“…Note that it is difficult to accurately estimate the basic reproduction number of the SARS-CoV-2, which may be influenced by the chosen methods, the available data, various populations/regions etc. For estimation of the basic reproduction number of the original SARS-CoV-2 (denoted by ), the estimated mean value is between 2 and 3 for mainland China ( Ahammed et al, 2021 ; Li et al, 2020 ; Riou & Althaus, 2020 ; Wu et al, 2020 ), and between 3 and 4 for European countries ( Flaxman et al, 2020 ; Gatto et al, 2020 ; Roques et al, 2020 ; Yuan et al, 2020 ). Here we chose , , and being in the interval of [1.4, 1.9] ( Davies et al, 2021 ) as the basis for estimating the basic reproduction number of the Delta variant.…”