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Background: The novel coronavirus,severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2,also called 2019-nCoV) causes different morbidity risks to individuals in different age groups.This study attempts to quantify the age-specific transmissibility using a mathematical model.Methods: An epidemiological model with five compartments (susceptible-exposed-symptomatic-asymptomatic-recovered/removed[SEIAR]) was developed based on observed transmission features.Coronavirus disease 2019 (COVID-19) cases were divided into four age groups: group 1,those ≤ 14years old;group 2,those 15 to 44years old;group 3,those 45 to 64years old;and group 4,those ≥ 65 years old.The model was initially based on cases (including imported cases and secondary cases) collected in Hunan Province from January 5 to February 19,2020.Another dataset,from Jilin Province,was used to test the model.Results: The age-specific SEIAR model fitted the data well in each age group (P ＜ 0.001).In Hunan Province,the highest transmissibility was from age group 4 to 3 (median:β43 =7.71 × 10-9;SAR43 =3.86 × 10-8),followed by group 3 to 4 (median:β34 =3.07 × 10-9;SAR34 =1.53 × 10-8),group 2 to 2 (median:β22 =1.24 × 10-9;SAR22 =6.21 × 10-9),and group 3 to 1 (median:β31 =4.10 × 10-10;SAR31 =2.08 × 10-9).The lowest transmissibility was from age group 3 to 3 (median:β33 =1.64 × 10-19;SAR33 =8.19 × 10-19),followed by group 4 to 4 (median: β44 =3.66 ×10-17;SAR44 =1.83 × 10-16),group 3 to 2 (median:β32=1.21 × 10-16;SAR32 =6.06 × 10-16),and group 1 to 4 (median:β14 =7.20 × 10-14;SAR14 =3.60 × 10-13).In Jilin Province,the highest transmissibility occurred from age group 4 to 4 (median: β43 =4.27 × 10-8;SAR43 =2.13 × 10-7),followed by group 3 to 4 (median: β34 =1.81 × 10-8;SAR34 =9.03 × 10-8).Conclusions: SARS-CoV-2 exhibits high transmissibility between middle-aged (45 to 64 years old) and elderly (≥ 65 years old) people.Children (≤ 14 years old) have very low susceptibility to COVID-19.This study will improve our understanding of the transmission feature of SARS-CoV-2 in different age groups and suggest the most prevention measures should be applied to middle-aged and elderly people.