“…For example, in some studies, most cases had a long or even unclear interval of exposure, making it difficult to determine the exact exposure date 3,5,13 and giving rise to error. Besides, the order of transmission (i.e., who is infector and who is infectee) in clustering cases, which is crucial to estimation of both parameters, is easy to be mistaken given the possibility of pre-symptomatic and asymptomatic transmission, [14][15][16][17] but previous studies rarely described how this issue was handled.…”