Lei Luo  1 , Dan Liu  2 , Xinlong Liao  1 , Xianbo Wu  2 , Qinlong Jing  1 , Jiazhen Zheng  2 , Fanghua Liu  1 , Shigui Yang  3 , Hua Bi  1 , Zhihao Li  2 , Jianping Liu  1 , Weiqi Song  2 , Wei Zhu  1 , Zhenghe Wang  2 , Xiru Zhang  2 , Qingmei Huang  2 , Peiliang Chen  2 , Huamin Liu  2 , Xin Cheng  2 , Miaochun Cai  2 , Pei Yang  2 , Xingfen Yang  2 , Zhigang Han  1 , Jinling Tang  4 , Yu Ma  1 , Chen Mao  2:

Results: Among 3410 close contacts, 127 (3.7% [95% CI, 3.1% to 4.4%]) were secondarily infected. Of these 127 persons, 8 (6.3% [CI, 2.1% to 10.5%]) were asymptomatic. Of the 119 symptomatic cases, 20 (16.8%) were defined as mild, 87 (73.1%) as moderate, and 12 (10.1%) as severe or critical. Compared with the household setting (10.3%), the secondary attack rate was lower for exposures in health care settings (1.0%; odds ratio [OR], 0.09 [CI, 0.04 to 0.20]) and on public transportation (0.1%; OR, 0.01 [CI, 0.00 to 0.08]). The secondary attack rate increased with the severity of index cases, from 0.3% (CI, 0.0% to 1.0%) for asymptomatic to 3.3% (CI, 1.8% to 4.8%) for mild, 5.6% (CI, 4.4% to 6.8%) for moderate, and 6.2% (CI, 3.2% to 9.1%) for severe or critical cases. Index cases with expectoration were associated with higher risk for secondary infection (13.6% vs. 3.0% for index cases without expectoration; OR, 4.81 [CI, 3.35 to 6.93]).