August 28, 2021

Our preprint article “Transmission of SARS-CoV-2 Delta Variant Among Vaccinated Healthcare Workers, Vietnam”

It has come to our attention that a recent post from the Children’s Health Defense on 23rd August 2021 shared the false claim that our paper demonstrated “vaccinated individuals carry 251 times the load of COVID-19 viruses in their nostrils compared to the unvaccinated”.

SARS-CoV-2 Delta variant infection is associated with high viral loads, which has been demonstrated in recent studies from various countries (including China (ref#1), Singapore (ref#2), the UK (ref#3), and the US (ref#4&5)). The study from China showed that viral loads in people infected with the Delta variant were 1000 times higher than those infected with the 19A/19B strains detected in China in early 2020. Additionally, the studies from the US, the UK and Singapore demonstrated that vaccinated and unvaccinated individuals infected with SARS-CoV-2 Delta variant carried the same amounts of the virus in their respiratory tracts, with a faster viral clearance rate observed in vaccinated people (according to the study from Singapore).

In our study (, we compared viral loads (inferred from PCR Ct values) from cases infected with the original SARS-CoV-2 strains detected in Vietnam between March and April 2020 (herein referred to original strains) with those from fully vaccinated healthcare workers infected with SARS-CoV-2 Delta variant.

Similar to data from the aforementioned studies, we showed that Delta variant infections in fully vaccinated healthcare workers were associated with high viral loads, and indeed were 250 times higher than those in people infected with the original strains. The differences in viral load were driven by the ability of the Delta variant to cause higher viral loads; they had nothing to do with the vaccination status of the infected individual. Thus the claim that vaccinated individuals carry 251 times the loads of SARS-CoV-2 in their respiratory tract compared to the unvaccinated people is a misrepresentation of the data.

There is overwhelming evidence for the effectiveness of vaccines in preventing severe disease and death from COVID-19. Our study provides no evidence to the contrary. We strongly endorse vaccination as a critical tool against COVID-19 and the terrible consequences of the pandemic.

Nguyen Van Vinh Chau, MD, PhD – Director of Hospital for Tropical Diseases, Ho Chi Minh City

Professor Guy Thwaites, MD, PhD  – Director of OUCRU

Le Van Tan, PhD – Head of Emerging Infections Group, OUCRU


1. Viral infection and transmission in a large, well-traced outbreak caused by the SARS-CoV-2 Delta variant,

2. Virological and serological kinetics of SARS-CoV-2 Delta variant vaccine-breakthrough infections: a multi-center cohort study,

3. Impact of Delta on viral burden and vaccine effectiveness against new SARS-CoV-2 infections in the UK,

4. Shedding of Infectious SARS-CoV-2 Despite Vaccination,

5. Outbreak of SARS-CoV-2 Infections, Including COVID-19 Vaccine Breakthrough Infections, Associated with Large Public Gatherings — Barnstable County, Massachusetts, July 202,

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