Transmission and Unvaccinated-Only Testing in Populations of Mixed Vaccination Status

Bubar, K. M.; Middleton, C. E.; Bjorkman, K.; Parker, R.; Larremore, D. B.

In populations with mixed vaccination status, testing programs focused on the unvaccinated have been proposed and enacted to mitigate the spread of SARS-CoV-2. While the benefits of universal SARS-CoV-2 screening are well established, it is unclear how the benefits of unvaccinated-only testing depend on population vaccination rate. Here, we introduce and analyze a model of SARS-CoV-2 transmission in which a variable fraction of the population is fully vaccinated and those who remain unvaccinated are proactively tested for infection, while varying transmission rates, vaccine performance parameters, and the degree of social mixing between vaccinated and unvaccinated populations. We find that unvaccinated testing programs are effective only when compliance is high and testing is frequent, and when vaccine coverage is low or moderate. However, in highly vaccinated populations, the impact of testing unvaccinated individuals decreases, and, by analyzing the possible modes of transmission within and between the unvaccinated and vaccinated populations, we show that the unvaccinated community ceases to be the dominant driver of transmission. By evaluating a wide range of scenarios, this work focuses on elucidating general principles, finding broadly that resources devoted to routine testing of the unvaccinated population could be reallocated to other needs when vaccine coverage is sufficiently high.