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We found no significant difference in cycle threshold values between vaccinated and unvaccinated, asymptomatic and symptomatic groups infected with SARS-CoV-2 Delta. Given the substantial proportion of asymptomatic vaccine breakthrough cases with high viral levels, interventions, including masking and testing, should be considered for all in settings with elevated COVID-19 transmission.

Competing Interest Statement

Dr. DeRisi reports being a scientific advisor to the Public Health Co. and a scientific advisor to Allen & Co. Dr. Havlir reports non-financial support from Abbott outside of the submitted work. The other authors declare no competing interests.

Funding Statement

This work was supported by the Chan Zuckerberg Biohub, Healthy Yolo Together, the University of California, San Francisco, the Chan Zuckerberg Initiative, and The University of California, Davis.

Author Declarations

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.


The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

HYT: The Genome Center laboratory that conducted COVID-19 testing was CLIA approved as an extension to the Student Health Center laboratory. The UC Davis IRB Administration determined that the study met criteria for public health reporting and was exempt from IRB review and approval. UeS: The UC San Francisco Committee on Human Research determined the study met criteria for public health surveillance. All participants provided informed consent for testing.

All necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived.


I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).


I have followed all appropriate research reporting guidelines and uploaded the relevant EQUATOR Network research reporting checklist(s) and other pertinent material as supplementary files, if applicable.


Paper in collection COVID-19 SARS-CoV-2 preprints from medRxiv and bioRxiv

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