Poster Presentation Asia-Pacific Vaccine and Immunotherapy Congress 2026

Fractional BNT162b2 boosters induce durable immune responses after non-mRNA priming in Mongolia: a randomised controlled trial. (#132)

Eleanor Frances Georgina Neal 1 2 , Tsetsegsaikhan Batmunkh 3 , Otgonjargal Amraa 3 , Nadia Mazarakis 1 2 , Bolor Altangerel 4 , Naranbaatar Avaa 4 , Lkhagvagaram Batbayar 5 , Khishigjargal Batsukh 6 , Kathryn J Bright 1 , Tsogjargal Burentogtokh 6 , Lien Anh Ha A H Do 1 2 , Gantuya Dorj 7 , John D Hart 1 2 , Otgonbold Jamiyandorj 8 , Khulan Jakhlantugs 9 , Sarantsetseg Jigjidsuren 6 , Frances Justice 1 , Shuo Li 1 , Khaliunaa Mashbaatar 3 , Kerryn Moore 1 , Narantuya Namjil 4 , Cattram Nguyen 1 2 , Batbayar Ochirbat 3 , Unursaikhan Surenjav 10 , Helen Thomson 1 , Bilegtsaikhan Tsolmon 3 7 , Paul V Licciardi 1 2 , Claire von Mollendorf 1 2 , Kim Mulholland 1
  1. MCRI, Parkville, VIC, Australia
  2. Department of Paediatrics, The University of Melbourne, Parkville, VIC, Australia
  3. National Centre for Communicable Diseases, Ulaanbaatar, Mongolia
  4. Onoshmed Laboratory, Ulaanbaatar, Mongolia
  5. Sukhbaatar District Health Centre, Ulaanbaatar, Mongolia
  6. General Laboratory of Clinical Pathology , First Central Hospital of Mongolia, Ulaanbaatar, Mongolia
  7. Mongolian National University of Medical Sciences, Ulaanbaatar, Mongolia
  8. City Health Department, Ulaanbaatar, Mongolia
  9. Bayangol District Health Centre, Ulaanbaatar, Mongolia
  10. National Centre for Public Health, Ulaanbaatar, Mongolia

Background: The COVID-19 pandemic catalysed rapid global vaccination efforts, with booster doses recommended to restores waning immunity. Optimising booster strategies is particularly important in low- and middle-income countries, where constrained supply and limited access to updated vaccines remain a barrier. Fractional booster dosing, using reduced vaccine volumes, has been proposed as a dose-sparing approach to extend supply and reduce costs without compromising safety or immunogenicity. After demonstrating non-inferiority of 15 μg versus 30 μg BNT162b2 at 28 days in Mongolian adults, we assessed 24-month immunogenicity and safety.

Methods: In this randomised, controlled, non-inferiority trial, adults in Mongolia who had been primed with ChAdOx1-S, BBIBP-CorV, or Gam-COVID-Vac were assigned (1:1) to 15 μg or 30 μg BNT162b2. Anti-spike IgG and surrogate virus neutralisation (sVNT) against Wuhan-Hu-1 and Omicron BA.1 were assessed to 24 months; a subset underwent IFN-γ release assays (Ag1/Ag2). Geometric mean ratios (GMRs; 15 μg/30 μg) were estimated using multivariable linear regression on log-transformed anti-spike IgG levels, adjusted for age group, priming strata, timing of blood draw, dosing intervals, and baseline levels. SARS-CoV-2 infections and serious adverse events (SAEs) were recorded. ClinicalTrials.gov: NCT05265065.

Findings: Of 601 enrolled participants (27 May–30 Sept 2022), 520 (86·5%) completed 24 months. IgG declined from six to 24 months but was similar between arms at 18 (GMR 1·08 [95% CI 0·97–1·22]) and 24 months (GMR 1·06 [95% CI 0·95–1·18]). IFN-γ peaked at 28 days and returned to baseline by 24 months (24-month GMRs: Ag1 1·17 [95% CI 0·82–1·66]; Ag2 1·06 [95% CI 0·73–1·54]). At 24 months, median sVNT inhibition remained high (Wuhan-Hu-1 88% (IQR 86–90) in both arms; Omicron BA.1 84% (IQR 71–88) with 30 μg and 85% (IQR 70–88) with 15 μg). Twenty-eight PCR-confirmed infections occurred; none required hospitalisation. Fifty-three SAEs occurred, balanced between arms, and none were vaccine related. 

Interpretation: This trial provides comprehensive 24-month immunogenicity and safety data from a low-resource setting. 15 μg and 30 μg BNT162b2 boosters yielded equivalent neutralising and cellular responses, while binding IgG declined. Safety was favourable, and there were no COVID-19–related hospitalisations or deaths documented, supporting fractional dosing as a pragmatic, cost-saving option. 

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