Characteristics of included studies
As shown in Figure 1A, data from more than 200 published clinical studies and preprints was screened. After a comprehensive review of the data in figures and tables, 85 reports were excluded due to lack of survivor or non-survivor sup-groups, examining infants/children/pediatrics, or had no DOI, resulting in 97 eligible retrospective studies. These criteria resulted in a total of 19014 patients (>20 years of age) including 14359 survivors and 4655 non-survivors. The population considered in these studies originated from China, Italy, Scotland, the United States, UK, Japan, South Korea, Iceland, Chile, the Netherlands and Germany.
Clinical outcomes
Clinical outcomes are shown in Table 1. Based on the studies reported age, patients (n=9375) aged 25.3 to 80.0 years (49.8; CI95% [46.9, 52.7]). Of them, 5448 were survivors (age: 46.6; CI95% [44.2, 48.9]) and 3927 were non-survivors (age: 71.5; CI95% [66.4, 76.5]). In non-survivors, the proportion of males was higher than females (33.3 vs. 17.7%). The prevalence of any comorbidities (65.9%), hypertension (64.5%), diabetes (65.5%), cardiovascular disease (78.8%), chronic obstructive lung (74.1%), cancer (79.9%), and renal disease (88.6%) was higher among COVID-19 non-survivors than among survivors.
Mortality incidence in mild cases was zero compared to 89.8% of mortality in patients with severe COVID-19. The percentage of non-survivors among patients receiving antibiotics or antiviral drugs was 39.0% and 48.4%, respectively. Non-ICU patients were found to have survived; while, 56.8% of ICU (only)-admitted patients died. Mortality in white or European ethnic groups (75.6%) was higher than in Asian (7.0%), African American (9.5%), and Hispanics-Latino ethnic groups (0.0%).
The number of neutrophils (NEUs) (3.52×109 L vs. 6.48×109 L found for survivors and non-survivors, respectively) and white blood cells (WBCs) (5.43×109 L vs. 8.55×109 L found for survivors and non-survivors, respectively) was higher in non-survivors than in survivors (P=0.0001). The number of lymphocytes (LYMs) (0.60×109 L vs. 1.23×109 L found for non-survivors and survivors, respectively) and PLTs (149.92×109 L vs. 187.76×109 L found for non-survivors and survivors, respectively) was lower in non-survivors than in survivors (P=0.0001).
Concentrations of aspartate transaminase (AST) (50.68 vs. 30.06 U/L, P=0.0003), creatinine (87.52 vs. 64.64 mol/L, P=0.0001), creatinine kinase (101.0 vs. 73.2 U/L, P=0.032), C-reactive protein (CRP) (96.39 vs. 22.32 mg/L, P=0.0001), and gamma-glutamyl transferase (GGT) (52.50 vs. 11.06 U/L, P=0.0001) were found to be higher in non-survivors compared to survivor. However, the concentrations of albumin (31.93 vs. 38.51g/ L, P=0.048) and hemoglobin (124.03 vs. 134.44 g/L, P=0.0001) were lower in non-survivors than in COVID-19 survivors.
Data showed that acute kidney injury (94.5%) was the most common complication among non-survivors, followed by respiratory failure (93.8%), septic shock (89.3%), heart failure (88.9%), acute cardiac injury (87.3%), coagulophaty (72.5%), acidosis (68.1%) and secondary infection (67.3%).
Quantitative synthesis of data
Meta-regression analysis
The multivariate meta-regression analysis showed that the risk factors (t, 4.77; CI95% [0.64, 1.68]; P=0.000) were associated with the estimated intervention effects on COVID-19 mortality while biochemical/hematological indices (t, 1.85; CI95% [-0.11, 1.60]; P=0.083) tended to be associated with this (Table 2). Moreover, complications were associated with the estimated intervention effects on COVID-19 mortality (t, 3.80; CI95% [1.07, 4.36]; P=0.005).
Meta-analysis of overall and individual hematological indices
The individual Hedges’g for each parameter and the combined effect size with CI95% are shown in Figure 1B. A random-effect model was used for the combined effect size as there was a significant statistical heterogeneity (P=0.000) between the parameters (Tau2 as the between-group variance and I2 as the proportion of total variation in the estimates of parameter effects). The overall increase in blood parameters of COVID-19 non-survivors (0.74 [0.02, 1.46]; Z=2.02; P=0.044; I2=100.0%; Tau2=2.13) was shown in the meta-analysis forest plot based on the random effect model (Figure 1B).
The number of NEUs (2.81[2.70, 2.91]; Z=53.97; P=0.000) and WBCs (2.38 [2.29, 2.47]; Z=50.05; P=0.000) and the concentrations of GGT (4.10 [3.81, 4.39]; Z=27.40; P=0.000), creatinine (2.40 [2.30, 2.49]; Z=49.67; P=0.000), CRP (2.28 [2.19, 2.38]; Z=46.23; P=0.000), AST (1.44 [1.34, 1.54]; Z=29.11; P=0.000), creatinine kinase (1.14 (1.03, 1.25); Z=19.58; P=0.000), IL-6 (0.95 [0.82, 1.08]; Z=14.04; P=0.000), blood urea nitrogen (BUN) (0.47 [0.38, 0.57]; Z=9.62; P=0.000), and bilirubin (0.20 [0.11, 0.29]; Z=4.46; P=0.000) were higher in non-survivor COVID-19.
The number of LYMs (-1.74 [-1.83, -1.66]; Z=41.36; P=0.000) and PLTs (-1.55 [-1.63, -1.47]; Z=36.89; P=0.000) and the concentration of hemoglobin (-1.26 [-1.35, -1.17]; Z=26.12; P=0.000), albumin (-0.80 [-0.90, -0.70]; Z=15.50; P=0.000) and procalcitonin (-0.12 [-0.20, -0.03]; Z=2.69; P=0.007) in non-survivors were lower than in survivors.
COVID-19 mortality increases with age, hypertension, cerebrovascular disease and diabetes
As shown in the meta-analysis forest plot based on the random effect model (Figure 2A), prevalence of pre-existing conditions increased COVID-19 mortality (1.16 [0.78, 1.55]; Z=5.87; P=0.000; I2=100.0%; Tau2=0.63).
Prevalence of individual pre-existing conditions, such as age (3.11 [3.05, 3.17]; Z=100.70; P=0.000); hypertension (2.30 (2.26, 2.35); Z=100.00; P=0.000), cerebrovascular disease (2.22 [2.13, 2.32]; Z=45.95; P=0.000), diabetes (2.11 [2.06, 2.15]; Z=96.66; P=0.000), any comorbidities (1.97 [1.99, 2.01]; Z=84.99; P=0.000), cardiovascular disease (1.55 [1.51, 1.59]; Z=76.90; P=0.000), COPD (1.16 [1.11, 1.20]; Z=56.68; P=0.000), renal disease (1.10 [1.06, 1.14]; Z=52.59; P=0.000), male sex (0.78 [0.75, 0.82]; Z=44.59; P=0.000), body mass index (BMI) (0.73 [0.46, 0.99]; Z=5.38; P=0.000), time from symptoms appearance to hospitalization (0.66 [0.61, 0.72]; Z=23.17; P=0.000), liver disease (0.52 [0.47, 0.56]; Z=22.42; P=0.0001), cancer (0.45 [0.41, 0.48]; Z=23.13; P=0.000) and smoking history (0.13 [0.02, 0.24]; Z=2.41; P=0.016) was higher among non-survivors. The prevalence of current drinkers was lower among non-survivors (-0.62 [-0.82, -0.42]; Z=6.01; P=0.000), which could be product of a relatively small number of non-survivors (n=101) used for meta-analysis (Figure 2A).
Meta-analysis determines common complications among non-survivors of COVID-19
The prevalence of complications among COVID-19 non-survivors (2.71 [1.91, 3.51]; Z=6.66; P=0.000; I2=100.0%; Tau2=1.48) increased as shown in the meta-analysis forest plot based on the random effect model (Figure 2B).
Heart failure (7.40 [7.15, 7.64]; Z=58.45; P=0.000) was the most common complication among non-survivors, followed by septic shock (4.49 [4.36, 4.63]; Z=65.90; P=0.000), acidosis (2.90 [2.64, 3.15]; Z=22.24; P=0.000), respiratory failure (2.80 [2.73, 2.87]; Z=78.36; P=0.000), acute cardiac injury (1.89 [1.83, 1.96]; Z=54.84; P=0.000), coagulopathy (1.79 [1.66, 1.93]; Z=25.32; P=0.000), acute kidney injury (1.64 [1.58, 1.69]; Z=58.91; P=0.000), secondary infection (1.31 [1.24, 1.37]; Z=39.24; P=0.000), and liver dysfunction (0.10 [0.01, 0.20]; Z=2.08; P=0.037) (Figure 2B).
Network analysis supports the results of the meta-analysis
The network correlation for blood indices (Figure 3A), risk factor (Figure 3B), and complication (Figure 3C) were shown at cutoff point of 50%. The number of PLTs and LYMs and the concentration of hemoglobin were associated with COVID-19 survivors at a maximum cutoff point of 72%, all parameters were disconnected after this cutoff point (Figure 3D). The number of NEUs, the concentration of GGT, and the incidence of COVID-19 mortality were associated together at a maximum cutoff point of 93% (Figure 3E).
Network analysis showed a relationship between COVID-19 mortality and age, hypertension, cerebrovascular disease, diabetes, any comorbidity, cardiovascular disease (at a maximum cutoff point of 79%, Figure 3F) and heart failure (at a maximum cutoff point of 97%, Figure 3G). These findings supported the results of the meta-analysis; however, the meta-analysis was able to rank the potent factors involved in COVID-19 mortality.