Cycle Threshold Values of SARS-CoV-2 RT-PCR during Outbreaks in Nursing Homes: A Retrospective Cohort Study

Abstract

Backgound/Objectives: Cycle threshold (Ct) values of SARS-CoV-2 real-time reverse transcriptase-polymerase chain reaction (RT-PCR) tests are associated with infectivity and viral load, and they could be an aid in forecasting the evolution of SARS-CoV-2 outbreaks. The objective was to know the Ct values related to the incidence and reinfection of SARS-CoV-2 in successive outbreaks, which took place in nursing homes in Castellon (Spain) during 2020–2022, and to test its usefulness as an instrument of epidemic surveillance in nursing homes. Methods: a retrospective cohort design with Poisson regression and multinomial logistic regression were used. Results: We studied four nursing home SARS-CoV-2 outbreaks, and the average infection rate, reinfection rate, and case fatality were 72.7%, 19.9%, and 5.5%, respectively; 98.9% of residents were vaccinated with three doses of a mRNA SARS-CoV-2 vaccine. Ct values for first infections and reinfections were 27.1 ± 6.6 and 31.9 ± 5.4 (p = 0.000). Considering Ct values ≥ 30 versus <30, residents with reinfections had Ct values higher than residents with a first infection, an adjusted relative risk of 1.66 (95% Confidence interval 1.10–2.51). A sensitivity analysis confirmed these results. Conclusions: Reinfection and SARS-CoV-2 vaccination (hybrid immunity) could protect against severe disease better than vaccination alone. High Ct values suggest lower transmission and severity. Its value can be useful for surveillance and forecasting future SARS-CoV-2 epidemics.