Background: ES-SCLC is an aggressive lung cancer with high rates of relapse following induction regimens. Immune checkpoint immunotherapy (ICI) seems promising in the refractory/relapsing setting based on a smaller number of studies with smaller sample sizes. Here we report a pooled estimate of clinical activity of ICI in above patients based on published phase II/III trials so far. Methods: PubMed, EMBASE, Cochrane review, and clinicaltrials.gov databases were searched for phase II/III randomized controlled trials studying immune checkpoint immunotherapy in second or third line setting for ES-SCLC treatment. Data were extracted based on Reviews and Meta- Analysis guidelines. DerSimonian-Laird (DL) random effects model was used to estimate the pooled proportion of patients achieving objective response rate (ORR), pooled proportion of patients experiencing therapy related adverse events (TRAEs grade ≥ 1), pooled median progression free survival (PFS) and pooled median overall survival (OS), and weights were estimated using inverse variance method. Statistical heterogeneity was calculated using the I² and chi-squared test. Outcomes were calculated by using ‘Metafor’ and ‘Meta’ packages by R-Studio and “CRAN” project. Results: Five studies with a total of 698 patients who received immune checkpoint immunotherapy were analyzed. Median age of patients who received treatment was 63 yrs. The pooled proportion of patients who achieved ORR was 15% (95% CI 0.11 to 0.20). Pooled Median PFS was about 1.54 (95% CI, 1.34- 1.74) months and median OS was 6.46 (95% CI, 5.13- 7.73) months. The pooled proportion of patients who experienced adverse events during the treatment was 49% (95% CI 0.38 to 0.60). Conclusions: ICI as second or third line treatment choice offers modest improvement in progression free survival and overall survival in ES-SCLC patients who have not received ICI as first line in combination with etoposide and platinum based therapies.