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Volume 26, Number 12—December 2020

Large SARS-CoV-2 Outbreak Caused by Asymptomatic Traveler, China

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To the Editor: Liu et al. (1) reported on a large outbreak of >70 cases of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. The origin of the outbreak was traced back to an asymptomatically infected traveler. However, delays in detecting SARS-CoV-2 infections in families B and C1 represent missed opportunities for earlier isolation and interruption of disease transmission.

After reading Lui et al. (1), we questioned whether April 7 was the first day of illness onset for the initial confirmed case, B2.3. Because viral load and infectiousness peak around the time of symptom onset and exposure of family C1 to case B2.3 was 9 days before that date, presymptomatic transmission would be highly unlikely (2). Although B2.2, who was an asymptomatic carrier, also could have played a role in exposing the family of C1, a close examination of publicly available records (3) altered this hypothesis.

Exposed on March 26, case B2.3 transmitted the virus to family C1 3 days later, on March 29, which appears to be 1 day before his first symptoms. Case B2.3 went to an outpatient clinic with a subjective fever on March 30 but was not tested for SARS-CoV-2. He was not isolated until he went to a clinic again on April 7 with worsening symptoms. Earlier isolation and testing of B2.3 could have prompted earlier contact tracing and triggered earlier diagnosis of C1 during his hospital stay, potentially preventing the chain of >60 SARS-CoV-2 transmissions in 2 hospitals.

The uncooperative behavior of cases B2.2 and B2.3 complicated efforts for early contact tracing (3), demonstrating cooperation with medical officers, coupled with proactive case-finding and earlier case isolation, clearly are crucial in curbing disease spread (4,5). If timely actions had been implemented, the outbreak could have been prevented or greatly reduced in size.


Andrei R. AkhmetzhanovComments to Author 
Author affiliation: College of Public Health, National Taiwan University, Taipei, Taiwan



  1. Liu  J, Huang  J, Xiang  D. Large SARS-CoV-2 outbreak caused by asymptomatic traveler, China. Emerg Infect Dis. 2020;26:22603.PubMedGoogle Scholar
  2. He  X, Lau  EHY, Wu  P, Deng  X, Wang  J, Hao  X, et al. Temporal dynamics in viral shedding and transmissibility of COVID-19. Nat Med. 2020;26:6725.PubMedGoogle Scholar
  3. Health Commission of Heilongjiang Province China. Patient trajectory: release of new confirmed cases, asymptomatic infection trajectory of 10 April [in Chinese] [cited 2020 Jul 25].
  4. Wang  CJ, Ng  CY, Brook  RH. Response to COVID-19 in Taiwan: big data analytics, new technology, and proactive testing. JAMA. 2020;323:13412. DOIPubMedGoogle Scholar
  5. Dinh  L, Dinh  P, Nguyen  PDM, Nguyen  DNH, Hoang  T. Vietnam’s response to COVID-19: prompt and proactive actions. J Trav Med. 2020;27(3):taaa047.


Cite This Article

DOI: 10.3201/eid2612.203437

Original Publication Date: November 16, 2020

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Please use the form below to submit correspondence to the authors or contact them at the following address:

Andrei R. Akhmetzhanov, Global Health Program & Institute of Epidemiology and Preventive Medicine, College of Public Health, National Taiwan University, No. 17, Xuzhou Rd, Zhongzheng District, Taipei City 100025, Taiwan

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Page created: September 18, 2020
Page updated: November 23, 2020
Page reviewed: November 23, 2020
The conclusions, findings, and opinions expressed by authors contributing to this journal do not necessarily reflect the official position of the U.S. Department of Health and Human Services, the Public Health Service, the Centers for Disease Control and Prevention, or the authors' affiliated institutions. Use of trade names is for identification only and does not imply endorsement by any of the groups named above.