Interpreting the viral mechanism of SARS-CoV-2 based on the human body level is critical for developing more efficient interventions. Due to the limitation of data, limited models consider the viral dynamics of the early phase of infection. The Human Challenge Study (Killingley et al., 2022) enables us to obtain data from inoculation to the 14th day after infection, which provides an overview of the dynamics of SARS-CoV-2 infection within the host. In the Human Challenge Study, each volunteer was inoculated with 10TCID50, approximately 55PFU, of a wild-type of virus (Killingley et al., 2022), and the data indicates that the viral load reduced below the detectable level within a day.

The simplified within-host models developed by Xu et al. (2023) explain the data from the Human Challenge Study (Killingley et al., 2022). However, they do not explain the viral decay from Day 0 to Day 1. Hence, in this paper, we aim to develop a new viral mechanism to explain this phenomenon. Based on the simplified within-host models developed by Xu et al. (2023), we consider that the virus will first go through an adjustment phase and then start to replicate. A new dose-response model is developed to evaluate the probability of infection by constructing a boundary problem. We will discuss this viral mechanism and fit the model to the data of the Human Challenge Study (Killingley et al., 2022) by adopting AMC-SMC (approximate Bayesian computation-sequential Monte Carlo). Based on the results of parameter inference, we estimate that the adjusted viral load is around 1% of the inoculated viral load.