Methods: We studied breakthrough infections among healthcare workers of a major infectious diseases hospital in Vietnam. We collected demographics, vaccination history and results of PCR diagnosis alongside clinical data. We measured SARS-CoV-2 (neutralizing) antibodies at diagnosis, and at week 1, 2 and 3 after diagnosis. We sequenced the viruses using ARTIC protocol.

Findings: Between 11th–25th June 2021 (week 7–8 after dose 2), 69 healthcare workers were tested positive for SARS-CoV-2. 62 participated in the clinical study. 49 were (pre)symptomatic with one requiring oxygen supplementation. All recovered uneventfully. 23 complete-genome sequences were obtained. They all belonged to the Delta variant, and were phylogenetically distinct from the contemporary Delta variant sequences obtained from community transmission cases, suggestive of ongoing transmission between the workers. Viral loads of breakthrough Delta variant infection cases were 251 times higher than those of cases infected with old strains detected between March-April 2020. Time from diagnosis to PCR negative was 8–33 days (median: 21). Neutralizing antibody levels after vaccination and at diagnosis of the cases were lower than those in the matched uninfected controls. There was no correlation between vaccine-induced neutralizing antibody levels and viral loads or the development of symptoms.

Interpretation: Breakthrough Delta variant infections are associated with high viral loads, prolonged PCR positivity, and low levels of vaccine-induced neutralizing antibodies, explaining the transmission between the vaccinated people. Physical distancing measures remain critical to reduce SARS-CoV-2 Delta variant transmission.

Funding: Wellcome (106680/B/14/Z and 204904/Z/16/Z).

Declaration of Interest: None to declare.

Ethical Approval: The study was approved by the Institutional Review Board of HTD and the Oxford Tropical Research Ethics Committee, University of Oxford, UK.

Keywords: Delta variant, Oxford-AstraZeneca, COVID-19, vaccine breakthrough, Vietnam

Suggested Citation:

Chau, Nguyen Van Vinh and Ngoc, Nghiem My and Nguyet, Lam Anh and Quang, Vo Minh and Ny, Nguyen Thi Han and Khoa, Dao Bach and Phong, Nguyen Thanh and Toan, Le Mau and Hong, Nguyen Thi Thu and Tuyen, Nguyen Thi Kim and Phat, Voong Vinh and Nhu, Le Nguyen Truc and Truc, Nguyen Huynh Thanh and That, Bui Thi Ton and Thao, Huynh Phuong and Thao, Tran Nguyen Phuong and Vuong, Vo Trong and Tam, Tran Thi Thanh and Tai, Ngo Tan and Bao, Ho The and Nhung, Huynh Thi Kim and Minh, Nguyen Thi Ngoc and Tien, Nguyen Thi My and Huy, Nguy Cam and Choisy, Marc and Man, Dinh Nguyen Huy and Ty, Dinh Thi Bich and Anh, Nguyen To and Uyen, Le Thi Tam and Tu, Tran Nguyen Hoang and Yen, Lam Minh and Dung, Nguyen Thanh and Hung, Le Manh and Truong, Nguyen Thanh and Thanh, Tran Tan and Thwaites, Guy and Tan, Le Van and Group, OUCRU COVID-19 Research, Transmission of SARS-CoV-2 Delta Variant Among Vaccinated Healthcare Workers, Vietnam. Available at

SSRN: https://ssrn.com/abstract=3897733 or http://dx.doi.org/10.2139/ssrn.3897733

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Monday, 23 August 2021

PROOF: The Vaccinated ARE the Super Spreaders

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