Our literature review yielded 3 retrospective studies [3, 20, 21] on 9022 unique patients including 590 patients who had a history of prior bariatric surgery (Fig. 1, Table 1).
Cleveland Clinic Study
Among 4365 patients who tested positive for SARS-CoV-2 between March 8, 2020, and July 22, 2020, in the Cleveland Clinic Health System, 33 patients were identified who had a prior history of bariatric surgery. The surgical patients were propensity matched 1:10 to nonsurgical patients to assemble a cohort of control patients (n = 330) with a body mass index (BMI) ≥ 40 kg/m2 at the time of SARS-CoV-2 testing. The primary endpoint was the rate of hospital admission. In the univariate analysis, 6 (18.2%) patients in the bariatric surgery group and 139 (42.1%) patients in the control group were admitted to the hospital (P = 0.013). In the multivariate analysis, prior history of bariatric surgery was associated with a lower hospital admission rate compared with control patients with obesity (OR 0.31 [95% CI 0.11–0.88], P = 0.028). While none of the 4 exploratory outcomes occurred in the bariatric surgery group, 43 (13.0%) patients in the control group required ICU admission (P = 0.021), 22 (6.7%) required mechanical ventilation, 5 (1.5%) required dialysis, and 8 (2.4%) patients died .
In the surgical group, the median interval between the bariatric surgery and the positive viral test was 46 months (interquartile range, 29–85). Body mass index of patients decreased from 49.1 ± 8.8 kg/m2 at the time of bariatric surgery to 37.2 ± 7.1 kg/m2 at the time of SARS-CoV-2 test (paired difference of 12.6 kg/m2, P < 0.001). Nine (27.3%) patients had type 2 diabetes at the time of bariatric surgery including 4 patients on insulin therapy. At the time of the SARS-CoV-2-positive test, diabetes was in remission in 7 patients. Similarly, of the 21 (63.6%) patients who had hypertension at the time of bariatric surgery, 9 patients were not on antihypertensive medications at the time of a positive viral test. In patients with a history of hypertension, the median number of antihypertensive medications decreased from 2 at the time of bariatric surgery to 1 at the time of positive viral test (P = 0.001) .
University of Minnesota Study
This retrospective analysis of electronic medical record data of patients with a positive SARS-CoV-2 test from March 1, 2020, to August 25, 2020, showed that a history of nonalcoholic fatty liver disease (NAFLD) was associated with increased odds of hospital admission after SARS-CoV-2 infection. Among 373 patients with NAFLD, a subgroup of patients who had undergone bariatric surgery (n = 16) had significantly decreased odds of hospitalization (OR 0.22 [95% CI 0.05–0.98], P < 0.05) .
Study from France
This is a retrospective population-based study based on administrative data of all inpatient cases identified by the French National Health Insurance database. In total, 8286 patients with a diagnosis of obesity were admitted for SARS-CoV-2 infection between January 1, 2020, and May 15, 2020. In total, 541 had a history of bariatric surgery between 2010 and 2019. The need for an invasive mechanical ventilation and death occurred in 7% and 3.5% in the bariatric surgery group versus 15% and 14.2% in the non-bariatric surgery group, respectively (both P < 0.0001). The logistic regression analysis showed that prior bariatric surgery was associated with a lower risk of invasive mechanical ventilation (OR 0.67 [95% CI 0.48–0.95], P = 0.025) and mortality (OR 0.50 [95% CI 0.31–0.80], P = 0.004) .
All 3 studies reported the mortality outcome [3, 20, 21]. The risk of mortality without previous bariatric surgery was 133 per 1000 cases and its risk with previous bariatric surgery was 33 per 1000 (summary OR 0.22, 95% CI 0.19–0.26, based on the random-effects meta-analysis). No heterogeneity was observed between the included studies (I2 = 0%, P = 0.98 for heterogeneity) (Fig. 2, Table 2).
Hospital Admission Outcome
The Cleveland Clinic  and the University of Minnesota  studies independently reported a lower risk of hospitalization in patients with prior bariatric surgery after contracting SARS-CoV-2 infection. In the pooled analysis, the hospitalization rate in patients without previous bariatric surgery was 412 per 1000 cases and its rate in patients with previous bariatric surgery was 164 per 1000 (summary OR 0.28, 95% CI 0.12–0.65, based on the random-effects meta-analysis). No heterogeneity was observed between the included studies (I2 = 0%, P = 0.71 for heterogeneity) (Fig. 3, Table 2).
Quality Assessment of Included Studies
The quality of the included studies was examined using the domain-based risk of bias assessment tool. There was a substantial risk of bias across the studies for confounding and selection bias (Fig. 4).