Evidence of transmission from fully vaccinated individuals in a large outbreak of the SARS-CoV-2 Delta variant in Provincetown, Massachusetts
Authors: Siddle Katherine J. , Krasilnikova Lydia A. , Moreno Gage K. , Schaffner Stephen F. , Vostok Johanna , Fitzgerald Nicholas A. , Lemieux Jacob E. , Barkas Nikolaos , Loreth Christine , Specht Ivan , Tomkins-Tinch Christopher H. , Silbert Jillian , Schaeffer Beau , Taylor Bradford P. , Loftness Bryn , Johnson Hillary , Schubert Petra L. , Shephard Hanna M. , Doucette Matthew , Fink Timelia , Lang Andrew S. , Baez Stephanie , Beauchamp John , Hennigan Scott , Buzby Erika , Ash Stephanie , Brown Jessica , Clancy Selina , Cofsky Seana , Gagne Luc , Hall Joshua , Harrington Rachel , Gionet Gabrielle L. , DeRuff Katherine C. , Vodzak Megan E. , Adams Gordon C. , Dobbins Sabrina T. , Slack Sarah D. , Reilly Steven K. , Anderson Lisa M. , Cipicchio Michelle C. , DeFelice Matthew T. , Grimsby Jonna L. , Anderson Scott E. , Blumenstiel Brendan S. , Meldrim James C. , Rooke Heather M. , Vicente Gina , Smith Natasha L. , Messer Katelyn S. , Reagan Faye L. , Mandese Zoe M. , Lee Matthew D. , Ray Marianne C. , Fisher Marissa E. , Ulcena Maesha A. , Nolet Corey M. , English Sean E. , Larkin Katie L. , Vernest Kyle , Chaluvadi Sushma , Arvidson Deirdre , Melchiono Maurice , Covell Theresa , Harik Vaira , Brock-Fisher Taylor , Dunn Molly , Kearns Amanda , Hanage William P. , Bernard Clare , Philippakis Anthony , Lennon Niall J , Gabriel Stacey B. , Gallagher Glen R. , Smole Sandra , Madoff Lawrence C. , Brown Catherine M. , Park Daniel J. , MacInnis Bronwyn L. , Sabeti Pardis C. 
Published: October 2021

Multiple summer events, including large indoor gatherings, in Provincetown, Massachusetts (MA), in July 2021 contributed to an outbreak of over one thousand COVID-19 cases among residents and visitors. Most cases were fully vaccinated, many of whom were also symptomatic, prompting a comprehensive public health response, motivating changes to national masking recommendations, and raising questions about infection and transmission among vaccinated individuals. To characterize the outbreak and the viral population underlying it, we combined genomic and epidemiological data from 467 individuals, including 40% of known outbreak-associated cases. The Delta variant accounted for 99% of sequenced outbreak-associated cases. Phylogenetic analysis suggests over 40 sources of Delta in the dataset, with one responsible for a single cluster containing 83% of outbreak-associated genomes. This cluster was likely not the result of extensive spread at a single site, but rather transmission from a common source across multiple settings over a short time. Genomic and epidemiological data combined provide strong support for 25 transmission events from, including many between, fully vaccinated individuals; genomic data alone provides evidence for an additional 64. Together, genomic epidemiology provides a high-resolution picture of the Provincetown outbreak, revealing multiple cases of transmission of Delta from fully vaccinated individuals. However, despite its magnitude, the outbreak was restricted in its onward impact in MA and the US, likely due to high vaccination rates and a robust public health response.