This study investigates the predictive value of lymphopenia (low lymphocyte percentage) in the severity and prognosis of COVID-19 patients. The authors analyzed blood test data from 15 deceased, 15 severe-cured, and 40 moderate-cured patients, focusing on the dynamic changes in lymphocyte percentage (LYM%) over time. They found that LYM% was significantly reduced in deceased patients and decreased to below 5% within 2 weeks of symptom onset. In contrast, severe patients initially showed a decrease in LYM% but then increased to over 10%, while moderate patients maintained a high LYM% throughout their illness. Based on these findings, the authors developed a Time-LYM% model (TLM) to classify patients into moderate, severe, and critical categories at two key time points: 10-12 days and 17-19 days after symptom onset. The TLM was validated using 90 hospitalized COVID-19 patients, showing high consistency with existing guidelines. The study suggests that LYM% is a reliable indicator for classifying disease severity and can help optimize resource allocation in the treatment of COVID-19. The authors also explore potential mechanisms for lymphopenia, including viral infection, organ damage, inflammatory cytokine dysregulation, and metabolic disorders. They recommend incorporating TLM into clinical guidelines for better management of COVID-19 patients.This study investigates the predictive value of lymphopenia (low lymphocyte percentage) in the severity and prognosis of COVID-19 patients. The authors analyzed blood test data from 15 deceased, 15 severe-cured, and 40 moderate-cured patients, focusing on the dynamic changes in lymphocyte percentage (LYM%) over time. They found that LYM% was significantly reduced in deceased patients and decreased to below 5% within 2 weeks of symptom onset. In contrast, severe patients initially showed a decrease in LYM% but then increased to over 10%, while moderate patients maintained a high LYM% throughout their illness. Based on these findings, the authors developed a Time-LYM% model (TLM) to classify patients into moderate, severe, and critical categories at two key time points: 10-12 days and 17-19 days after symptom onset. The TLM was validated using 90 hospitalized COVID-19 patients, showing high consistency with existing guidelines. The study suggests that LYM% is a reliable indicator for classifying disease severity and can help optimize resource allocation in the treatment of COVID-19. The authors also explore potential mechanisms for lymphopenia, including viral infection, organ damage, inflammatory cytokine dysregulation, and metabolic disorders. They recommend incorporating TLM into clinical guidelines for better management of COVID-19 patients.