Transmission of Severe Acute Respiratory Syndrome Coronavirus 2 to Close Contacts, China, January–February 2020

T ransmissibility of an emerging infectious dis- ease is a key factor for determining transmission dynamics in a population. The basic reproductive number, R 0 , indicates the average number of new cases resulting from 1 infected person in a completely susceptible population (1). In December 2019, an outbreak of coronavirus disease (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was identifi ed in Wuhan, Hubei Province, China (2). The mean R 0 of COVID-19 was estimated to be in the range of 1.90-6.49 (3), indicating a high contagiousness that led to its rapid spread across the world (4). Another indicator of infectiousness is secondary attack rate (SAR), which is the probability that infection occurs among susceptible persons within a reasonable incubation period after known contact with an infectious person or an infectious source Few estimates are available for the SAR for COVID-19 and its variation by type of contact, characteristics of index case-patients and contacts, and other factors. Information about factors associated with variation in SAR could help identify persons at high risk of transmitting the virus or acquiring COVID-19. Studies have reported transmission during the incubation period of COV-ID-19 (7-10) but with unclear quantifi cation of risk. We estimated the SAR for COVID-19 and factors associated with risk for transmission.