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Background: Breakthrough severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in coronavirus disease 2019 (COVID-19) vaccinees typically produces milder disease than infection in unvaccinated individuals., Methods: To explore disease attenuation, we examined COVID-19 symptom burden and immuno-virologic responses to symptomatic SARS-CoV-2 infection in participants (AZD1222: n=177/17,617; placebo: n=203/8,528) from a 2:1 randomized, placebo-controlled, phase 3 study of two-dose primary series AZD1222 (ChAdOx1 nCoV-19) vaccination (NCT04516746)., Results: We observed that AZD1222 vaccinees had an overall lower incidence and shorter duration of COVID-19 symptoms compared with placebo recipients, as well as lower SARS-CoV-2 viral loads and a shorter median duration of viral shedding in saliva. Vaccinees demonstrated a robust antibody recall response versus placebo recipients with low-to-moderate inverse correlations with virologic endpoints. Vaccinees also demonstrated an enriched polyfunctional spike-specific Th-1-biased CD4+ and CD8+ T-cell response that was associated with strong inverse correlations with virologic endpoints., Conclusion: Robust immune responses following AZD1222 vaccination attenuate COVID-19 disease severity and restrict SARS-CoV-2 transmission potential by reducing viral loads and the duration of viral shedding in saliva. Collectively, these analyses underscore the essential role of vaccination in mitigating the COVID-19 pandemic., Competing Interests: JM, BA, AAA, TE, KS, AMS, MP, PAS, DW, TV, JAG, and EJK are current employees of AstraZeneca and hold or may hold AstraZeneca stock. HB and NP are contractors to AstraZeneca via Bogier consulting. BJ is a contractor to AstraZeneca via Cytel. SS is a contractor to AstraZeneca via Joule/System One. ARF has received institutional grants for research from Pfizer, Merck, Sharpe and Dohme, Janssen, and BioFire Diagnostics and has received fees for serving on Novavax COVID-19 vaccine Data and Safety Monitoring Board. MES declares grants from the NIH and NIAID during the conduct of the study and institutional research grants from the Bill and Melinda Gates Foundation, Gilead Sciences, Janssen Global Services, LLC, Merck, and Sanofi Pasteur Inc. AFL has received institutional research grants from AstraZeneca and Gilead Sciences. GCP has received institutional research grants from AstraZeneca, Pfizer, and Moderna. SAR has received institutional grants from the NIH and NIAID from clinical trial enrolment and provides pro bono consultancy to Novimmune for novimab. MLR declares funding for consultancy from the Walter Reed Army Institute of Research and for serving on their behalf in Operation Warp Speed. C-PR has received institutional research grants from Gilead Sciences and ViiV Healthcare, honoraria for lectures from Gilead Sciences, ViiV Healthcare and Vindico CME, and sits on advisory boards for Janssen, Gilead Sciences, and ViiV Healthcare. BES has received research funding from the US government under the COVID-19 Prevention Network initiative, institutional research grants from Gilead Sciences, ViiV Healthcare, and the University of Chicago and payment or honoraria from MATEC. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. This study received funding from AstraZeneca. The funder had the following involvement with the study with input from the external authors: study design; collection, analysis and interpretation of data; the writing of this article and the decision to submit it for publication. The first draft of the manuscript was written under the direction of the authors by a medical writer funded by AstraZeneca., (Copyright © 2023 Maaske, Sproule, Falsey, Sobieszczyk, Luetkemeyer, Paulsen, Riddler, Robb, Rolle, Sha, Tong, Ahani, Aksyuk, Bansal, Egan, Jepson, Padilla, Patel, Shoemaker, Stanley, Swanson, Wilkins, Villafana, Green and Kelly.)

Severe COVID-19 can be associated with a prothrombotic state, increasing risk of morbidity and mortality. The SARS-CoV-2 spike glycoprotein is purported to directly promote platelet activation via the S1 subunit and is cleaved from host cells during infection. High plasma concentrations of S1 subunit are associated with disease progression and respiratory failure during severe COVID-19. There is limited evidence on whether COVID-19 vaccine-induced spike protein is similarly cleaved and on the immediate effects of vaccination on host immune responses or hematology parameters. We investigated vaccine-induced S1 subunit cleavage and effects on hematology parameters using AZD1222 (ChAdOx1 nCoV-19), a simian, replication-deficient adenovirus-vectored COVID-19 vaccine. We observed S1 subunit cleavage in vitro following AZD1222 transduction of HEK293x cells. S1 subunit cleavage also occurred in vivo and was detectable in sera 12 hours post intramuscular immunization (1x10 10 viral particles) in CD-1 mice. Soluble S1 protein levels decreased within 3 days and were no longer detectable 7-14 days post immunization. Intravenous immunization (1x10 9 viral particles) produced higher soluble S1 protein levels with similar expression kinetics. Spike protein was undetectable by immunohistochemistry 14 days post intramuscular immunization. Intramuscular immunization resulted in transiently lower platelet (12 hours) and white blood cell (12-24 hours) counts relative to vehicle. Similarly, intravenous immunization resulted in lower platelet (24-72 hours) and white blood cell (12-24 hours) counts, and increased neutrophil (2 hours) counts. The responses observed with either route of immunization represent transient hematologic changes and correspond to expected innate immune responses to adenoviral infection., Competing Interests: AD is an employee of LabCorp Early Development Laboratories Limited, a contract research organization that supported AstraZeneca with immunohistochemistry analyses during this investigation. All other authors report a relationship with AstraZeneca that includes employment and stock options. This study received funding from AstraZeneca. The funder had the following involvement with the study: study design; collection, analysis and interpretation of data; the writing of this article and the decision to submit it for publication., (Copyright © 2022 Stebbings, Jones, Cotton, Armour, Maguire, Skellett, Tang, Goodman, Brady, Takahashi, Daunt, Lapointe and Cohen.)