Background: Vaccine-induced clinical protection against SARS CoV-2 variants is an evolving target. There is limited genomic level data on SARS CoV-2 breakthrough infections and vaccine effectiveness (VE) since the global spread of the B.1.617.2 (Delta) variant.
Methods: In a retrospective study from November 1st, 2020 - August 31st , 2021, divided as pre-Delta and Delta-dominant periods, laboratory-confirmed SARS CoV-2 infections among Healthcare personnel (HCP) at a large tertiary cancer center in New York City (NYC) were examined to compare the weekly infection rate-ratio in vaccinated, partially vaccinated, and unvaccinated HCP. We describe the clinical and genomic epidemiologic features of post-vaccine infections to assess for selection of VOC/VOI in the early post-vaccine period and impact of B.1.617.2 (Delta) variant domination on VE.
Results: Among 13,658 HCP in our cohort, 12,379 received at least one dose of an mRNA vaccine. In the pre-Delta period overall VE was 94.5%. WGS of 369 isolates in the pre-Delta period did not reveal a clade bias for VOC/VOI specific to post-vaccine infections. VE in the Delta dominant phase was 75.6%. No hospitalizations occurred among vaccinated HCP in the entire study period, compared to 17 hospitalizations and one death among unvaccinated HCP.
Conclusions: Findings show high VE among HCP in NYC in the pre-Delta phase, with moderate decline in VE post-Delta emergence. SARS CoV-2 clades were similarly distributed among vaccinated and unvaccinated infected HCP without apparent clustering during the pre-Delta period of diverse clade circulation. Strong vaccine protection against hospitalization was maintained through the entire study period.
Keywords: SARS-CoV-2; breakthrough infections; vaccine effectiveness.
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