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This phase 3, randomized, placebo-controlled study for AZD1222 (ChAdOx1 nCoV-19) primary-series vaccination enrolled 32,450 participants in the USA, Chile, and Peru between August 2020 and January 2021 (NCT04516746). Endpoints included the 2-year follow-up assessment of safety, efficacy, and immunogenicity. After 2 years, no emergent safety signals were observed for AZD1222, and no cases of thrombotic thrombocytopenia syndrome were reported. The assessment of anti-SARS-CoV-2 nucleocapsid antibody titers confirmed the durability of AZD1222 efficacy for up to 6 months, after which infection rates in the AZD1222 group increased over time. Despite this, all-cause and COVID-19-related mortality remained low through the study end, potentially reflecting the post-Omicron decoupling of SARS-CoV-2 infection rates and severe COVID-19 outcomes. Geometric mean titers were elevated for anti-SARS-CoV-2 neutralizing antibodies at the 1-year study visit and the anti-spike antibodies were elevated at year 2, providing further evidence of increasing SARS-CoV-2 infections over long-term follow-up. Overall, this 2-year follow-up of the AZD1222 phase 3 study confirms that the long-term safety profile remains consistent with previous findings and supports the continued need for COVID-19 booster vaccinations due to waning efficacy and humoral immunity.<\/jats:p>","DOI":"10.3390\/vaccines12080883","type":"journal-article","created":{"date-parts":[[2024,8,5]],"date-time":"2024-08-05T18:21:40Z","timestamp":1722882100000},"page":"883","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":3,"title":["Long-Term Safety and Immunogenicity of AZD1222 (ChAdOx1 nCoV-19): 2-Year Follow-Up from a Phase 3 Study"],"prefix":"10.3390","volume":"12","author":[{"given":"Kathryn","family":"Shoemaker","sequence":"first","affiliation":[{"name":"Biometrics, Vaccines & Immune Therapies, BioPharmaceuticals R&D, AstraZeneca, Gaithersburg, MD 20878, 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