A study published in Signal Transduction and Targeted Therapy (2020) investigates the role of lymphopenia (low lymphocyte count) in predicting the severity of COVID-19. The research analyzed blood test data from patients who recovered and those who died, finding that a significant decrease in lymphocyte percentage (LYM%) within two weeks of symptom onset was a strong predictor of poor prognosis. Patients with LYM% < 5% at the second time point (day 17–19 after symptom onset) were classified as critically ill with high mortality. The study developed a Time-LYM% model (TLM) to classify patients based on their LYM% at two time points: TLM-1 (day 10–12) and TLM-2 (day 17–19). Patients with LYM% > 20% at TLM-1 were classified as moderate and likely to recover, while those with LYM% < 20% at TLM-1 were classified as severe or critically ill. At TLM-2, patients with LYM% < 5% were classified as critically ill. The TLM was validated against existing classification guidelines, showing good consistency. The study suggests that LYM% can be used as a reliable indicator for classifying the severity of COVID-19, independent of other tests. Possible mechanisms for lymphopenia include direct viral infection of lymphocytes, damage to lymphatic organs, excessive inflammatory cytokines, and metabolic factors like hyperlactic acidemia. The study concludes that lymphopenia is an effective indicator of disease severity and hospitalization in COVID-19 patients, and the TLM should be included in diagnostic and therapeutic guidelines. The research was supported by grants from the National Natural Science Foundation of China and other funding sources. The authors include doctors, researchers, and public health workers who contributed to the study. The study was conducted in China and involved patients from Wuhan and other locations. The research was published in an open-access journal, and the authors declare no competing interests.A study published in Signal Transduction and Targeted Therapy (2020) investigates the role of lymphopenia (low lymphocyte count) in predicting the severity of COVID-19. The research analyzed blood test data from patients who recovered and those who died, finding that a significant decrease in lymphocyte percentage (LYM%) within two weeks of symptom onset was a strong predictor of poor prognosis. Patients with LYM% < 5% at the second time point (day 17–19 after symptom onset) were classified as critically ill with high mortality. The study developed a Time-LYM% model (TLM) to classify patients based on their LYM% at two time points: TLM-1 (day 10–12) and TLM-2 (day 17–19). Patients with LYM% > 20% at TLM-1 were classified as moderate and likely to recover, while those with LYM% < 20% at TLM-1 were classified as severe or critically ill. At TLM-2, patients with LYM% < 5% were classified as critically ill. The TLM was validated against existing classification guidelines, showing good consistency. The study suggests that LYM% can be used as a reliable indicator for classifying the severity of COVID-19, independent of other tests. Possible mechanisms for lymphopenia include direct viral infection of lymphocytes, damage to lymphatic organs, excessive inflammatory cytokines, and metabolic factors like hyperlactic acidemia. The study concludes that lymphopenia is an effective indicator of disease severity and hospitalization in COVID-19 patients, and the TLM should be included in diagnostic and therapeutic guidelines. The research was supported by grants from the National Natural Science Foundation of China and other funding sources. The authors include doctors, researchers, and public health workers who contributed to the study. The study was conducted in China and involved patients from Wuhan and other locations. The research was published in an open-access journal, and the authors declare no competing interests.