What is the transmissibility of coronavirus disease 2019 (COVID-19) to close contacts?
In this case-ascertained study of 100 cases of confirmed COVID-19 and 2761 close contacts, the overall secondary clinical attack rate was 0.7%. The attack rate was higher among contacts whose exposure to the index case started within 5 days of symptom onset than those who were exposed later.
High transmissibility of COVID-19 before and immediately after symptom onset suggests that finding and isolating symptomatic patients alone may not suffice to interrupt transmission, and that more generalized measures might be required, such as social distancing.
The dynamics of coronavirus disease 2019 (COVID-19) transmissibility are yet to be fully understood. Better understanding of the transmission dynamics is important for the development and evaluation of effective control policies.
To delineate the transmission dynamics of COVID-19 and evaluate the transmission risk at different exposure window periods before and after symptom onset.
Design, Setting, and Participants
This prospective case-ascertained study in Taiwan included laboratory-confirmed cases of COVID-19 and their contacts. The study period was from January 15 to March 18, 2020. All close contacts were quarantined at home for 14 days after their last exposure to the index case. During the quarantine period, any relevant symptoms (fever, cough, or other respiratory symptoms) of contacts triggered a COVID-19 test. The final follow-up date was April 2, 2020.
Main Outcomes and Measures
Secondary clinical attack rate (considering symptomatic cases only) for different exposure time windows of the index cases and for different exposure settings (such as household, family, and health care).
We enrolled 100 confirmed patients, with a median age of 44 years (range, 11-88 years), including 44 men and 56 women. Among their 2761 close contacts, there were 22 paired index-secondary cases. The overall secondary clinical attack rate was 0.7% (95% CI, 0.4%-1.0%). The attack rate was higher among the 1818 contacts whose exposure to index cases started within 5 days of symptom onset (1.0% [95% CI, 0.6%-1.6%]) compared with those who were exposed later (0 cases from 852 contacts; 95% CI, 0%-0.4%). The 299 contacts with exclusive presymptomatic exposures were also at risk (attack rate, 0.7% [95% CI, 0.2%-2.4%]). The attack rate was higher among household (4.6% [95% CI, 2.3%-9.3%]) and nonhousehold (5.3% [95% CI, 2.1%-12.8%]) family contacts than that in health care or other settings. The attack rates were higher among those aged 40 to 59 years (1.1% [95% CI, 0.6%-2.1%]) and those aged 60 years and older (0.9% [95% CI, 0.3%-2.6%]).
Conclusions and Relevance
In this study, high transmissibility of COVID-19 before and immediately after symptom onset suggests that finding and isolating symptomatic patients alone may not suffice to contain the epidemic, and more generalized measures may be required, such as social distancing.
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CME Disclosure Statement: Unless noted, all individuals in control of content reported no relevant financial relationships. If applicable, all relevant financial relationships have been mitigated.
Accepted for Publication: April 21, 2020.
Published Online: May 1, 2020. doi:10.1001/jamainternmed.2020.2020
Correction: This article was corrected on September 8, 2020, to fix transposed numbers of men and women study participants in the Abstract and in the Results section; an error in the definition of serial interval in the Introduction; the misspelling of 2 names in the Group Information section; and typographical data errors in the Supplement.
Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2020 Cheng H-Y et al. JAMA Internal Medicine.
Corresponding Author: Hsien-Ho Lin, MD, ScD, National Taiwan University, 17 Xuzhou Rd, Taipei 100, Taiwan (firstname.lastname@example.org).
Author Contributions: Drs Cheng and Lin had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.
Concept and design: Cheng, Jian, Huang, Lin.
Acquisition, analysis, or interpretation of data: All authors.
Drafting of the manuscript: Cheng, Jian, Ng, Lin.
Critical revision of the manuscript for important intellectual content: Cheng, Jian, Liu, Huang, Lin.
Statistical analysis: Cheng, Jian, Ng, Lin.
Obtained funding: Lin.
Administrative, technical, or material support: Jian, Liu, Ng, Huang, Lin.
Supervision: Liu, Huang, Lin.
Conflict of Interest Disclosures: None reported.
Funding/Support: The study was funded by Taiwan Ministry of Science and Technology (MOST 107-2314-B-002-187-MY2 and MOST 108-2628-B-002-022).
Role of the Funder/Sponsor: The funder had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.
Group Information: The Taiwan COVID-19 Outbreak Investigation Team members include the following: Taiwan CDC: Office of Preventive Medicine and Taiwan Field Epidemiology Training Program: Wan-Ting Huang, Wan-Chin Chen, Angela Song-En Huang, Chia-Ping Su, Pin-Hui Lee, Pei-Chun Chan, Hao-Hsin Wu, Shih-Tse Huang, Tsung-Pei Tsou, Ying-Shih Su, and Yang Li. Taipei Regional Control Center: Hsin-Yi Wei, Meng-Yu Chen, Shiao-Ping Tung, Yu-Fang Tsai, Xiang-Ting Huang, and Chien-Yu Chou. North Regional Control Center: Pei-Yuan Wu, Fang-Tzu Chang, Chia-Ying Yen, Hsueh-Mei Chiang, Ju-Hui Lin, and Ming-Chu Tai. Central Regional Control Center: Kung-Chin Wang, Ching-Fen Ko, Pei-Fang Lai, Du-Ling Lin, Min-Tsung Lin, and Zhi-Jie Ding. South Regional Control Center: Huai-Te Tsai, Ping-Jung Liu, Pei-Yi Lin, Shu-Chen Chang, and Yi-Ying Lin. Kao-Ping Regional Control Center: Hsin-Chun Lee, Chi-Nan Hung, Ching-Li Lin, Chi-Mei Lai, and Hsiao-Mei Liu.
Additional Contributions: We thank the Taiwan COVID-19 Outbreak Investigation Team, the staffs of regional control centers of the Taiwan Centers for Disease Control (CDC), and partners from other public health bureaus (Taipei City, New Taipei City, Taoyuan City, Taichung City, Tainan City, Kaohsiung City, Changhua, Nantou, Hsinchu, Miaoli, Yunlin, and Ilan County) for their dedicated outbreak investigation and meticulous date collection. Our study could not have been done without their efforts. We also thank Chia-Lin Lee, MSc, and Yu-Lun Liu, MD, MSc, for the development of electronic contact tracing system (Epidemic Intelligence Center, Taiwan CDC); Ching-Hung Wang (TonyQ) for the consultation of system development; Angela Song-En Huang, MD, MPH (Office of Preventive Medicine, Taiwan CDC), for writing assistance. They did not receive compensation outside of their salary.
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