{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,22]],"date-time":"2026-04-22T06:47:50Z","timestamp":1776840470546,"version":"3.51.2"},"reference-count":43,"publisher":"Frontiers Media SA","license":[{"start":{"date-parts":[[2026,4,22]],"date-time":"2026-04-22T00:00:00Z","timestamp":1776816000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":["frontiersin.org"],"crossmark-restriction":true},"short-container-title":["Front. Immunol."],"abstract":"<jats:p>Despite the role of booster doses in sustaining protection against emerging SARS-CoV-2 variants, global uptake remains low, highlighting the need for dose-sparing strategies that maintain durable immunity. We conducted a randomized controlled trial in Mongolian adults to examine long-term cellular immune responses to standard and fractional doses of BNT162b2 given as a third dose. A total of 601 participants, primed with ChAdOx1-S, BBIBP-CorV, or Gam-COVID-Vac, were randomized (1:1) to receive 15 \u03bcg (fractional dose) or 30 \u03bcg (standard dose) of BNT162b2. A subset of participants (N\u00a0=\u00a0256) were enrolled for cell-mediated immunity analysis (n = 101 ChAdOx1-S primed, n = 117 BBIBP-CorV primed, n = 38 Gam-COVID-Vac primed). Antibodies were measured for binding anti-Spike IgG and neutralizing antibodies, and T-cell responses were measured using activation-induced marker (AIM), intracellular cytokine staining (ICS), and IFN-\u03b3 against SARS-CoV-2 Spike-specific wild-type and JN.1, and followed-up for 12 months. At 12 months post-vaccination, wild-type and JN.1 IgG levels were sustained and remained approximately 1.7\u20132.7-fold higher than baseline levels, and neutralizing antibodies were maintained (89% inhibition) for each priming stratum and standard and fractional dose groups. Across all study visits, total AIM CD4mem (expressing CD69+, OX40+, or CD137+) and CD8mem (CD69+CD137+), total ICS CD4mem and CD8mem (IL-2+, TNF-\u03b1+, or IFN-\u03b3+), and ELISpot IFN-\u03b3 remained similar by study arm and priming strata for wild-type and JN.1 responses. CD4mem AIM responses peaked 6 months post-vaccination; by 12 months, responses to wild-type were maintained, whereas JN.1 responses had declined to day 28 levels or lower. For memory T-cell ICS responses, durability was maintained over 12 months post-vaccination for both wild-type and JN.1 responses. Overall, fractional booster dosing produced comparable and robust long-term humoral and cellular immune responses to a standard dose, including against the JN.1 variant.<\/jats:p>","DOI":"10.3389\/fimmu.2026.1779435","type":"journal-article","created":{"date-parts":[[2026,4,22]],"date-time":"2026-04-22T05:49:52Z","timestamp":1776836992000},"update-policy":"https:\/\/doi.org\/10.3389\/crossmark-policy","source":"Crossref","is-referenced-by-count":0,"title":["Cellular immune responses 12 months after fractional or standard dose BNT162b2 booster vaccination in Mongolian adults"],"prefix":"10.3389","volume":"17","author":[{"given":"Nadia","family":"Mazarakis","sequence":"first","affiliation":[{"name":"Infection, Immunity, and Global Health, Murdoch Children\u2019s Research Institute","place":["Melbourne, VIC, Australia"]},{"name":"Department of Paediatrics, The University of Melbourne","place":["Parkville, VIC, Australia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Zheng Quan","family":"Toh","sequence":"additional","affiliation":[{"name":"Infection, Immunity, and Global Health, Murdoch Children\u2019s Research Institute","place":["Melbourne, VIC, Australia"]},{"name":"Department of Paediatrics, The University of Melbourne","place":["Parkville, VIC, Australia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Tsetsegsaikhan","family":"Batmunkh","sequence":"additional","affiliation":[{"name":"National Centre for Communicable Diseases","place":["Ulaanbaatar, Mongolia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Jeremy","family":"Anderson","sequence":"additional","affiliation":[{"name":"Infection, Immunity, and Global Health, Murdoch Children\u2019s Research Institute","place":["Melbourne, VIC, Australia"]},{"name":"Department of Paediatrics, The University of Melbourne","place":["Parkville, VIC, Australia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Leanne","family":"Quah","sequence":"additional","affiliation":[{"name":"Infection, Immunity, and Global Health, Murdoch Children\u2019s Research Institute","place":["Melbourne, VIC, Australia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Yan Yung","family":"Ng","sequence":"additional","affiliation":[{"name":"Infection, Immunity, and Global Health, Murdoch Children\u2019s Research Institute","place":["Melbourne, VIC, Australia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Otgonjargal","family":"Amraa","sequence":"additional","affiliation":[{"name":"National Centre for Communicable Diseases","place":["Ulaanbaatar, Mongolia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Tsogjargal","family":"Burentogtokh","sequence":"additional","affiliation":[{"name":"General Laboratory of Clinical Pathology, First Central Hospital of Mongolia","place":["Ulaanbaatar, Mongolia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Sarantsetseg","family":"Jigjidsuren","sequence":"additional","affiliation":[{"name":"General Laboratory of Clinical Pathology, First Central Hospital of Mongolia","place":["Ulaanbaatar, Mongolia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Bolor","family":"Altangerel","sequence":"additional","affiliation":[{"name":"Onoshmed Clinical Laboratory","place":["Ulaanbaatar, Mongolia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Narantuya","family":"Namjil","sequence":"additional","affiliation":[{"name":"Onoshmed Clinical Laboratory","place":["Ulaanbaatar, Mongolia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Khaliunaa","family":"Mashbaatar","sequence":"additional","affiliation":[{"name":"National Centre for Communicable Diseases","place":["Ulaanbaatar, Mongolia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Rachael","family":"Carissa","sequence":"additional","affiliation":[{"name":"Infection, Immunity, and Global Health, Murdoch Children\u2019s Research Institute","place":["Melbourne, VIC, Australia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Skyy","family":"Luu","sequence":"additional","affiliation":[{"name":"Infection, Immunity, and Global Health, Murdoch Children\u2019s Research Institute","place":["Melbourne, VIC, Australia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Kerryn A.","family":"Moore","sequence":"additional","affiliation":[{"name":"Infection, Immunity, and Global Health, Murdoch Children\u2019s Research Institute","place":["Melbourne, VIC, Australia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Eleanor F. G.","family":"Neal","sequence":"additional","affiliation":[{"name":"Infection, Immunity, and Global Health, Murdoch Children\u2019s Research Institute","place":["Melbourne, VIC, Australia"]},{"name":"Department of Paediatrics, The University of Melbourne","place":["Parkville, VIC, Australia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Cattram","family":"Nguyen","sequence":"additional","affiliation":[{"name":"Infection, Immunity, and Global Health, Murdoch Children\u2019s Research Institute","place":["Melbourne, VIC, Australia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Shuo","family":"Li","sequence":"additional","affiliation":[{"name":"Infection, Immunity, and Global Health, Murdoch Children\u2019s Research Institute","place":["Melbourne, VIC, Australia"]}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"John","family":"Hart","sequence":"additional","affiliation":[{"name":"Infection, Immunity, and Global Health, Murdoch 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