{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,2,19]],"date-time":"2026-02-19T07:13:32Z","timestamp":1771485212252,"version":"3.50.1"},"reference-count":33,"publisher":"Frontiers Media SA","license":[{"start":{"date-parts":[[2026,2,19]],"date-time":"2026-02-19T00:00:00Z","timestamp":1771459200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100016105","name":"Quzhou Municipal Science and Technology Bureau","doi-asserted-by":"publisher","id":[{"id":"10.13039\/501100016105","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":["frontiersin.org"],"crossmark-restriction":true},"short-container-title":["Front. Immunol."],"abstract":"<jats:sec>\n                    <jats:title>Background<\/jats:title>\n                    <jats:p>Data on antibody dynamics and protective threshold generated by infection with Omicron subvariants of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), particularly among populations that have primarily (&amp;gt;90%) received inactivated coronavirus disease 2019 (COVID-19) vaccines, remain limited. Using data from two large-scale paired serosurveys, we analyzed real-world changes in SARS-CoV-2 antibody levels associated with specific hybrid immunity.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Methods<\/jats:title>\n                    <jats:p>The history of COVID-19 vaccination and SARS-CoV-2 infection were recorded for each participant. Serum samples collected at three-month intervals were analyzed for antibody levels against nucleocapsid (N) and spike (S) proteins using chemiluminescent microparticle immunoassay (CMIA). Linear mixed-effects model (LMM) and restricted cubic spline analysis were applied to assess antibody dynamics and determine protection thresholds, respectively.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results<\/jats:title>\n                    <jats:p>A total of 4,065 participants were recruited in February and May 2023, with 2,894 completing both sampling rounds. Over three months, median IgG antibody levels declined by 28% (from 396.39 to 285.80 AU\/mL). Prior infection and vaccination were significantly associated with higher antibody levels, while increasing age correlated with an annual decay of 0.38 AU\/mL (most prominent in adults \u226560 years, 32.7% of whom fell below the protective threshold by follow-up). The IgG threshold for protection against XBB reinfection was 400 AU\/mL, and each additional vaccination reduced reinfection risk by 13\u201315%.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Conclusion<\/jats:title>\n                    <jats:p>The 400 AU\/mL IgG threshold provides a actionable quantitative guideline for prioritizing booster doses in Chinese adults aged \u226560 years (the fastest antibody decay subgroup) and high-risk groups. Routine antibody testing combined with this threshold could optimize targeted COVID-19 vaccination strategies in inactivated vaccine-predominant populations.<\/jats:p>\n                  <\/jats:sec>","DOI":"10.3389\/fimmu.2026.1768679","type":"journal-article","created":{"date-parts":[[2026,2,19]],"date-time":"2026-02-19T06:42:39Z","timestamp":1771483359000},"update-policy":"https:\/\/doi.org\/10.3389\/crossmark-policy","source":"Crossref","is-referenced-by-count":0,"title":["400 AU\/mL IgG protective threshold against SARS-CoV-2 XBB reinfection in Chinese inactivated vaccine recipients: implications for booster vaccination"],"prefix":"10.3389","volume":"17","author":[{"given":"Zhiying","family":"Yin","sequence":"first","affiliation":[{"name":"Department of Immunity, Quzhou Center for Disease Control and Prevention","place":["Quzhou, Zhejiang, 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