SARS-CoV-2 variant Delta rapidly displaced variant Alpha in the United States and led to higher viral loads

Alexandre Bolze, Shishi Luo, Simon White, Elizabeth T. Cirulli, Dana Wyman, Andrew Dei Rossi, Henrique Machado, Tyler Cassens, Sharoni Jacobs, Kelly M. Schiabor Barrett, Francisco Tanudjaja, Kevin Tsan, Jason Nguyen, Jimmy M. Ramirez, Efren Sandoval, Xueqing Wang, David Wong, David Becker, Marc Laurent, James T. LuMagnus Isaksson, Nicole L. Washington, William Lee

Research output: Contribution to journalArticlepeer-review

40 Scopus citations


We report on the sequencing of 74,348 SARS-CoV-2 positive samples collected across the United States and show that the Delta variant, first detected in the United States in March 2021, made up the majority of SARS-CoV-2 infections by July 1, 2021 and accounted for >99.9% of the infections by September 2021. Not only did Delta displace variant Alpha, which was the dominant variant at the time, it also displaced the Gamma, Iota, and Mu variants. Through an analysis of quantification cycle (Cq) values, we demonstrate that Delta infections tend to have a 1.7× higher viral load compared to Alpha infections (a decrease of 0.8 Cq) on average. Our results are consistent with the hypothesis that the increased transmissibility of the Delta variant could be due to the ability of the Delta variant to establish a higher viral load earlier in the infection as compared to the Alpha variant.

Original languageEnglish
Article number100564
JournalCell Reports Medicine
Issue number3
StatePublished - 15 Mar 2022
Externally publishedYes


  • Alpha
  • COVID-19
  • Delta
  • Gamma
  • Iota
  • Mu
  • SARS-CoV-2
  • surveillance
  • transmissibility
  • viral load


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