SARS-CoV-2 Transmission and Impacts of Unvaccinated-Only Screening in Populations of Mixed Vaccination Status

Abstract

In populations with mixed vaccination status, testing programs focused on only the unvaccinated population are being enacted to mitigate SARS-CoV-2 spread. However, it is not well understood how viral spread occurs in mixed-status populations, including the possible benefits of unvaccinated-only testing. Here, we analyze a model of SARS-CoV-2 transmission in which a variable fraction of the population is fully vaccinated and unvaccinated individuals are proactively tested for infection, while varying transmission rates, vaccine effectiveness (VE) parameters, and prior infection rates. This analysis reveals principles of viral spread in communities of mixed vaccination status, with implications for testing policies. As vaccination rates increase, the proportions of infections occurring in unvaccinated individuals and the amount of transmission driven by the unvaccinated both decrease, such that at ∼65-75% vaccine coverage, most infections are vaccine breakthroughs, and at ∼76-83% vaccine coverage, most community spread is driven by breakthrough infections, under baseline mRNA VE assumptions. These ranges shift lower with waning VE and higher with boosted VE. In highly vaccinated communities proactive unvaccinated-only testing had little impact on community spread. Instead, the benefits of weekly unvaccinated-only testing were restricted to regimes with high or moderate ongoing transmission due to lower vaccination rates and strongly depended on near-perfect test compliance. By evaluating a wide range of scenarios, this work finds broadly that resources devoted to routine unvaccinated-only testing could be reallocated when vaccine coverage is sufficiently high.

SARS-CoV-2 transmission and impacts of unvaccinated-only screening in populations of mixed vaccination status

“…Model simulation data used in this manuscript can be found in the "dataframes" folder in the open source Github repository at https://zenodo.org/badge/latestdoi/419096560 53 .…”

“…All code is open source and provided by the authors at https://zenodo.org/badge/ latestdoi/419096560 53 .…”

Optimizing COVID-19 testing strategies on college campuses: evaluation of the health and economic costs

“…Building on prior cost effectiveness analyses of COVID-19 screening policies (21)(22)(23) and university COVID-19 policies (7,13,24), we developed this approach to support planning efforts at one of the largest public universities in the US during the summer of 2021. Using the University of Texas at Austin as a case study, we derive cost-effective testing strategies to prevent campus closures in a partially-vaccinated community of 50,000 students during the emergence of a novel variant (Delta).…”