Study Selection and Characteristics
We retrieved 965 records from the combination of two independent searches. After the removal of duplications and irrelevant literature, 56 full-text articles were assessed for eligibility. A total of 35 studies [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45] (32 published studies [11,12,13,14,15,16,17,18,19,20,21,22,23,24, 26,27,28,29, 31,32,33,34,35, 37,38,39,40,41,42,43,44,45] and three studies available as a preprint [25, 30, 36]) that corresponded to the inclusion and exclusion criteria were included in the systematic review and meta-analyses (Fig. 1).
All the included studies [11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45] are observational studies, reported as cohort/case–control studies or database reviews. All except seven studies [18, 26, 29, 34, 35, 38, 39] were based directly on the hospital (or nursing home) medical records, and 12 of the included studies [13, 20, 22, 23, 25, 31, 33, 36, 41,42,43,44] are single-centered observational studies. These hospital- (or nursing home)-based studies [11,12,13,14,15,16,17, 19,20,21,22,23,24,25, 27, 28, 30,31,32,33,34, 36, 37, 40,41,42,43,44,45] covered a moderate number of participants per study (median 840.0, interquartile range 286.5–1997.5). The remaining seven studies [18, 26, 29, 34, 35, 38, 39] analyzed data from the nation-level registries (retrospective database reviews), and included a range of 1868–64,781 participants. The regions where the included studies were performed span Asia (China [n = 4] [11, 12, 27, 42], Iran [n = 1] [31], and Korea [n = 1] [39]), Europe (Italy [n = 4] [14, 23, 28, 37], the United Kingdom [n = 2] [25, 30], Belgium [n = 1] [15], France [n = 1] [19], Spain [n = 3] [24, 35, 40], Denmark [n = 1] [26], Poland [n = 1] [44], Sweden [n = 1] [38], and Belgium [n = 1] [32]), and North America (the United States of America [n = 14] [13, 16,17,18, 20,21,22, 29, 33, 34, 36, 41, 43, 45]). Seven studies [15, 19, 32,33,34, 38, 42] (could have) included both laboratory-confirmed and clinically or radiologically confirmed patients with COVID-19, while the remaining studies [11,12,13,14, 16,17,18, 20,21,22,23,24,25,26,27,28,29,30,31, 33,34,35,36,37, 39,40,41, 43,44,45] included only laboratory-confirmed patients with COVID-19. Eleven studies [12, 13, 18, 20, 27, 32, 35, 36, 39, 42, 44] confirmed the continuation/de novo initiation of statins during hospitalization for COVID-19. Table 1 summarizes the main characteristics of all the included studies.
Table 1 Characteristics of included studies Assessment with the Newcastle–Ottawa Scale revealed that other than three studies [11, 19, 20], which are of moderate quality (5/9 to 6/9), all studies [12,13,14,15,16,17,18, 21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45] are of good quality (at least 7/9). None of the included studies are of poor quality.
Use of Statins and All-Cause Mortality in Patients with COVID-19
The outcome of all-cause mortality is available in 32 studies [12,13,14, 16,17,18,19,20,21, 23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45] comparing statin users versus non-statin users with COVID-19, of which ten studies [12,13,14, 20, 24, 26, 27, 31, 36, 45] provided adjusted measure as HR, whereas 22 studies [16,17,18,19, 21, 23, 25, 28,29,30, 32,33,34,35, 37,38,39,40,41,42,43,44] provided adjusted measure as OR. The meta-analysis of studies with an HR, which included 21,127 patients with COVID-19, revealed that the use of statins was associated with a significantly lower hazard of all-cause mortality in patients with COVID-19 compared to non-use of statins (Fig. 2B; HR = 0.70; 95% CI 0.58–0.84). The pooled OR is consistent with the pooled HR, which also demonstrated significantly reduced odds of all-cause mortality with the use of statins in patients with COVID-19 compared to non-use of statins (Fig. 2A; OR = 0.63; 95% CI 0.51–0.79; n = 115,097). Visual inspection of the funnel plot (Fig. 4A) identified some degree of asymmetry.
Subgroup analyses with studies originating from Asia (HR = 0.56; 95% CI 0.42–0.75) [12, 27, 31], Europe (OR = 0.77; 95% CI 0.64–0.94) [19, 23, 25, 28, 30, 32, 35, 37, 38, 40, 44], and North America (OR = 0.62; 95% CI 0.54–0.73), respectively [16,17,18, 21, 29, 33, 34, 41, 43], observed significantly reduced risks of mortality with the use of statins in patients with COVID-19 compared to non-use of statins. Findings from the sensitivity analyses with studies [12,13,14, 16,17,18, 20, 21, 23,24,25,26,27,28,29,30,31, 35,36,37, 39,40,41, 43,44,45] including only laboratory-confirmed patients with COVID-19 (OR = 0.67; 95% CI 0.56–0.79 and HR = 0.70; 95% CI 0.58–0.84) and studies [12, 13, 20, 27, 32, 35, 36, 39, 42, 44] that confirmed the continuation of statins during hospitalization for COVID-19 (OR = 0.61; 95% CI 0.52–0.73 and HR = 0.59; 95% CI 0.41–0.84), are consistent with the main analyses.
Use of Statins and Development of Severe Illness in Patients with COVID-19
A total of 15 studies [11, 12, 15,16,17, 19, 22, 26,27,28, 31, 35,36,37, 43] reported the composite endpoint of severe illness of COVID-19, and all except four studies [12, 26, 27, 36] provided adjusted measure as OR. The definition of severe illness varied across the 15 studies: in the study by Yan et al. [11], the definition is based on that given in the Diagnosis and Treatment Protocol for Novel Coronavirus Pneumonia by the Chinese National Health Commission; in the studies by Zhang et al. [12], Butt et al. [26], Fan et al. [27], Mitacchione et al. [28], and Greco et al. [37] respectively, the definition is based on admission into an intensive care unit or pulmonology department; in the study by De Spiegeleer et al. [15], the definition is based on long-stay hospital admission or death; in the studies by Gupta et al. [16], Cariou et al. [19], and Peymani et al. [31] respectively, the definition is based on the requirement of intubation/mechanical ventilation; in the studies by Song et al. [17] and Lohia et al. [43] respectively, the definition is based on admission into an intensive care unit or the requirement of intubation; in the studies by Daniels et al. [22] and Memel et al. [36] respectively, the definition is based on admission into an intensive care unit or death; in the study by Torres-Peña et al. [35], the definition is based on development of acute respiratory distress syndrome.
The meta-analysis of studies [11, 15,16,17, 19, 22, 28, 31, 35, 37, 43] with ORs, which altogether included 10,081 patients with COVID-19, observed that the use of statins was associated with significantly lower odds of development of severe illness in patients with COVID-19 compared to non-use of statins (Fig. 3A; OR = 0.80; 95% CI 0.73–0.88). The pooled HR is consistent with the pooled OR (n = 10,738), which demonstrated a non-significantly reduced hazard of development of severe illness with the use of statins in patients with COVID-19 compared to non-use of statins (Fig. 3B; HR = 0.80; 95% CI 0.56–1.14). Visual inspection of the funnel plot (Fig. 4B) identified some degree of asymmetry.
Subgroup analyses with studies originating from Europe (OR = 0.79; 95% CI 0.71–0.89) [15, 19, 28, 35, 37] and North America (OR = 0.79; 95% CI 0.59–1.05), respectively [16, 17, 22, 43], observed reduced risks of mortality with the use of statins in patients with COVID-19 compared to non-use of statins. Sensitivity analyses with studies [11, 16, 17, 22, 28, 31, 35, 37, 43] including only laboratory-confirmed patients with COVID-19 (OR = 0.79; 95% CI 0.72–0.88), studies [12, 27, 36] that confirmed the continuation of statins during hospitalization for COVID-19 (HR = 0.71; 95% CI 0.55–0.92), and studies in which the definition of severe illness is based on the requirement of mechanical ventilation or intubation [16, 17, 19, 31, 43] (OR = 0.81; 95% CI 0.69–0.96) also revealed consistent findings with the main analyses.