In total, 625 articles were identified through the abovementioned databases, and 3 articles were identified by manual searching. After removing 81 duplicate studies and reviewing the titles and abstracts, 131 articles remained for full-text screening. Finally, 55[6, 9–62] papers were included in this systematic review and meta-analysis (Fig. 1). Among these studies, there were 3 case reports[12, 36, 44], 3 ICU studies[22, 25, 55], 8 surveillance studies[15, 17, 31, 37–39, 42, 53] and 41 retrospective studies. Most of the included studies were from China, including the Wuhan area and outside of the Wuhan area.
Forty-eight studies including 8134 patients reported the estimated CFR, and the sample sizes ranged from 3 in a case report[12] to 1265 in a surveillance study[31]. The estimated CFR ranged from 0 to 61.5%, while the pooled CFR was 3.3% (95% CI: 1.5, 5.4). The proportions of ARDS and ICU admission were reported in 16 and 19 studies, respectively. The pooled proportion of ICU admission was 24.9% (95% CI: 13.0, 39.1), and the pooled proportion of ARDS was 20.9% (95% CI: 10.8, 33.3). The pooled incidence rate of severe cases was 26.6% (95% CI: 20.1, 33.6) for 35 studies. The between-studies estimate of heterogeneity for CFR, calculated with I2 statistics, was 93%, and it was 97% for the rest of the abovementioned proportion. The forest plots are shown in Fig. 2.
The risk of death, ARDS, ICU admission and severe cases were stratified by study design and geographic region. The results of subgroup analyses are presented in Table 1. The CFR in ICU studies and case reports was obviously larger than that in retrospective studies and surveillance studies. ICU studies yielded the highest pooled risks of ARDS, ICU admission and severe cases. When comparing data between the Wuhan area and outside of the Wuhan area, the risk of death, ARDS, ICU admission and severe cases were all larger in studies that were conducted in Wuhan than in those conducted outside of Wuhan.
Table 1
subgroup
|
CFR(95% CI)(%)
|
Risk of ARDS(95% CI) (%)
|
Risk of ICU admission(95% CI) (%)
|
Risk of severe cases(95% CI) (%)
|
design
|
|
|
|
|
retrospective study
|
3.3(1.5,5.8)
|
14.1(6.2,24.7)
|
11.4(4.6,20.6)
|
28.3(20.6,36.7)
|
ICU study
|
20.0(0.0,78.7)
|
87.4(50.5,100.0)
|
100(98.6,100))
|
100(92.2,100)
|
surveillance study
|
0.5(0.1,1.3)
|
3.4(2.4,4.5)
|
5.0(3.8,6.4)
|
10.1(5.9,15.3)
|
case report
|
16.5(2.7,38.6)
|
0.0(0.0,0.6)
|
33.3(0.2,85.6)
|
40.0(6.0,81.3)
|
region
|
|
|
|
|
Wuhan
|
10.0(5.9,14.8)
|
28.3(17.9,40.0)
|
38.5(20.7,58.0)
|
40.0(27.4,53.2)
|
outside of Wuhan
|
0.0(0.0,0.1)
|
12.6(0.7,35,8)
|
13.2(0.8,37.4)
|
18.3(12.2,25.3)
|
Age and gender distributions related to the severity of COVID-19 are shown in Fig. 3. Eight studies assessed older age as a risk factor for severe patients. The pooled OR for the association between male sex and severe COVID-19 was 1.62 (95% CI: 1.32, 1.99).
A meta-analysis of the association between seven selected comorbidities is shown in Fig. 4. The pooled ORs were larger than 1 for six underlying comorbidities, including hypertension, diabetes, renal disease, heart disease, respiratory disease and cerebrovascular disease. However, for liver disease, the pooled OR was 0.63 (95% CI: 0.36, 1.10). Among these underlying medical conditions, the association between respiratory disease and the severity of COVID-19 had the highest OR value of 3.74 (95% CI: 2.15, 6.49), followed by renal disease with an OR of 3.60 (95% CI: 1.53, 8.46). Heterogeneity was only present in the comorbidities of hypertension (I2 = 64%, P = 0.001).
D-dimer and interleukin-6 (IL-6) levels were regarded as risk factors and were also assessed by a small number of studies. For D-dimer levels, the combined results of eight studies were statistically significant (SMD = 0.62, 95% CI: 0.28, 0.96), and the I2 value was 76%. In terms of IL-6 levels, there was an apparent difference between the severe group and the nonsevere group (SMD = 2.21, 95% CI: 0.11, 4.31), but the included studies showed substantial heterogeneity (I2 = 99%, P < 0.00001). Figure 5 shows the forest plots.