptomatic infection) have reported negative vaccine effectiveness (VE), suggesting vaccines are facilitating the spread of infection and disease for the Omicron variant. However, the true biological efficacy of vaccines is unlikely to be negative. Instead, these observed negative measurements likely arose due to uncontrolled sources (or mechanisms) of bias that can be broadly categorized as either influencing the differences in underlying infections (e.g. differences in contact or susceptibility) or the differences in the observations of infection (e.g. differences in testing behaviour). In this talk, I focus on two examples of these mechanisms of bias: vaccinated contact heterogeneity, defined as an increase in the contact rate only between vaccinated individuals and, vaccinated testing heterogeneity, defined as an increase in the tendency to test for vaccinated individuals. Using simulations from transmission dynamics models, I explore how these bias mechanisms could underestimate VE measurements and whether their effects on observed VE differ over time and across different levels of biological efficacy. I demonstrate how these mechanisms could have feasibly produced negative measurements during the Omicron period and highlight their general ability to bias real-world immunity studies.
Bio: Korryn Bodner is a Research Associate with the Mishra Lab at the MAP Centre for Urban Health Solutions. Her general research focuses on emerging SARS-CoV-2 Variants of Concern (VOCs) and how heterogeneities in contact patterns, testing and vaccination strategies can alter transmission dynamics and/or our observations of those dynamics. This work has recently extended into how heterogeneities in contact and testing can also cause bias in observational studies focused on vaccines and prior SARS-CoV-2 infection. In the future, Korryn plans to use this research as a foundation for developing better forecasting models for emerging infectious diseases.
Prior to her position with the Mishra Lab, Korryn completed her PhD in Ecology and Evolutionary Biology at the University of Toronto. Her PhD focused on creating frameworks to understand variations in ecological interactions from mechanisms to networks and improving how to design and evaluate explanatory and predictive models.
Korryn is also the co-founder of the Canadian Ecological Forecasting Initiative, whose main objective is to bridge the divide between forecasting and decision-making through capacity and skill building.