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Background: AZD2816 is a variant-adapted COVID-19 vaccine that expresses the full-length SARS-CoV-2 beta variant spike protein but is otherwise similar to AZD1222 (ChAdOx1 nCoV-19). This study aimed to evaluate the safety and immunogenicity of AZD1222 or AZD2816 (or both) primary-series vaccination in a cohort of adult participants who were previously unvaccinated., Methods: In this phase 2/3, randomised, multinational, active-controlled, non-inferiority, immunobridging study, adult participants previously unvaccinated for COVID-19 were enrolled at 16 study sites in Brazil, South Africa, Poland, and the UK. Participants were stratified by age, sex, and comorbidity and randomly assigned 5:5:5:2 to receive a primary series of AZD1222 (AZD1222 group), AZD2816 (AZD2816 [4-week] group), or AZD1222-AZD2816 (AZD1222-AZD2816 group) at 4-week dosing intervals, or AZD2816 at a 12-week interval (AZD2816 [12-week] group) and evaluated for safety and immunogenicity through 180 days after dose 2. Primary outcomes were safety (rates of solicited adverse events occurring during 7 days and unsolicited adverse events occurring during 28 days after each dose) and immunogenicity (non-inferiority of pseudovirus neutralising antibody geometric mean titre [GMT], GMT ratio margin of 0·67, and seroresponse rate, rate difference margin of -10%, recorded 28 days after dose 2 with AZD2816 [4-week interval] against beta vs AZD1222 against ancestral SARS-CoV-2) in participants who were seronegative at baseline. This trial is registered with ClinicalTrials.gov, NCT04973449, and is completed., Findings: Between July 7 and Nov 12, 2021, 1449 participants were assigned to the AZD1222 group (n=413), the AZD2816 (4-week) group (n=415), the AZD1222-AZD2816 group (n=412), and the AZD2816 (12-week) group (n=209). Ten (2·6%) of 378 participants who were seronegative at baseline in the AZD1222 group, nine (2·4%) of 379 in the AZD2816 (4-week) group, eight (2·1%) of 380 in the AZD1222-AZD2816 group, and 11 (5·8%) of 191 in the AZD2816 (12-week) group had vaccine-related unsolicited adverse events. Serious adverse events were recorded in one (0·3%) participant in the AZD1222 group, one (0·3%) in the AZD2816 (4-week) group, two (0·5%) in the AZD1222-AZD2816 group, and none in the AZD2816 (12-week) group. Co-primary immunogenicity endpoints were met: neutralising antibody GMT (ratio 1·19 [95% CI 1·08-1·32]; lower bound greater than 0·67) and seroresponse rate (difference 1·7% [-3·1 to 6·5]; lower bound greater than -10%) at 28 days after dose 2 were non-inferior in the AZD2816 (4-week) group against beta versus in the AZD1222 group against ancestral SARS-CoV-2. Seroresponse rates were highest with AZD2816 against beta (12-week interval 94·3% [95% CI 89·4-97·3]; 4-week interval 85·7% [81·5-89·2]) and with AZD1222 (84·6% [80·3-88·2]) against ancestral SARS-CoV-2., Interpretation: Primary series of AZD1222 and AZD2816 were well tolerated, with no emergent safety concerns. Both vaccines elicited robust immunogenicity against beta and ancestral SARS-CoV-2 with greater responses demonstrated when testing against SARS-CoV-2 strains that matched those targeted by the respective vaccine. These findings demonstrate the continued importance of ancestral COVID-19 vaccines in protecting against severe COVID-19 and highlight the feasibility of using the ChAdOx1 platform to develop COVID-19 vaccines against future SARS-CoV-2 variants., Funding: AstraZeneca., Competing Interests: Declaration of interests SACC declares participation in the clinical development, with University of Oxford, of the vaccines described in this Article, membership on CureVac and Clover advisory boards, and institutional research grants from AstraZeneca. BJ is an employee of Cytel and is currently on assignment to AstraZeneca. QEB declares institutional funding, the provision of study materials, or both from the Wits Health Consortium, the Bill & Melinda Gates Foundation, the South African Medical Research Council, and AstraZeneca, as well as research grants or contracts from Regeneron, Novo Nordisk, Avillion, GlaxoSmithKline, Novavax, Sinovac, Johnson & Johnson, and Pfizer pharmaceuticals. PKA declares institutional grants to support the conduct of the study from AstraZeneca and the UK Vaccine Taskforce via the UK National Institute for Health Research (NIHR). HB is an employee of Bogier consulting and is currently on assignment to AstraZeneca. EJK is an employee of Sanofi. SM is an employee of Exploristics and is currently on assignment to AstraZeneca. EPM declares no competing interests. TL declares consulting fees from Vaccitech on an unrelated project, an honorarium from Seqirus on an unrelated project, an institutional grant from the UK Vaccine Taskforce via the NIHR, work-related investments, and is a named inventor on a patent application for a vaccine against SARS-CoV-2. JJL declares no competing interests. MNR declares institutional support for the study from AstraZeneca. AJP is Chair of the UK Department of Health and Social Care’s Joint Committee on Vaccination and Immunisation (JCVI) but does not participate in the JCVI COVID-19 committee. AJP also declares membership of the WHO’s Strategic Advisory Group of Experts on Immunization until January, 2022; provision of advice to Shionogi on COVID-19; and funding from the NIHR, AstraZeneca, the Bill & Melinda Gates Foundation, Wellcome, the UK Research and Innovation Medical Research Council, the Serum Institute of India, and the Coalition for Epidemic Preparedness Innovations. University of Oxford has entered a partnership with AstraZeneca for the development of COVID-19 vaccines; TL, MNR, SB, PKA, and AJP are contributors to the intellectual property licensed by Oxford University Innovation to AstraZeneca. AA, TA, EJK, MK, UO, SS, KS, AS, TV, and JAG are, or were, employees of and may hold (or have held) stock or stock options in AstraZeneca. MSMdOP declares no competing interests., (Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

AZD1222 (ChAdOx1 nCoV-19) is a replication-deficient adenoviral vectored coronavirus disease-19 (COVID-19) vaccine that is manufactured as SII-ChAdOx1 nCoV-19 by the Serum Institute of India Pvt Ltd following technology transfer from Oxford University/AstraZeneca. The non-inferiority of SII-ChAdOx1 nCoV-19 with AZD1222 was previously demonstrated in an observer-blind, phase 2/3 immuno-bridging study (trial registration: CTRI/2020/08/027170). In this analysis of immunogenicity and safety data 6 months post first vaccination (Day 180), 1,601 participants were randomized 3:1 to SII-ChAdOx1 nCoV-19 or AZD1222 (immunogenicity/reactogenicity cohort n  = 401) and 3:1 to SII-ChAdOx1 nCoV-19 or placebo (safety cohort n  = 1,200). Immunogenicity was measured by anti-severe acute respiratory syndrome coronavirus 2 spike (anti-S) binding immunoglobulin G and neutralizing antibody (nAb) titers. A decline in anti-S titers was observed in both vaccine groups, albeit with a greater decline in SII-ChAdOx1 nCoV-19 vaccinees (geometric mean titer [GMT] ratio [95% confidence interval (CI) of SII-ChAdOx1 nCoV-19 to AZD1222]: 0.60 [0.41-0.87]). Consistent similar decreases in nAb titers were observed between vaccine groups (GMT ratio [95% CI]: 0.88 [0.44-1.73]). No cases of severe COVID-19 were reported following vaccination, while one case was observed in the placebo group. No causally related serious adverse events were reported through 180 days. No thromboembolic or autoimmune adverse events of special interest were reported. Collectively, these data illustrate that SII-ChAdOx1 nCoV-19 maintained a high level of immunogenicity 6 months post-vaccination. SII-ChAdOx1 nCoV-19 was safe and well tolerated.

Background: Immunity to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is now widespread; however, the degree of cross-immunity between SARS-CoV-2 and endemic, seasonal human coronaviruses (HCoVs) remains unclear., Methods: SARS-CoV-2 and HCoV cross-immunity was evaluated in adult participants enrolled in a US sub-study in the phase III, randomized controlled trial (NCT04516746) of AZD1222 (ChAdOx1 nCoV-19) primary-series vaccination for one-year. Anti-HCoV spike-binding antibodies against HCoV-229E, HCoV-HKU1, HCoV-OC43, and HCoV-NL63 were evaluated in participants following study dosing and, in the AZD1222 group, after a non-study third-dose booster. Timing of SARS-CoV-2 seroconversion (assessed via anti-nucleocapsid antibody levels) and incidence of COVID-19 were evaluated in those who received AZD1222 primary-series by baseline anti-HCoV titers., Results: We evaluated 2,020/21,634 participants in the AZD1222 group and 1,007/10,816 in the placebo group. At the one-year data cutoff (March 11, 2022) mean duration of follow up was 230.9 (SD: 106.36, range: 1-325) and 94.3 (74.12, 1-321) days for participants in the AZD1222 (n = 1,940) and placebo (n = 962) groups, respectively. We observed little elevation in anti-HCoV humoral titers post study-dosing or post-boosting, nor evidence of waning over time. The occurrence and timing of SARS-CoV-2 seroconversion and incidence of COVID-19 were not largely impacted by baseline anti-HCoV titers., Conclusion: We found limited evidence for cross-immunity between SARS-CoV-2 and HCoVs following AZD1222 primary series and booster vaccination. Susceptibility to future emergence of novel coronaviruses will likely persist despite a high prevalence of SARS-CoV-2 immunity in global populations., Competing Interests: Authors AA, AS, DW, DL, EK, JG, and KS are/were employees of AstraZeneca and may hold AstraZeneca stock. AA declares patents in connection with Meso Scale Diagnostics. AF reports receiving institutional grants for research from Pfizer, Merck, Sharpe and Dohme, Janssen CyanVac, Moderna VaxCo, and BioFire Diagnostics, fees for serving on Novavax COVID-19 vaccine Safety Monitoring Board, travel fees from GlaxoSmithKline and Moderna, serving on a board of directors for ADMA biologics and consulting/personal fees from ADMA Biologics, GFS, and Sanofi Pasteur. DL is an employee of Cytel Cambridge, MA, USA. EK is an employee of Sanofi Swiftwater, PA, USA. H-VT declares grants from the NIH and NIAID during the study and grants from Shionogi and GlaxoSmithKline. MS 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, Merck, and Sanofi Pasteur, as well as a travel grant from the Infectious Diseases Society of America. 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 © 2024 Stanley, Aksyuk, Wilkins, Green, Lan, Shoemaker, Tieu, Sobieszczyk, Falsey and Kelly.)

Background: This study aimed to evaluate AZD2816, a variant-updated COVID-19 vaccine expressing the full-length SARS-CoV-2 beta (B.1.351) variant spike protein that is otherwise similar to AZD1222 (ChAdOx1 nCoV-19), and AZD1222 as third-dose boosters., Methods: This phase 2/3, partly double-blinded, randomised, active-controlled study was done at 19 sites in the UK and four in Poland. Adult participants who had received a two-dose AZD1222 or mRNA vaccine primary series were randomly assigned by means of an Interactive Response Technology-Randomisation and Trial Supply Management system (1:1 within each primary-series cohort, stratified by age, sex, and comorbidities) to receive AZD1222 or AZD2816 (intramuscular injection; 5 × 10 10 viral particles). Participants, investigators, and all sponsor staff members involved in study conduct were masked to randomisation. AZD1222 and AZD2816 doses were prepared by unmasked study staff members. The primary objectives were to evaluate safety and humoral immunogenicity (non-inferiority of day-29 pseudovirus neutralising antibody geometric mean titre [GMT] against ancestral SARS-CoV-2: AZD1222 booster vs AZD1222 primary series [historical controls]; margin 0·67; SARS-CoV-2-seronegative participants). This study is registered with ClinicalTrials.gov, NCT04973449, and is completed., Findings: Between June 27 and Sept 30, 2021, 1394 participants of the 1741 screened were randomly assigned to AZD1222 or AZD2816 following an AZD1222 (n=373, n=377) or mRNA vaccine (n=322, n=322) primary series. In SARS-CoV-2-seronegative participants receiving AZD1222 or AZD2816, 78% and 80% (AZD1222 primary series) and 90% and 93%, respectively (mRNA vaccine primary series) reported solicited adverse events to the end of day 8; 2%, 2%, 1%, and 1% had serious adverse events and 12%, 12%, 10%, and 11% had adverse events of special interest, respectively, to the end of day 180. The primary immunogenicity non-inferiority endpoint was met: day-29 neutralising antibody GMT ratios (ancestral SARS-CoV-2) were 1·02 (95% CI 0·90-1·14) and 3·47 (3·09-3·89) with AZD1222 booster versus historical controls (AZD1222 and mRNA vaccine primary series, respectively). Responses against beta were greater with AZD2816 versus AZD1222 (GMT ratios, AZD1222, mRNA vaccine primary series 1·84 [1·63-2·08], 2·22 [1·99-2·47])., Interpretation: Both boosters were well tolerated, with immunogenicity against ancestral SARS-CoV-2 similar to AZD1222 primary-series vaccination. AZD2816 gave greater immune responses against beta versus AZD1222., Funding: AstraZeneca., Competing Interests: Declaration of interests MNR declares institutional support for the study from AstraZeneca. EJK, SS, AC, MK, GTT, JV, PAS, MNP, TV, NM-A, and JAG are, or were, employees of and may hold (or have held) stock or stock options in AstraZeneca. PID declares institutional support for the study from AstraZeneca, and institutional grants from AstraZeneca, Janssen, Moderna, and Atea. RP declares institutional support for the study from AstraZeneca. MA declares support for the study from AstraZeneca and IQVIA to the Clinical Infection Research Group. PKA declares institutional grants to support the conduct of the study from AstraZeneca and the UK Vaccine Taskforce via National Institute for Health Research (NIHR). BJ is an employee of Cytel and is currently on assignment to AstraZeneca. SM is an employee of Exploristics and is currently on assignment to AstraZeneca. TL reports consulting fees from Vaccitech on an unrelated project, an honorarium from Seqirus, grant support from the Vaccine Taskforce for this trial, work-related investments, and is named as an inventor on a patent application for a vaccine against SARS-CoV-2. AJP was a member of WHO's Strategic Advisory Group of Experts on Immunization until January, 2022 and remains chair of the UK Department of Health and Social Care's Joint Committee on Vaccination and Immunisation (JCVI) but does not participate in the JCVI COVID-19 committee; and reports providing advice to Shionogi on COVID-19, and funding from the NIHR, AstraZeneca, the Bill & Melinda Gates Foundation, Wellcome, the Medical Research Council, and the Coalition for Epidemic Preparedness Innovations. Oxford University has entered into a partnership with AstraZeneca for the development of COVID-19 vaccines. VL and SB declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 license. Published by Elsevier Ltd.. All rights reserved.)

Background: Long duration trial data for two-dose COVID-19 vaccines primary series' are uncommon due to unblinding and additional doses. We report one-year follow-up results from a phase 1/2 trial of AZD1222 (ChAdOx1 nCoV-19) in Japan., Methods: Adults (n = 256) seronegative for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) were stratified by age, 18-55 (n = 128), 56-69 (n = 86) and ≥70-year-old (n = 42), and randomized 3:1 to AZD1222 or placebo. Safety, immunogenicity, and exploratory efficacy data were collected until study Day 365., Results: Safety was consistent with previous reports. In AZD1222 vaccinees, humoral responses against SARS-CoV-2 steadily declined over time. By Day 365, anti-SARS-CoV-2 spike-binding (spike) and receptor-binding domain (RBD) mean antibody titers remained above Day 15 levels and pseudovirus neutralizing antibodies were undetectable in many participants., Conclusions: AZD1222 is immunogenic and well tolerated in Japanese adults. Expected waning in anti-SARS-CoV-2 humoral responses was observed; spike and RBD antibody titers remained elevated. (ClinicalTrials.gov: NCT04568031)., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Justin Green reports financial support for the study was provided by AstraZeneca K.K. Justin Green reports a relationship with AstraZeneca that includes: employment and may hold equity or stocks. Kensuke Ishikawa reports a relationship with AstraZeneca that includes: employment and may hold equity or stocks. Maria-Claudia Nascimento reports a relationship with AstraZeneca that includes: employment and may hold equity or stocks. Michiko Asano reports a relationship with AstraZeneca that includes: employment and may hold equity or stocks. Yohji Itoh reports a relationship with AstraZeneca that includes: employment and may hold equity or stocks. Elizabeth J. Kelly reports a relationship with AstraZeneca that includes: employment and may hold equity or stocks. Akiko Matsui reports a relationship with AstraZeneca that includes: employment and may hold equity or stocks. Urban Olsson reports a relationship with AstraZeneca that includes: employment and may hold equity or stocks. Kathryn Shoemaker reports a relationship with AstraZeneca that includes: employment and may hold equity or stocks., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

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.)

Background: Immunogenicity and safety of the AZD1222 (ChAdOx1 nCoV-19) vaccine was evaluated in Japanese adults in an ongoing phase 1/2, randomized, double-blind, parallel-group, placebo-controlled, multi-centre trial (NCT04568031)., Methods: Adults (n=256, age ≥18 years) seronegative for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) were stratified by age into 18-55- (n=128), 56-69- (n=86) and ≥70-year-old cohorts (n=42), and randomized 3:1 to receive AZD1222 or placebo (two intramuscular injections 4 weeks apart). Immunogenicity and safety were coprimary endpoints. Data collected up to Day 57 are reported., Results: Positive seroresponses to SARS-CoV-2 spike and receptor-binding domain antigens were seen in all 174 participants who received two doses of AZD1222. Neutralizing antibody seroresponses were seen in 67.5%, 60.3% and 50.0% of participants receiving AZD1222 aged 18-55, 56-69 and ≥70 years, respectively. Solicited adverse events (AEs) were typically mild/moderate in severity and included pain and tenderness at the injection site, malaise, fatigue, muscle pain and headache. Common unsolicited AEs included pain and tenderness at the injection site, fatigue and elevated body temperature. No vaccine-related serious AEs or deaths were reported., Conclusions: AZD1222 elicited a strong humoral immune response against SARS-CoV-2, and was well tolerated in Japanese participants, including elderly participants., Competing Interests: Conflict of interest statement JV was an employee and stockholder of AstraZeneca at the time of the study. All other authors are employees of, and hold or may hold stock in, AstraZeneca., (Copyright © 2021 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Background: The safety and efficacy of the AZD1222 (ChAdOx1 nCoV-19) vaccine in a large, diverse population at increased risk for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in the United States, Chile, and Peru has not been known., Methods: In this ongoing, double-blind, randomized, placebo-controlled, phase 3 clinical trial, we investigated the safety, vaccine efficacy, and immunogenicity of two doses of AZD1222 as compared with placebo in preventing the onset of symptomatic and severe coronavirus disease 2019 (Covid-19) 15 days or more after the second dose in adults, including older adults, in the United States, Chile, and Peru., Results: A total of 32,451 participants underwent randomization, in a 2:1 ratio, to receive AZD1222 (21,635 participants) or placebo (10,816 participants). AZD1222 was safe, with low incidences of serious and medically attended adverse events and adverse events of special interest; the incidences were similar to those observed in the placebo group. Solicited local and systemic reactions were generally mild or moderate in both groups. Overall estimated vaccine efficacy was 74.0% (95% confidence interval [CI], 65.3 to 80.5; P<0.001) and estimated vaccine efficacy was 83.5% (95% CI, 54.2 to 94.1) in participants 65 years of age or older. High vaccine efficacy was consistent across a range of demographic subgroups. In the fully vaccinated analysis subgroup, no severe or critical symptomatic Covid-19 cases were observed among the 17,662 participants in the AZD1222 group; 8 cases were noted among the 8550 participants in the placebo group (<0.1%). The estimated vaccine efficacy for preventing SARS-CoV-2 infection (nucleocapsid antibody seroconversion) was 64.3% (95% CI, 56.1 to 71.0; P<0.001). SARS-CoV-2 spike protein binding and neutralizing antibodies increased after the first dose and increased further when measured 28 days after the second dose., Conclusions: AZD1222 was safe and efficacious in preventing symptomatic and severe Covid-19 across diverse populations that included older adults. (Funded by AstraZeneca and others; ClinicalTrials.gov number, NCT04516746.)., (Copyright © 2021 Massachusetts Medical Society.)