Severe Acute Respiratory Syndrome Coronavirus 2 (SARS‐CoV‐2) is regarded as one of the most critical threats to human health and the global economy. Since its first report in 2019, its impact will continue as the Omicron strain mutates and spreads (1). Although COVID-19 initially invades the respiratory system, neurological manifestations have been reported in most patients, including anosmia, ageusia, headache, fatigue, myalgia, dizziness, and psychiatric disorders (2, 3). However, it is still necessary to understand whether the clinical manifestation of COVID-19 patients will change as Omicron mutates. Current evidence shows that COVID-19 infection induces a rapidly coordinated immune response and subsequent inflammatory cytokine storm, which is associated with disease progression (3–5).

Several studies have confirmed the impaired immune system caused by COVID-19 and a significant reduction of lymphocytes, especially a decrease of B cells, natural killer (NK) cells, CD3+ T cells, CD4+ T cells and CD8+ T cells in peripheral blood examination (3, 6–8). At the beginning of the COVID-19 outbreak, some early studies suggested that T lymphocyte cells and their subsets could predict disease severity and mortality (4, 8, 9). Cantenys‐Molina et al. (10) suggested that decreased CD4+ and CD8+T cell counts and expansion of NK lymphocytes proportion at admission were prognostic factors for COVID-19 patients. Mirsharif et al. (3) included 100 COVID-19 patients compared to 70 healthy controls to identify biomarkers for assigning the risk of mortality, and demonstrated that CD8+ HLA‐DR+ T cells count significantly decreased in severe patients and may be the best biomarker for mortality outcome. However, it is unclear whether T lymphocyte subsets can predict the outcome of COVID-19 patients who are treated with antiviral drugs.

Nirmatrelvir is an oral antiviral drug that can effectively inhibit the SARS-CoV-2 3-chymotrypsin–like cysteine protease enzyme (11). Nirmatrelvir has been authorized for COVID-19 patients in many countries worldwide (12, 13). Current clinical evidence suggests that Nirmatrelvir can reduce the risk of 28-days hospitalization or death of outpatients, while showing a well-curative effect in hospitalized patients (12–14). However, the supply of Nirmatrelvir cannot meet the global COVID-19 patients’ demand, and it needs to be used for the most suitable patients. Therefore, in this retrospective study, we focus on the clinical manifestations and immunophenotyping characteristics of COVID-19 patients in the context of the prevalence of Omicron variants. Additionally, we aimed to investigate the potential use of lymphocyte subsets as biomarkers to evaluate the risk of mortality in COVID-19 patients who were undergoing Nirmatrelvir treatment.

This retrospective study investigated hospitalized patients during the Omicron variant epidemic. We analyzed the demographic, immunophenotype, and clinical characteristics of 131 COVID-19 patients treated with Nirmatrelvir. Neurological manifestations such as anosmia, ageusia, and psychiatric disorders were not common in enrolled patients. It may also be the widespread of full vaccination, which protects the nervous system from Omicron damage. A recent study shows that children who are not fully vaccinated may develop omicron-related neurological complications (16). In composite-outcome patients, the median counts of lymphocytes decreased, and CD3+ T cell, CD4+ T cell, and CD8+T cell counts were almost decreased to half of median counts of non-composite-outcome patients. Similar results were observed in patients who died in the hospital compared to discharged patients. Meanwhile, inflammatory cytokines were significantly higher in patients with composite outcome and death than in patients with non-composite outcome and discharge, respectively. The median concentrations of CRP, IL-6, and IL-10 in died patients were more than two-fold of that of discharged patients. Previous evidence has shown that lymphocyte subsets significantly reduced, including CD3+T cells, CD4+T cells, and CD8+T cells, B cells, NK cells, and cytokine storms such as CRP are common in COVID-19 patients (8, 10, 17). In this study, we explored the association of peripheral blood T-lymphocyte subsets with the prognosis of COVID-19 patients treated with Nirmatrelvir. We found that lower counts (/μL) of T lymphocyte subsets CD8+T cells (<201) were associated with a higher risk of composite outcome, and lower counts (/μL) of CD4+T cells (<368) and CD8+T cells (<201) were significantly associated with the death outcome of COVID‐19 patients with Nirmatrelvir therapy. Identifying biomarkers that predict the curative effect of Nirmatrelvir may help physicians conduct evidence-based treatments for COVID-19.

The immune response is closely related to the pathogenesis, progression, and prognosis of COVID-19 patients, especially the activation of adaptive immune function (18). CD4+ and CD8+ T cells are the most basic components of adaptive immunity, with various helper and effector functionalities and the ability to kill infected cells respectively (7). Previous studies on CD4+ and CD8+ T cells in COVID-19 patients have mostly focused on the progression and prognosis of the disease from mild to severe. However, there are no reports on CD4+ and CD8+T cells regarding progression and mortality for COVID-19 patients who treated with Nirmatrelvir. One study on 701 COVID-19 patients reported that the counts of CD4+ T cells (≤500) and CD8+ T cells (100) were significantly associated with mortality (10). The earliest study by Du et al. showed a significant decrease in 21 deceased patients compared with 158 survivors, and when CD8+T cells reduced (≤75), the risk of death increased more than five-fold (9). Xu et al. in the evaluation of 187 hospitalized patients with COVID‐19, reported that the counts of total lymphocytes, CD3+ T cells, CD4+ T cells, CD8+ T cells, B cells, and NK cells decreased significantly. And the sensitivity analysis indicated that the count of lymphocytes (<500), CD3+ T cells (<100), CD4+ T cells (<100), CD8+ T cells (<100), and B cells (<50), were risk factors for COVID-19 patients’ death (8). We note that several studies reported the significant decrease in CD4+ T cells, CD8+ T cells of COVID-19 patients with increasing disease severity (17, 19, 20).

Our study also has some limitations. Firstly, we conducted a single-center retrospective study in Hunan province, limiting the generalizability of our findings. Future studies on the association of T lymphocyte subsets with the prognosis of COVID-19 patients in different regions and ethnicities should be carried out. Secondly, the supply of other antiviral drugs was insufficient, and hence we only evaluated the association between T lymphocyte subsets and prognosis in COVID-19 patients treated with Nirmatrelvir. Thirdly, our population of COVID-19 patients was mainly composed of females (71.8%), which may not be sufficient to predict the prognosis of Nirmatrelvir in male patients. Fourth, the absence of data before and after the administration of Nirmatrelvir may limit the ability to fully assess its impact on lymphocyte subsets as biomarkers. Finally, due to the small sample size, we did not perform ROC curve analysis to confirm the warning values of T lymphocytes. To our best knowledge, we are the first to explore the association of T lymphocyte subsets with the prognosis of COVID-19 patients treated with Nirmatrelvir. Flow cytometry is a quick and convenient method to detect peripheral blood lymphocyte subsets for hospitalized COVID-19 patients, which could help to identify the most suitable patients treated with Nirmatrelvir.

In summary, our study reveals a significant correlation between decreased CD8+ T cell counts and in-hospital progression and mortality of COVID-19 patients treated with Nirmatrelvir. Specifically, patients with lower CD8+ T cell counts (/μL < 201) exhibited a higher risk of in-hospital severity and death. These findings may provide valuable references for physicians in optimizing the use of Nirmatrelvir.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The Xiangya Hospital Institutional Review Committee (202002024) approved this research, and all participants in the retrospective cohort study remained anonymous with no need for individual informed consent.

Conception and design: GD and QG. Acquisition of data: YS and YD. Interpretation of data, statistical analysis and manuscript writing: YS and GD. Revision of manuscript and administrative, technical, or material support: GD, QG, YS, and YD. All authors contributed to the article and approved the submitted version.