new study from the University of California, Davis, Genome Center, UC San Francisco and the Chan Zuckerberg Biohub shows no significant difference in viral load between vaccinated and unvaccinated people who tested positive for the delta variant of SARS-CoV-2. It also found no significant difference between infected people with or without symptoms.
The findings underscore the continuing need for masking and regular testing alongside vaccination, especially in areas of high prevalence, the authors wrote. The study is currently available online as a preprint from MedRxiv.
“Our study adds to existing data about levels of virus in vaccine breakthroughs in two settings of high ongoing community prevalence of the delta variant,” said Professor Richard Michelmore, director of the UC Davis Genome Center.
The study was conducted with positive samples from asymptomatic testing at UC Davis for Healthy Yolo Together and at the Unidos en Salud walk-up testing site in the Mission District of San Francisco.
The researchers looked at 869 positive samples, 500 from Healthy Yolo Together and 369 from Unidos en Salud. All the Healthy Yolo Together samples were from people who were asymptomatic at the time of positive test result, and three-quarters were from unvaccinated individuals. The Unidos en Salud samples included both asymptomatic and symptomatic cases. Just over half (198) of the Unidos en Salud samples were unvaccinated.