Introduction

As of December 2020, almost 80 million people have been infected by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes coronavirus disease 2019 COVID-19 [1, 2]. This equates to more than 1% of the world population [3]. The total number of COVID-19-related deaths has now reached more than 1.7 million, which translates to a rate of 2.2% of the cases. In the face of this, the last several weeks have seen the rollout of new vaccines for protection against COVID-19, which have been produced in record time due to an unprecedented worldwide effort [4–6]. Although massive worldwide networks have already been put in place to distribute these vaccines across the globe, we are still several months away from seeing a drop in daily COVID-19 case numbers, hospitalisations, and mortality. Therefore, there is still an urgent need to control the disease spread by testing of new drugs and compounds and by repurposing existing therapeutics [7, 8].

Epidemiological studies have shown that age and preexisting medical conditions such as hypertension, cardiopulmonary diseases, diabetes and obesity are associated with a more severe disease course and higher mortality in patients hospitalized with COVID-19 disease [9–13]. However, it is still not known why some of these individuals experience a more severe form of the diseasewhile others in the same high-risk groups only show mild symptoms or are asymptomatic. One possible explanation is due to the protective effects of medications routinely used to treat these conditions, although this has been debated [14, 15].

Statins are inexpensive, easily available, and are already in widespread use for treatment of cardiovascular disorders, which has created considerable interest in further exploration of their repurposing as a new treatment for managing the severity of COVID-19 infections [16–21]. Statins are known to reduce the risk of major cardiovascular events and to counteract inflammation, the immune response, and oxidative stress damage [22–25], which may reduce the risk of a more severe disease course and improve outcomes in COVID-19 patients. However, it should be emphasised that there is still a lack of clinical consensus on the efficacy of statins in improving disease outcomes in COVID-19 patients, particularly with respect to whether or not the drug has been administered before or after diagnosis.

To address these points, we have carried out a systematic review and meta-analysis to investigate the effect of statins on COVID-19 disease outcomes. In particular, we focused on the rate of intensive care unit (ICU) admission, the need for mechanical ventilation, and mortality. It was also of special interest to determine whether or not there was a difference in these outcomes in patients who had received statins before or after their COVID-19 diagnosis.

Material and methods

Study selection

We performed a 4-step process carried out in parallel by 2 authors (AHS and AVA) to determine eligible criteria for inclusion. First, we searched the PubMed, Web of Science, Scopus, and ProQuest databases and identified 1094 potentially relevant studies. Six additional studies were identified via Internet searches, bringing the total number of articles to 1100. Next, we exported the identified records into the EndNote X9 software, reviewed the full list, and excluded duplicate studies, leaving 954 records. The titles and abstracts of these articles were then screened, and relevant studies selected for full text evaluation. The full texts of the 66 remaining studies were assessed for eligibility, leaving 26 studies. Finally, 24 studies that met all inclusion and exclusion criteria were selected for the analysis (Figure 1).

Figure 1

Study selection process

Results

ICU admission

The OR from 10 studies was 0.78 (95% CI: 0.58–1.06, p = 0.11), with significant heterogeneity between studies (τ² = 0.16, I² = 85.80%, H² = 7.04, Q_(df=9) = 107.72, p_Q < 0.001) (Figure 2 A). Assessment for bias by Egger’s (p = 0.815) and Begg’s (p = 0.142) tests showed no significant small-study effects. Further visual inspection of the funnel plot suggested a slight degree of publication bias (Figure 2 B). Extending the results of the analyses for the nonparametric trim-and-fill method of accounting for publication bias showed that there should be 2 additional studies for inclusion (Figure 2 C). A re-estimation of the overall OR after these studies was included, which resulted in an OR = 0.70 (95% CI: 0.52–0.94).

Figure 2

A – Forest plot of individual effect sizes within each study for ICU admission outcome. The overall effect is shown by the green symbol. B – Funnel plot showing publication bias for ICU admission outcome. C – Extending the results using the nonparametric trim and fill method to account for publication bias. Two additional studies were included in this analysis (shown in red)

Tracheal intubation

The OR from 7 studies was 0.79 (95% CI: 0.57–1.11, p = 0.18) based on the random effects model analysis, with significant heterogeneity between studies (τ² = 0.16, I² = 88.99%, H² = 9.08, Q_(df=6) = 34.9, p_Q < 0.001) (Figure 3 A). Assessment for bias by Egger’s (p = 0.447) and Begg’s (p = 0.115) tests showed no significant small-study effects, and visual inspection of the funnel plot suggested no publication bias (Figure 3 B). Therefore, we did not extend the results of the analyses for nonparametric trim and fill method of accounting for publication bias in this case.

Figure 3

A – Forest plot of individual effect sizes within each study for tracheal intubation outcome. The overall effect is shown by the green symbol. B – Funnel plot showing publication bias for tracheal intubation outcome

Death

The OR from 21 studies which assessed the effect of statins on all-cause mortality was 0.70 (95% CI: 0.55–0.88, p < 0.01) based on a random effects model, with significant heterogeneity between studies (τ² = 0.18, I² = 82.55%, H² = 5.73, Q_{(df = 20)} = 97.04, p_Q < 0.001). Further investigation detected 2 outliers with a large effect size [34, 35]. We removed these from further analyses for this outcome, which resulted in a slight reduction in effect size (OR = 0.64, 95% CI: 0.52–0.79, p < 0.01); although heterogeneity was reduced, it still remained significant (τ² = 0.11, I² = 76.50%, H² = 4.23, Q_(df=18) = 76.5, p_Q < 0.001) (Figure 4 A). Assessment for bias by Egger’s (p = 0.903) and Begg’s (p = 0.349) tests showed no significant small-study effects, and visual inspection of the funnel plot suggested no publication bias (Figure 4 B).

Figure 4

A – Forest plot showing the effect across the studies assessing pre-hospital (n = 18) and in-hospital (n = 3) use of statins for the death outcome (overall for each subgroup shown as red symbols). The overall effect is shown by the green symbol. B – Funnel plot showing the publication bias for all 21 studies which assessed the death outcome

Subgroup analysis

The forest plot of individual ORs of the predetermined subgroup analyses indicated a significant effect in the pre-hospital use (OR = 0.70, 95% CI: 0.56–0.86, n = 18) and the in-hospital use (OR = 0.40, 95% CI: 0.22–0.73, n = 3) subgroups, with a non-significant difference between the subgroups (Q_{(df = 1)} = 2.86, p_Q = 0.09) (Figure 5 C). Additionally, although the heterogeneity was reduced slightly in the 2 subgroups, the amount of reduction was not considerable and remained significant in the pre-hospital group (p < 0.05). The heterogeneity was not significant in the in-hospital group (p = 0.08), which may be due to the smaller number of studies in this subgroup and thus a lower power of the heterogeneity test.

Figure 5

(Upper figure) Proposed mechanisms for the beneficial effects of statins in SARS-CoV-2 infection. Cellular pathways altered by statins during the course of infection in COVID-19 patients. Surface expression of ACE2 in epithelial and endothelial cells is enhanced by statins. Statins also mitigate the MYD88–NF-κB pathway and the ensuing proinflammatory cytokine response following interaction of SARS-CoV-2 with toll-like receptors (TLRs). The anti-thrombotic effects of statins are mediated by suppressive effects on tissue factor, and cytosolic phospholipase A2 (cPLA2)-induced thromboxane A2 (TXA2) synthesis and inactivated endothelial cells and platelets, respectively. Statins have also been suggested to decrease plasma membrane cholesterol content in the host cells, which can interfere with the replication of virus particles. (Lower figure) Proposed mechanisms for the anti-thrombotic effects of statins in SARS-CoV-2 infection SARS-CoV-2 induces activation of endothelial cells and the release of pro-inflammatory cytokines, which can lead to:1) over-expression of adhesion molecules (e.g. P-selectins, intercellular adhesion molecule 1 (ICAM-1), von Willebrand factor, αvβ3) and further release of proinflammatory cytokines; 2) increased recruitment of leukocytes and platelets; 3) activated endothelial cells also express tissue factor, which activates factor VII, factor Xa, and thrombin formation; 4) fibrinogen is cleaved into fibrin by thrombin; 5) thrombin is essential for thrombus formation; 6)pro-inflammatory factors can also promote coagulation and accelerate platelet activation and thrombus formation; and 7)statins downregulate proinflammatory cytokine release from endothelial cells and mitigate coagulation cascade. Moreover, statins interfere with SARS-CoV-2 entry intoACE2- expressing endothelial cells and ensuing endothelial activation (reproduced with permission from [18])

Discussion

Overall, this systematic review and meta-analysis shows a significant benefit of statin use in reducing the severity of COVID-19 disease. We found a 22–30% reduction of the risk of ICU admission and a 30% reduction in the risk of death due to COVID-19 disease. In addition, our subgroup analysis showed that in-hospital use of statins was associated with as much as a 60% reduction in mortality while pre-hospital use was associated with a 30% reduction, although the difference between the subgroups did not reach significance. Statin use was not associated with the tracheal intubation outcome.

Statins may help by preventing the development of severe lung injury and acute respiratory distress syndrome (ARDS) by protecting against inflammation via modulation of cytokine over-expression, angiotensin-converting enzyme 2 (ACE2) expression, and the immune response in COVID-19 patients [18, 36, 37]. Statins have been shown to inhibit nuclear factor κB (NF-κB), which mediates inflammatory responses during infections [38, 39]. Statins may also be used to counteract the cytokine storm, which can also occur in severe cases of viral infection such as in COVID-19 cases [40, 41]. These properties support statin continuation for mitigating ARDS and multiorgan failure, which can occur in the acute phases of COVID-19 illness [42]. In addition, statins can exert direct antiviral effects [43]. We recently produced in-silico evidence showing that statins can bind with high affinity to the main Mpro protease, which mediates viral replication and transcription and could potentially exert anti-SARS-CoV-2 activity through inhibition of viral replication [44]. Statins also exert anti-thrombotic actions through modulation of endothelial cell activation and platelet aggregation, and thereby mitigate coagulopathies associated with infection, the cytokine storm, and impending organ failure [18, 19] (Figure 5).

Our findings regarding the reduced deaths in COVID-19 patients using statins are consistent with mechanistic findings on the anti-thrombotic, anti-inflammatory, antiviral, and immunomodulatory actions of statins [18, 43] as well as those of a recent meta-analysis of 6 studies, which found a pooled 30% risk reduction of severe disease or mortality in COVID-19 patients taking statins [45]. However, 2 of the studies within the previous analyses based statin use on historical record with no mention of pre- or in-hospital administration [46, 47]. Differences in the type of statin used may also account for discrepancies across different studies. This possibility was highlighted in a study by Rossi et al., which showed that administration of simvastatin and atorvastatin reduced mortality in COVID-19 patients, whereas those treated with pravastatin and rosuvastatin showed no such difference [48]. In contrast, the CORONADO study found that statin use was associated with an increased risk of death in diabetes patients with COVID-19 [49], although this conflicted with another study, which found that statin application reduced mortality in COVID-19 diabetic patients [50]. These discrepancies may be due to the use of different statins, because the statin type was not listed in the former study and the latter mainly involved the use simvastatin and atorvastatin.

Another possibility for discrepancies in the pre- hospital use of statins with respect to COVID-19 severity could be due to the presence of diseases, such as hypertension, obesity, or lipid disorders, which are significant risk factors for worse outcomes [10–12]. A study carried out in Denmark showed that recent statin use in COVID-19 patients was not associated with an effect on all-cause mortality or severe infection [51]. It could be that the potential benefit of statin-use was offset by the presence of co-morbidities. Although the researchers attempted to adjust for the potential effects of medication differences, they could not exclude a possible impact on the association between statin use and outcomes, or the presence of pre-existing co-morbidities. Here, we have attempted to address this in our subgroup analysis which compared the effects of pre- and in-hospital use of statins on COVID-19 outcomes. However, future studies should also include an assessment of whether or not pre-existing diseases are present.

The findings of this study should be interpreted by considering a number of limitations. First, the study did not allow any assessment of a cause-and-effect relationship and, therefore, only associations are reported. Second, confounding factors cannot be excluded such as the pre-existing or post-diagnosis development of co-morbidities, including insulin resistance, ARDS, and coagulopathies, which may have affected some outcomes. Third, the findings have not been adjusted for concomitant medications. It should also to be emphasised, that based on the data from the included studies, we were not able to include the sub-analyses on the statin preparations and the investigated outcomes. Finally, our finding that in-hospital statin use was associated with decreased death compared to pre-hospital use did not reach significance. However, this is also likely to be due to the fact that there were only 3 studies [52–54] in the in-hospital use group and this was therefore likely to be statistically underpowered.

In conclusion, we showed that statin therapy might be useful in the context of a lower risk of the severe course of the COVID-19 with a reduction of ICU admission and mortality. Further studies are urgently needed to establish the role of statins in patients with COVID-19. Such studies should assess the importance of pre-existing medical conditions as well as medications on clinical outcomes. These findings also highlight the need for systematic clinical studies to assess both pre- and in-hospital use of statins as the best means of reducing COVID-19 disease severity and mortality. Successful randomised clinical studies proving that statins offer a viable therapeutic option for COVID-19 disease might pave the way for their rapid deployment in the field as needed, due to their low cost, high availability, and well-established safety and tolerability profiles. Furthermore, the knowledge that we have gained over the past several months regarding COVID-19 and repurposing existing therapeutics such as statins will provide important insights and strategic measures to successfully control future epidemics and pandemics.

Conflict of interest

Maciej Banach: speakers bureau: Amgen, DaichiiSankyo, Esperion, Herbapol, KRKA, MSD, Mylan, Novartis, Novo-Nordisk, Sanofi-Aventis, Servier; consultant to Abbott Vascular, Akcea, Amgen, Daichii Sankyo, Esperion, Lilly, MSD, Resverlogix, Sanofi-Aventis; Grants from Amgen, Mylan, Sanofi and Valeant. All other authors have no conflict of interest.