The coronavirus disease 2019 (COVID-19) caused by the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) was first observed in Wuhan, China in December 2019 and was later declared a pandemic by World Health Organization on 11 th March 2020. 1 Severe COVID-19 is characterized as a syndrome of hypoxemic pneumonitis with a profound inflammatory response. This inflammatory response is associated with elevated levels of inflammatory cytokines including interleukin-6 (IL-6) and leads to diffuse alveolar damage and acute respiratory distress syndrome (ARDS). [2] [3] [4] Patients with severe disease often have high IL-6 levels. 5 Therefore, the treatment of cytokine storm with IL-6 blocking agents for severely ill COVID-19 patients seems logical. Trials are ongoing to assess the effectiveness of three IL-6 blocking agents; tocilizumab, siltuximab, and sarilumab in COVID-19 patients. 6-8 Tocilizumab is FDA approved for chimeric antigen receptor (CAR) T cell-induced severe or life-threatening cytokine release syndrome. 9 Currently, there are no published randomized trials on the use of tocilizumab in COVID-19 patients. The best evidence thus far is from observational studies which often report aggregate data. We performed a systematic review of individual patient data to summarize the baseline characteristics COVID 19 This article is protected by copyright. All rights reserved.
patients who received tocilizumab in order to identify nuanced predictors of response to therapy and other clinical outcomes.
The Preferred Reporting Items for Systematic Reviews and Meta-Analyses of individual participant data (the PRISMA-IPD) statement was followed for the conduct and reporting of this systematic review. 10
DA and ZK performed a systematic search of PubMed, Embase, and Medline from 22 nd April 2020 and again on 27 th April 2020. The following search terms or keywords were used alone or in combination: "COVID-19", "coronavirus", "SARS-CoV-2", "COVID", "anti-interleukin 6 receptor antibodies", "anti-IL-6", "tocilizumab", "sarilumab", "siltuximab". We did not apply age, geographic, language, publication type, or date restrictions. Following the search and duplicate record removal, two independent reviewers (DA & ZK) screened titles and abstracts for inclusion in the review. Primary outcome of interest was in-hospital mortality. Secondary outcomes included in-hospital complications, recovery rates, effect of patient characteristics on the primary outcome and changes in levels of inflammatory markers.
Included studies reported a diagnosis of reverse-transcriptase polymerase chain reactionconfirmed COVID-19 in patients who received either tocilizumab, sarilumab, or siltuximab alone or in addition to regular care for COVID-19. A single study currently This article is protected by copyright. All rights reserved.
under review was also added (Siddiqui et al, submitted). All included studies reported individual patient data. We excluded reviews, letters, comments, editorials, society guidelines, non-human studies, and all studies which did not report individual patient data.
Eligible studies were critically appraised by the two independent reviewers at the study level for methodological and reporting quality by adapting the ROBIN-I tool for assessment of risk of bias to the selected observational studies. 11 Where the two independent reviewers disagreed on whether a study met the inclusion criteria or not, we resolved the disagreement through discussion which involved a third author when necessary. We assessed inter-reviewer reliability in selecting studies with Fleiss' kappa.
Where required, authors of papers were contacted to request missing or additional data for clarification.
Two reviewers (DA and ZK) independently extracted relevant data using a data extraction form designed for this systematic review. Extracted data included study characteristics such as the name of the first author, study type, country of origin of the study, and the clinical setting of the patient encounter. Patient characteristics included age, sex, pre-existing medical conditions, day of tocilizumab administration from the day of hospitalization, dose of tocilizumab, and administration of hydroxychloroquine, corticosteroids, antivirals, or antibiotics during index hospitalization. When available, we also extracted information on in-hospital complications, levels of C-reactive protein This article is protected by copyright. All rights reserved.
(CRP) and IL-6 before and after the administration of tocilizumab, adverse effects of tocilizumab when reported, as well as, clinical outcomes.
Continuous variables were summarised as mean (standard deviations [SD]) or median (interquartile range [IQR]), where applicable and categorical variables were presented as number (percentage). We compared baseline demographics amongst some diving and month survival in patients using Fisher's exact test. Differences in the serum levels of IL-6 and CRP before and after administration of tocilizumab were assessed with paired Student's t-test or Wilcoxon-signed rank test depending upon whether the normality assumptions were met. All tests were performed as 2-tailed and statistical significance levels set at a P-value < .05 with Stata (StataCorp version 14, College Station, Texas) and XLSTAT (Addinsoft, New York, N.Y.).
A total of 351 records were identified through the systematic search. A single study currently under review was also added (Siddiqui et al, submitted) . Thus, 352 records were screened for eligibility for further full-text review. Of these, 30 studies underwent fulltext review for inclusion in the analysis. Among these, 11 studies were identified for inclusion that encompassed 29 patients and were available for further analysis (Fig 1) .
Fleiss' kappa for inter-reviewer reliability in selecting studies was 0.85, with sensitive of This article is protected by copyright. All rights reserved.
100% and specificity of 89%. Disagreement was on the inclusion of 2 studies. This was resolved after extensive discussion among three reviewers.
Majority of the patients came from case series of which 55% came from a single Chinese report. 12 Table 1 illustrates the characteristics of each included study. 12-21 These studies 
Baseline patient characteristics are outlined in Table 2 21 There was a considerable heterogeneity in the dosing and duration of tocilizumab therapy. Weight-based dosing ranged from 6-8 mg/kg per dose, and frequency ranged from one to four times during the hospitalization. There was no uniform indication for the initiation of tocilizumab. However, most patients received the medication when there was an increased oxygen requirement along with a This article is protected by copyright. All rights reserved.
rise in inflammatory markers such as CRP. Of these 29 patients, 52% received only tocilizumab and the rest of the patients received corticosteroid, hydroxychloroquine, lopinavir-ritonavir and/or other antiviral therapy in addition to tocilizumab (Table 2) .
Serum IL-6 levels before and after initiation of tocilizumab were available for 17 patients. There were inconsistencies in reporting of the time when first IL-6 levels were drawn relative to the index dose of tocilizumab. However, follow up IL-6 levels were obtained about 2 days after the first dose of tocilizumab. IL-6 level was significantly higher after the initiation of tocilizumab with median (IQR) of 376.6 (148-900.6) pg/mL compared to the baseline of 71.1 (31.9-122.8) pg/mL (p=0.002) as seen in Fig 2A. CRP levels before and after tocilizumab initiation were available for 22 patients. CRP levels were drawn 2.4 (±1.6) days before the initiation of tocilizumab and repeat measures were drawn 3.4 (±2.5) days after the index dose of tocilizumab. The mean (SD) CRP levels before and after tocilizumab was 140.4 (77) mg/L and 24.6 (26.9) mg/L respectively ( Fig   2B) . CRP level was significantly lower after initiation of tocilizumab (P< 0.0001).
Over an average of 5.4 hospital days, 6 (20.7%) of the 29 patients had died. Two-way comparisons of the baseline demographics were not significantly different amongst survivors and non-survivors by Fisher's exact test. Among survivors, about 10% had worsened disease, 17% recovered and 4 (17.3%) were discharged home. One patient was not hospitalized due to mild symptoms. That patient was however, taking tocilizumab prior to COVID-19 diagnosis. Disposition information on 18 patients were not available, although most appear to remain hospitalized at the end of the reports. The most common in-hospital complication was ARDS (27.6%) requiring mechanical ventilation (Table 2) which is certainly an indication of disease progression rather than a consequence of This article is protected by copyright. All rights reserved.
treatment. There were three reports of medication-related complications including hypertriglyceridemia in two patients and an incident of QT interval prolongation believed to be due to an adverse effect of hydroxychloroquine. 14,20
This systematic review was based on 11 studies which encompassed 29 patients who received tocilizumab during their admission. The results demonstrated that most patients were treated in the ICU which is in line with most of the institutional guidelines where tocilizumab is usually administered at later or advanced stages of illnesses. The sex distribution was heavily skewed towards males which is a common observation in this disease. 2, 22, 23 Other demographic features of the patients involved in present study included the presence of comorbidities including hypertension, diabetes, obesity, cerebrovascular accident, cardiac and lung diseases. The presence of these illnesses has important prognostic value and as such these patients were noted to have rapidly deteriorated and required admission to ICU and administration of anti-IL6 agents. 24 A considerable proportion of these patients further developed ARDS and required mechanical ventilation. Review of clinical outcomes data was limited because of limitations encountered secondary to event reporting limitations and limited timeframe.
An interesting observation was an improvement in serum CRP levels which could serve as a surrogate for dampening inflammation and clinical improvement. An initial rise in IL-6 levels after initiation of tocilizumab is temporarily expected as its receptors have been blocked by Tocilizumab. 25 There may exist an upper threshold beyond which this temporary feedback mechanism fails and elevated IL-6 levels persist as seen in two This article is protected by copyright. All rights reserved.
patients in this study. 12 Therefore it seems likely that clinical improvement noted in some of these patients is the result of rapid control of hyper inflammation resulted from tocilizumab treatment.
The use of tocilizumab in COVID-19 patients who develop respiratory failure and other complications is currently off label and hence its safety profile needs to be evaluated with further randomized control trials. Currently, FDA has approved phase III clinical trial of tocilizumab for COVID 19 pneumonia. There were several instances where successful treatment of patients with this drug was carried out as illustrated in this study. The small sample size from these observational studies and the heterogeneity of the results including the scarcity of the clinical outcomes data limit the generalizability of this study.
In conclusion, severe COVID-19 patients may develop this dreaded hyper-inflammatory cytokine release syndrome which involves a complex interplay of IL-6 and other chemokines. IL-6 seems to play a pivotal role in this, and the use of anti-IL-6 treatment seems beneficial. Data from randomized control trials are needed for proper evaluation of clinical outcomes using this promising therapy.