Fact Check-Study did not find vaccinated healthcare workers carry 251 times the viral load of those who were unvaccinated

Social media users are sharing articles that discuss a study examining the transmission of the Delta variant of COVID-19 and falsely claiming the study found that fully vaccinated healthcare workers carry 251 times the viral load compared of those who are unvaccinated.

Examples can be seen here and here .

Many of the posts share an article ( posted by the anti-vaccine campaign group Children’s Health Defense on Aug. 23, 2021 with the headline: “Study: Fully Vaccinated Healthcare Workers Carry 251 Times Viral Load, Pose Threat to Unvaccinated Patients, Co-Workers.” The subheading says the study found that “vaccinated individuals carry 251 times the load of COVID-19 viruses in their nostrils compared to the unvaccinated.”

This, however, was not the premise of the study. Rather, it sought to compare viral loads among workers with Delta variant vs previous variants of SARS-CoV-2.

The study discussed in the article is visible here as a preprint, which is an early-stage research paper that has not yet been peer-reviewed.

The paper examines breakthrough infections in healthcare workers in Vietnam, all of whom contracted the Delta variant after receiving the Oxford-AstraZeneca COVID-19 vaccine.

The paper does not conclude that fully vaccinated healthcare workers carry 251 times the viral load of the virus compared to unvaccinated healthcare workers. Rather, it concludes that “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.”

It adds that it found no correlation between vaccine-induced neutralizing antibody levels, and viral loads or developing symptoms.

On page 2 of the paper, the authors say that breakthrough infections of the Delta variant “are associated with high viral loads, prolonged PCR positivity, and low levels of vaccine-induced neutralizing antibodies,” which helps explain the transmission between those who are vaccinated.

Chi Ngo, senior communications officer for the Centre for Tropical Medicine at the Oxford University Clinical Research Unit, told Reuters via email that it does not support any statement shared by the Children’s Health Defense website.

“We support vaccination and believe that vaccination is highly effective against hospitalisation and death from COVID-19,” Ngo said. “This article states that with the Delta variant, safety measures (mask-wearing, hand-washing, social distancing) are still advised to reduce transmission (even if you have been fully vaccinated).”

The Centers for Disease Control and Prevention (CDC) explains on its website here that the Delta variant is more than twice as contagious as previous variants and that vaccinated individuals with breakthrough infections of the variant appear to produce the same amount of virus load as those who are unvaccinated.

The viral load does decrease faster in vaccinated individuals compared to unvaccinated individual, meaning that the vaccinated are thought to be contagious for a shorter period of time, according to the CDC.

“This study is about the Delta variant and explaining ‘breakthrough cases’ among vaccinated healthcare workers because of the Delta variant,” Ngo said. “There is no focus on unvaccinated versus the vaccinated cases.”

Reuters emailed the Children’s Health Defense for comment but has not yet heard back.


Misleading. The study found that viral loads of breakthrough COVID-19 cases of the Delta variant were 251 higher compared to cases with other variants detected between March and April 2020. It did not focus on vaccinated vs unvaccinated cases.

This article was produced by the Reuters Fact Check team. Read more about our fact-checking work here .