SARS-CoV-2 variant B.1.617.2 (delta) is associated with higher viral loads  and increased transmissibility relative to other variants, as well as partial escape from polyclonal and monoclonal antibodies . The emergence of the delta variant has been associated with increasing case counts and test-positivity rates, indicative of rapid community spread. Since early July 2021, SARS-CoV-2 cases in the United States have increased coincident with delta SARS-CoV-2 becoming the predominant lineage nationwide .
Understanding how and why the virus is spreading in settings where there is high vaccine coverage has important public health implications. It is particularly important to assess whether vaccinated individuals who become infected can transmit SARS-CoV-2 to others. In Wisconsin, a large local contract laboratory provides SARS-CoV-2 testing for multiple local health departments, providing a single standard source of data using the same assay to measure virus burdens in test-positive cases. This includes providing high-volume testing in Dane County, a county with extremely high vaccine coverage. These PCR-based tests provide semi-quantitative information about the viral load, or amount of SARS-CoV-2 RNA, in respiratory specimens. Here we use this viral load data to compare the amount of SARS-CoV-2 present in test-positive specimens from people who self-report their vaccine status and date of final immunization, during a period in which the delta variant became the predominant circulating variant in Wisconsin.
We find no difference in viral loads when comparing unvaccinated individuals to those who have vaccine “breakthrough” infections. Furthermore, individuals with vaccine breakthrough infections frequently test positive with viral loads consistent with the ability to shed infectious viruses. Our results, while preliminary, suggest that if vaccinated individuals become infected with the delta variant, they may be sources of SARS-CoV-2 transmission to others.