Tehran University of Medical Sciences Journal of Biostatistics and Epidemiology 2383-4196 8 1 2022 03 15 1 7 Mojtaba Sepandi Exercise Physiology Research Center, Life Style Institute, Baqiyatallah University of Medical Sciences, Tehran, Iran Yousef Alimohamadi Health Research Center, Life Style Institute, Baqiyatallah University of Medical Sciences, Tehran, Iran Firooz Esmaeilzadeh Department of Public Health, School of Public Health, Maragheh University of Medical Sciences, Maragheh, Iran 2021 10 27 2022 01 01 Introduction: An essential concept in assessing the extent to which an infectious outbreak spread is the concept of basic reproductive number (R0). The current systematic review and meta-analysis aimed to estimate the R0 of the Delta variant of SARS-CoV-2 based on studies published from 1 January 2021 to 23 September 2021. Methods: International databases (including Google Scholar, Science Direct, PubMed, and Scopus) were searched using keywords: "Basic reproduction number, R0, COVID-19, SARS-COV-2, Severe Acute Respiratory Syndrome Coronavirus, NCOV, 2019 NCOV, coronavirus, Delta variant, B.1.617.2". Due to significant heterogeneity, DerSimonian-Laird random-effects model was used to estimate the pooled value of R0. Results: A total of 245 reports were identified. After assessing the inclusion criteria, three studies were selected. The pooled R0 for the Delta variant was estimated as 5.10 (95% CI, 3.04 to 7.17). (I2 =86.77%, T2:2.68, p-value from the chi-square test for heterogeneity was<0.001). Conclusions: Considering the estimated value of R0 for the Delta variant of SARS-CoV-2, the amount of vaccine coverage required to achieve herd immunity appears to be higher than previous variants of the virus. https://jbe.tums.ac.ir/index.php/jbe/article/view/782