Pfizer recipients infected with delta have same viral load as the unvaccinated, Oxford study says

COVID-19 vaccines are less effective in preventing delta variant infection, according to a preprint study released Aug. 19 by the University of Oxford. The findings suggest that vaccinating large swaths of the population may not be helpful for protecting unvaccinated people.

The research team examined the COVID-19 vaccines produced by Pfizer and AstraZeneca, comparing the vaccines' protection from infection before and after May 17, when the delta variant became the most common virus strain in the U.K. Using nose and throat swabs, researchers analyzed 2.58 million test results from 384,543 adults between Dec. 1 and May 16, and 811,624 test results from 358,983 adults between May 17 and Aug. 1.

The study found that people who were infected with the delta variant after receiving two doses of Pfizer's or AstraZeneca's vaccine had similar peak virus levels to those in unvaccinated people. For fully vaccinated people infected with the alpha variant, peak virus levels were much lower than those in unvaccinated people.

Two doses of Pfizer's vaccine provide greater protection against infection than AstraZeneca's vaccine initially, but the protection offered by Pfizer's vaccine declined faster than AstraZeneca's vaccine, according to the study.

"We don’t yet know how much transmission can happen from people who get COVID-19 after being vaccinated — for example, they may have high levels of virus for shorter periods of time," Sarah Walker, PhD, one of the researchers, said in a statement.

There is currently not enough data to understand how the vaccines protect against severe COVID-19 and hospitalization over time.


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