Skip directly to site content Skip directly to page options Skip directly to A-Z link Skip directly to A-Z link Skip directly to A-Z link
Volume 26, Number 7—July 2020
Research Letter

Severe Acute Respiratory Syndrome Coronavirus 2 Shedding by Travelers, Vietnam, 2020

Thi Quynh Mai Le1, Taichiro Takemura1, Meng Ling Moi, Takeshi Nabeshima, Le Khanh Hang Nguyen, Vu Mai Phuong Hoang, Thi Hong Trang Ung, Thi Thanh Le, Vu Son Nguyen, Hong Quynh Anh Pham, Tran Nhu Duong, Hai Tuan Nguyen, Duy Nghia Ngu, Cong Khanh Nguyen, Kouichi Morita, Futoshi HasebeComments to Author , and Duc Anh DangComments to Author 
Author affiliations: National Institute of Hygiene and Epidemiology, Hanoi, Vietnam (T.Q.M. Le, L.K.H. Nguyen, V.M.P. Hoang, T.H.T. Ung, T.T. Le, V.S. Nguyen, H.Q.A. Pham, T.N. Duong, H.T. Nguyen, D.N. Nguyen, C.K. Nguyen, D.A. Dang); World Health Organization Collaborating Center for Reference and Research on Tropical and Emerging Virus Diseases, Institute of Tropical Medicine, Nagasaki University, Nagasaki, Japan (T. Takemura, M.L. Moi, T. Nabeshima, K. Morita, F. Hasebe)

Cite This Article


We analyzed 2 clusters of 12 patients in Vietnam with severe acute respiratory syndrome coronavirus 2 infection during January–February 2020. Analysis indicated virus transmission from a traveler from China. One asymptomatic patient demonstrated virus shedding, indicating potential virus transmission in the absence of clinical signs and symptoms.

During the past 2 months, emergence of 2019 novel coronavirus disease (COVID-19) has caused global public health concern (1,2). In light of the rapid global expansion of the disease, we performed a detailed epidemiologic and clinical assessment to determine the transmission patterns of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outside China. As of March 6, 2020, imported SARS-CoV-2 infections have been identified in Vietnam; 17 cases (none severe) have been confirmed. Patients were 3 months to 65 years of age. Although transmission of SARS-CoV-2 may occur within days after illness onset, data on the early viremia kinetics in travelers are limited. We describe the virus shedding patterns in a cluster of travelers and in a cluster of patients who had close contact with the travelers.

On November 15, 2019, eight employees from a company in northern Vietnam (7 from Vinh Phuc Province, 1 from Thanh Hoa Province) were sent to Wuhan, China, for technical training for ≈2 months. On January 17, 2020, they returned to Vietnam via a flight from Wuhan to Guangzhou, China, followed by a flight from Guangzhou to Hanoi, Vietnam. During January 21–27, 2020, fever and cough developed in 6 of those travelers. Real-time PCR confirmed SARS-CoV-2 infection in all 6 travelers. Virus isolation and next-generation sequencing were performed for samples that were positive for viral RNA. The remaining 2 employees that had been on the same flight were quarantined, but real-time PCR indicated that they were negative for viral RNA. Patients were hospitalized at the National Hospital of Tropical Diseases, Thanh Hoa Provincial Hospital, and Tam Dao District Hospitals in Vinh Phuc Province, where they were closely monitored in isolated wards and followed up. Patient throat swab samples were sent to the Institute of Hygiene and Epidemiology, Hanoi, Vietnam, for laboratory diagnosis. The patients provided consent to have their details shared.

Among those tested for viral RNA by SARS-CoV–specific reverse transcription PCR, results were negative for 155 persons who had been in close contact with the 6 SARS-CoV–positive travelers and 1,092 persons exhibiting clinical signs, including cough and fever (3). Those with positive results were 6 of the persons sent to Wuhan for training (cluster 1), another 5 (cluster 2) who had been in close contact with a patient from cluster 1, and 1 patient in cluster 2 (patient 12) who had been in close contact with 2 other patients from cluster 2 (Table). We monitored SARS-CoV-2 viremia in 30 throat swab specimens obtained from the 12 patients in hospitals throughout Vietnam (3 male and 9 female; average age 31.2 years [range 3 months–55 years]).

Clinical signs, including fever and cough, were demonstrated by 11 patients an average of 9.9 (± 5.4) days after travel or close contact with patients, indicating that the incubation period was 1–2 weeks after exposure (4) (Appendix). In these patients, virus shedding was detected from day 1 after illness onset through day 19 (4.6 days) after potential initial exposure (Table). Of note, 1 patient in cluster 2 (patient 9, a 55-year-old man) was asymptomatic, but virus shedding was detected for up to 9 days (Table). This finding confirms virus shedding in asymptomatic patients and indicates possible transmission during the asymptomatic period. In this context, virus was isolated from 3 patients by inoculation of throat swab samples onto Vero cells. Phylogenetic analyses of an isolate from patient 3 showed that the full-length genome had high sequence homology (99.96%) to a SARS-CoV-2 isolate identified in Wuhan, China (5).

Although close-contact transmission of SARS-CoV-2 between family members has been identified (6), evidence of virus circulation within the community in Vietnam is limited. We describe 6 cases of close-contact transmission between family members and those living in close proximity and determined virus shedding patterns of 12 patients in Vietnam. Because the virus was not circulating locally, our data provide insight into viral RNA shedding patterns from a potential point of exposure. Although patients were discharged after 2 consecutive negative PCR results, further assessment of the correlation of virus shedding with infectivity will be key for determining the risk for transmission during the viral RNA–positive phase. Given that the possibility of virus transmission between patients within the same cluster could not be ruled out and that the incubation period varies among individuals, uncertainties remain surrounding the estimated incubation period based on contact with patients. Further epidemiologic data are expected to improve the estimates of incubation period. We found limited community transmission of SARS-CoV-2 in Vietnam, and our data indicate that viremic travelers may pose a risk for introduction of virus strains that could potentially lead to outbreaks within a local community.

Dr. Le is deputy director at the National Institute of Hygiene and Epidemiology, Hanoi, Vietnam. Her research interest is the epidemiology of tropical and emerging infectious diseases. Dr. Takemura is assistant professor at the Institute of Tropical Medicine, Nagasaki University, Japan. His research interests include molecular epidemiologic studies on viral and bacterial diseases.



We thank healthcare practitioners of the clinics and hospitals in Vietnam who supported the study.

This research was supported by Vingroup Innovation Foundation, Vietnam (VINIF.2020.COVID-19.DA01), the Japan Agency for Medical Research and Development Japan Initiative for Global Research Network on Infectious Diseases (JP19fm010800), the Japan Agency for Medical Research and Development Research on Emerging and Re-emerging Infectious Diseases (19fk0108035j0003 and 19fk0108109h0001), and the Japan Society for the Promotion of Science KAKENHI (JP19K24679).



  1. World Health Organization. Statement on the second meeting of the International Health Regulations (2005) Emergency Committee regarding the outbreak of novel coronavirus (2019-nCoV) [cited 2020 Feb 18].
  2. Zhu  N, Zhang  D, Wang  W, Li  X, Yang  B, Song  J, et al.; China Novel Coronavirus Investigating and Research Team. A novel coronavirus from patients with pneumonia in China, 2019. N Engl J Med. 2020;382:72733. DOIPubMedGoogle Scholar
  3. Corman  VM, Landt  O, Kaiser  M, Molenkamp  R, Meijer  A, Chu  DKW, et al. Detection of 2019 novel coronavirus (2019-nCoV) by real-time RT-PCR. Euro Surveill. 2020;25:25. DOIPubMedGoogle Scholar
  4. Rothe  C, Schunk  M, Sothmann  P, Bretzel  G, Froeschl  G, Wallrauch  C, et al. Transmission of 2019-nCoV infection from an asymptomatic contact in Germany. N Engl J Med. 2020;382:9701. DOIPubMedGoogle Scholar
  5. Chen  N, Zhou  M, Dong  X, Qu  J, Gong  F, Han  Y, et al. Epidemiological and clinical characteristics of 99 cases of 2019 novel coronavirus pneumonia in Wuhan, China: a descriptive study. Lancet. 2020;395:50713. DOIPubMedGoogle Scholar
  6. Phan  LT, Nguyen  TV, Luong  QC, Nguyen  TV, Nguyen  HT, Le  HQ, et al. Importation and human-to-human transmission of a novel coronavirus in Vietnam. N Engl J Med. 2020;382:8724. DOIPubMedGoogle Scholar




Cite This Article

DOI: 10.3201/eid2607.200591

Original Publication Date: April 02, 2020

1These authors contributed equally to this article.

Table of Contents – Volume 26, Number 7—July 2020

EID Search Options
presentation_01 Advanced Article Search – Search articles by author and/or keyword.
presentation_01 Articles by Country Search – Search articles by the topic country.
presentation_01 Article Type Search – Search articles by article type and issue.



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

Futoshi Hasebe, WHO Collaborating Center for Reference and Research on Tropical and Emerging Virus Diseases, Institute of Tropical Medicine, Nagasaki University, 1-12-4 Sakamoto, Nagasaki 852-8523, Japan; ; Duc Anh Dang, National Institute of Hygiene and Epidemiology, 1 Yersin St, Hanoi 10000, Vietnam

Send To

10000 character(s) remaining.


Page created: April 02, 2020
Page updated: June 18, 2020
Page reviewed: June 18, 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.