VIRAL DYNAMIC MODELS PROVIDE AN EXPLANATION FOR SARS-CoV-2 REBOUND AFTER NIRMATRELVIR

ABSTRACT

Background: A 5-day course of nirmatrelvir-ritonavir (N/R) can significantly reduce the hospitalization and death rates and the duration of infectiousness in high-risk SARS-CoV-2 patients. However, in a fraction of treated individuals virus rebounds following an initial recovery after treatment. The mechanism driving rebound is not well understood. We hypothesize that treatment with N/R near the time of symptom onset halts the depletion of target cells, but does not fully eliminate the virus, and thus can lead to viral rebound. Method(s) Previously, we and others have developed viral dynamic models and successfully used them to fit data on SARS-CoV-2 infection. Here we expand these models and incorporate N/R pharmacokinetic and pharmacodynamic effects and an adaptive immune response. Result(s) We fit this model to the data presented in Charness et al., NEJM (2022) where longitudinal quantitative PCR data is available for 3 individuals who experienced viral rebounds after taking N/R. We found that the model fit the data well. By varying model parameters from their best-fit values, we show the occurrence of viral rebound is sensitive to model parameters, and the time treatment is initiated, which may explain why only a fraction of individuals rebound. Finally, the model with its best-fit parameter values was used to test the therapeutic effects of treatment extended to 10 days or a second 5-day course of N/R initiated one day after symptoms reoccur. Conclusion(s) Our model fits predicted that virus is not fully eliminated during N/R treatment and supported our initial hypothesis that at the end of treatment target cells are available to allow viral resurgence. Simulating the effect of starting treatment later, we find the probability of viral rebound occurring decreases, suggesting that delaying treatment may be a strategy to reduce viral rebound. However, N/R treatment accelerates viral clearance and hence potentially can reduce viral transmission. Thus, delaying treatment may have a detrimental effect on public health and could also have impact on the severity of disease in the high-risk patients for whom N/R is recommended. Increasing treatment from 5 to 10 days continues to preserve target cells and thus may still allow viral rebound if viable virus is present at the end of treatment and sufficient adaptive immunity has not developed. Simulating giving a second course of treatment one day after symptoms reappear, did not prevent rebound.