Evidence of transmission from fully vaccinated individuals in a large outbreak of the SARS-CoV-2 Delta variant in Provincetown, Massachusetts

Abstract

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.

Updated vaccine protects from infection with SARS-CoV-2 variants, prevents transmission and is immunogenic against Omicron in hamsters

“…To our knowledge, this constitutes the first experimental evidence for full protection from SARS-CoV-2 transmission by any vaccine. The block conferred by S0* appears to be more complete than that observed in humans by current vaccines (41).…”

Transmission of SARS-CoV-2 during indoor clubbing events: A clustered randomized, controlled, multicentre trial protocol

“…A recent prospective cohort study in the UK found a modest protective effect on COVID-19 transmission (18): The secondary attack rate (SAR) in household contacts exposed to the delta variant was 25 and 38% in vaccinated and unvaccinated contacts, respectively. SAR among household contacts exposed to fully vaccinated index cases (25%; 95% CI 15-35) was similar to household contacts exposed to unvaccinated index cases (23%; [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31].…”

One health genomic surveillance and response to a university-based outbreak of the SARS-CoV-2 Delta AY.25 lineage, Arizona, 2021

“…While hundreds of Delta variant sub-lineages were characterized throughout 2021, AY.25 was not a dominant sub-lineage across the U.S.; however, in-depth phylogenetic analysis of outbreak cases associated with large public summer gatherings in Massachusetts revealed that AY.25 was responsible for a single cluster containing 84% of outbreak-associated genomes [ 33 ]. A second analysis of Delta variant associated vaccine breakthrough infections in New York identified a small cluster of AY.25 with a signature mutation in the spike protein, S112L [ 7 ].…”