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

Background: As the COVID-19 pandemic continues, efforts to better understand 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 RT-PCR cycle threshold values (CT values) as a proxy for infectious viral titers from individuals infected with SARS-CoV-2 is yet to be fully understood. This retrospective observational cohort study compares quantitative infectious viral titers derived from a focus-forming viral titer assay with SARS-CoV-2 RT-PCR CT 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 CT value < 27 collected from June 26th to October 17th of 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 titers were determined using a micro-focus forming assay under BSL-3 containment. Results: 119 specimens from 22 unvaccinated and 97 vaccinated individuals met all inclusion criteria and had sufficient residual volume to undergo viral titering. A negative correlation between RT-PCR CT values and viral titers was observed in both unvaccinated and vaccinated groups. No difference in mean CT value or viral titer was detected between vaccinated and unvaccinated groups. Viral titers 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 CT values, vaccination does not appear to impact an individual’s potential infectivity.