Determining the impact of vaccination on SARS-CoV-2 RT-PCR cycle threshold values and infectious viral titres

BACKGROUND. As the COVID-19 pandemic continues, efforts to better understand severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viral shedding and transmission in both unvaccinated and vaccinated populations remain critical to informing public health policies and vaccine development. The utility of using real time RT-PCR cycle threshold values (C(T) values) as a proxy for infectious viral litres from individuals infected with SARS-CoV-2 is yet to be fully understood. This retrospective observational cohort study compares quantitative infectious viral litres derived from a focus-forming viral titre assay with SARS-CoV-2 RT-PCR C(T) values in both unvaccinated and vaccinated individuals infected with the Delta strain. METHODS. Nasopharyngeal swabs positive for SARS-CoV-2 by RT-PCR with a C(T) value <27 collected from 26 June to 17 October 2021 at the University of Vermont Medical Center Clinical Laboratory for which vaccination records were available were included. Partially vaccinated and individuals <18 years of age were excluded. Infectious viral litres were determined using a micro-focus forming assay under BSL-3 containment. RESULTS. In total, 119 specimens from 22 unvaccinated and 97 vaccinated individuals met all inclusion criteria and had sufficient residual volume to undergo viral titring. A negative correlation between RT-PCR C(T) values and viral litres was observed in both unvaccinated and vaccinated groups. No difference in mean C(T) value or viral titre was detected between vaccinated and unvaccinated groups. Viral litres did not change as a function of time since vaccination. CONCLUSIONS. Our results add to the growing body of knowledge regarding the correlation of SARS-CoV-2 RNA levels and levels of infectious virus. At similar C(T) values, vaccination does not appear to impact an individual’s potential infectivity when infected with the Delta variant.