"Abstract: We found no significant difference in cycle threshold values between vaccinated and unvaccinated persons infected with severe acute respiratory syndrome coronavirus 2 Delta, overall or stratified by symptoms...
Discussion: In our study, mean viral loads as measured by Ct value were similar for large numbers of vaccinated and unvaccinated individuals infected with SARS-CoV-2 during the Delta variant surge, regardless of symptom status, at two distinct California testing sites. These results are in contrast to a large ongoing UK community cohort, in which the median Ct value was higher for vaccinated individuals (27.6) than for unvaccinated individuals (23.1), but are consistent with multiple other reports from the United Kingdom, Singapore, and the United States. Our study contributes data from a large number of persons, including children, who were asymptomatic at the time of testing to inform this topic; most importantly, we demonstrate that 20% of positive, vaccinated individuals had low Ct values (<20), a third of whom were asymptomatic when tested.
Given that low Ct values are indicative of high levels of virus, culture positivity (including among vaccine breakthrough infections), and increased transmission, our detection of low Ct values in asymptomatic, fully vaccinated individuals is consistent with the potential for, although it does not prove, transmission from breakthrough infections before any emergence of symptoms."
© The Author(s) 2022. Published by Oxford University Press on behalf of Infectious Diseases Society of America. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs licence (https://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reproduction and distribution of the work, in any medium, provided the original work is not altered or transformed in any way, and that the work is properly cited.