The research team, led by Frederica Dedo Partey of the Noguchi Memorial Institute for Medical Research at the University of Ghana, recruited healthy adults aged 18 to 60 from the Legon community in Accra who had completed a primary vaccination series at least six months earlier with one of four vaccines: AstraZeneca’s ChAdOx1-S, Moderna’s mRNA-1273, Pfizer-BioNTech’s BNT162b2, or Janssen’s Ad26.COV2.S. Blood was drawn at four time points — before the booster and at three, six, and nine months afterward — and peripheral blood mononuclear cells were isolated by density-gradient centrifugation, cryopreserved in liquid nitrogen, and later thawed for functional testing. A critical feature of the cohort is that every participant was nucleocapsid-seropositive at enrollment, indicating prior natural infection, and antibody levels remained stable throughout follow-up, confirming that no new infections occurred during the study. The findings therefore describe how boosters reshape cellular immunity on a background of hybrid immunity — the combination of vaccination and prior infection that most of the world’s population now carries.

Rather than testing crude pools of viral peptides, the researchers took a precision approach. They used NetMHCpan-4.1, a machine-learning tool trained on peptide binding and eluted-ligand data, to predict 10-mer peptides from the spike and nucleocapsid proteins that bind strongly to seven globally prevalent HLA class I supertypes: A01, A02, A03, A24, B07, B40, and B58. Because participants were not HLA-typed, selecting epitopes across these supertypes maximized population coverage and was particularly relevant for a West African cohort with extensive HLA diversity. The predicted epitopes were matched to pre-synthesized overlapping 15-mer peptides spanning the full spike protein (1,273 amino acids) and nucleocapsid protein (419 amino acids), based on the Wuhan reference strain. Each peptide was then tested individually in an ex vivo interferon-gamma ELISpot assay, with 200,000 peripheral blood mononuclear cells per well stimulated for 24 hours. This single-peptide design meant that every positive response could be assigned to a defined viral sequence — a level of resolution that proved essential to the study’s central discovery.

That discovery concerns what actually drives the decline of T-cell immunity over time. Across the whole cohort, time had a powerful effect on total spike-specific interferon-gamma responses, with an omnibus F-test yielding F(3, 96.6) = 33.16 and a p-value of 8 × 10⁻¹⁵. Yet the median per-peptide response — the strength of each individual epitope-specific reaction — did not change significantly over the same period (F(3, 95.8) = 0.62, p = 0.606). The dissociation is striking: the waning of T-cell immunity after boosting was not caused by individual T-cell responses growing weaker, but by the immune system progressively dropping entire epitope specificities from its repertoire. The same pattern held within both booster groups separately, and the residual response stabilized after month six, with no significant difference between months six and nine.

The two booster platforms produced measurably different immune profiles. Participants boosted with the adenoviral vector Ad26.COV2.S showed significantly broader spike-specific epitope recognition — 70 positive peptide responses at month three compared with 34 in the BNT162b2 group, a difference with a p-value of 4.4 × 10⁻⁵ — and a subset of their responses persisted through month nine. The BNT162b2 recipients, by contrast, mounted higher cumulative interferon-gamma magnitude through month six, an advantage that was already present before the booster was administered (p = 0.012), suggesting it reflected their prior vaccination history rather than the booster itself. By month nine the picture had reversed, with the adenoviral-vector group showing higher — though not statistically significant — interferon-gamma levels. The authors propose a biological explanation rooted in platform mechanics: mRNA is inherently transient and subject to interferon-driven degradation, limiting the duration of antigen presentation, whereas adenoviral vectors support more prolonged antigen expression that may recruit subdominant T-cell clones and sustain a wider memory repertoire.

Vaccination strategy — whether the primary series and booster used the same platform — also left its mark. Within the Ad26.COV2.S group, homologous boosting (viral vector followed by viral vector) was associated with significantly broader epitope recognition at months zero, three, and nine, and this subgroup contributed the highest proportion of persistent responses at months six and nine. Two participants, J028 and J171, recognized at least one spike epitope at every time point, with J171 alone accounting for eight of the ten positive responses recorded at month nine. In the BNT162b2 group, no homologously boosted participant showed sustained responses across all post-booster time points, and no significant differences emerged between homologous and heterologous subgroups — a null result the authors attribute partly to limited statistical power. Notably, the finding that homologous adenoviral boosting outperformed heterologous regimens in breadth runs counter to the common assumption that anti-vector immunity undermines repeat adenoviral dosing.

Mapping the responses onto the spike protein revealed a dynamic immunodominance hierarchy. At baseline, recognition clustered in the N-terminal domain and the fusion peptide region — structurally exposed domains previously shown to harbor conserved T-cell epitopes. After boosting, recognition shifted toward the receptor-binding domain and subdomains 1/2, with peptides such as KLPDDFTGCV (amino acids 421–435) and IHADQLTPTW (amino acids 621–635) among the most frequently recognized at month three. By month nine, only nine spike peptides remained immunoreactive: three each from the N-terminal domain and fusion peptide region, and one each from the receptor-binding domain, subdomain 1/2, and the buried heptad repeat 2. The persistence of N-terminal and fusion-derived epitopes underscores their structural stability, while the rare recognition of a heptad repeat 2 peptide — a region largely hidden in the prefusion spike — hints at unusual HLA binding or exposure to post-fusion conformations.

The nucleocapsid arm of the study offered a natural experiment in memory maintenance without antigen restimulation. Because all administered vaccines encode only the spike protein, any nucleocapsid-specific T-cell responses must have been acquired through prior infection. Despite stable anti-nucleocapsid antibodies and no incident infection, these responses declined from 57 positive peptide reactions at baseline to just three at month nine — evidence that spike-based boosting does nothing to sustain cellular memory against non-spike antigens. Yet two nucleocapsid regions, YGANKDGIIW (amino acids 121–135) and VTPSGTWLTY (amino acids 321–335), were recognized at every one of the four time points. Their repeated immunogenicity, combined with the nucleocapsid protein’s high sequence conservation across coronaviruses, makes them leading candidates for inclusion in multi-epitope T-cell vaccines.

The authors are candid about the study’s limitations. The timing and severity of participants’ pre-enrollment infections could not be established; the interval between primary vaccination and booster ranged from six to 24 months; HLA typing was not performed, so genotype-specific responses could not be assessed; and months six and nine testing was restricted to peptides positive at month three, meaning delayed or newly emerging responses would have been missed. The observed contraction in breadth should therefore be read as a narrowing among the month-three-positive epitopes rather than definitive proof of global epitope loss. Subgroup comparisons were also modestly powered. Even so, the study’s central message stands: the platform and strategy chosen for boosting shape not just how strong T-cell immunity is, but how wide and how long-lasting it remains. For next-generation vaccine design, the implication is clear — durable cellular protection may depend less on maximizing peak response magnitude than on preserving epitope breadth, and conserved targets in the spike N-terminal domain, the fusion peptide, and the nucleocapsid protein deserve priority in the vaccines meant to outpace viral evolution.

Subject of Research: Longitudinal durability and epitope breadth of SARS-CoV-2-specific T-cell responses after mRNA versus adenoviral-vector booster vaccination

Article Title: T-cell recognition of SARS-CoV-2 antigen peptides following booster vaccination with mRNA and viral-vectored COVID-19 vaccines

Article References: Maru, A. K., Kumi, P. K., Pobee, A. N. A., Appiah, N. Y. A., Koranteng, N. A. O., Brenko, T., Osei, F., Teye-Adjei, D., Agyekum, G., Akuffo, L., Dzabeng, F., Kyei-Baafour, E., Obiri-Yeboah, D., Aniweh, Y., Ofori, M. F., Kusi, K. A., & Partey, F. D. (2026). T-cell recognition of SARS-CoV-2 antigen peptides following booster vaccination with mRNA and viral-vectored COVID-19 vaccines. Frontiers in Cellular and Infection Microbiology, 16, Article 1947651. https://doi.org/10.3389/fcimb.2026.1947651

Image Credits: AI Generated

DOI: 10.3389/fcimb.2026.1947651

Keywords: SARS-CoV-2, T-cell immunity, booster vaccination, BNT162b2, Ad26.COV2.S, ELISpot, epitope breadth, hybrid immunity, nucleocapsid protein, spike protein, HLA class I, vaccine design

The trial was designed as a prospective, open-label, multicenter, active-controlled study conducted across 31 clinical sites in India, enrolling healthy infants who had never received a pneumococcal conjugate vaccine. Participants aged 6 to 8 weeks at enrollment received three primary doses of either BE-PCV14 or Prevenar 13, followed by a booster dose administered at 12 to 15 months of age, a regimen commonly described as a 3p plus 1 schedule. The investigators, led by Subhash Thuluva of Biological E Limited in Hyderabad, measured serotype-specific immunoglobulin G antibodies against all 14 vaccine serotypes as well as the cross-protective serotype 6A, assessed 28 days after the booster dose. The primary immunological benchmarks were seroresponse rates, defined as the proportion of children achieving serotype-specific IgG concentrations of at least 0.35 micrograms per milliliter, a threshold long used as a correlate of protection against invasive pneumococcal disease, along with geometric mean concentrations of antibody.

The immunogenicity analysis included 559 children who received BE-PCV14 and 147 who received PCV13, providing a substantial evidence base for the comparison. The results were striking in their consistency: after the booster dose, seroresponse rates with BE-PCV14 reached at least 92.8 percent for every one of the 14 vaccine serotypes, and climbed to 96.2 percent for serotype 6A, which is not itself in the vaccine but is known to elicit cross-protection through immunological relatedness to vaccine serotype 6B. For the serotypes shared between the two vaccines, the immune responses elicited by BE-PCV14 were comparable to those generated by Prevenar 13, and post hoc statistical analyses formally demonstrated non-inferiority for all 14 serotypes by both seroresponse and antibody concentration criteria.

The non-inferiority assessment deserves particular attention because of its methodological rigor. The investigators pre-specified that BE-PCV14 would be considered non-inferior to PCV13 for a given serotype if the lower bound of the 95 percent confidence interval for the difference in seroresponse rates exceeded minus 10 percent, and if the lower bound of the confidence interval for the ratio of geometric mean concentrations exceeded 0.5. These are the kinds of thresholds that regulators and vaccine advisory bodies use to judge whether a new product can reasonably substitute for an established one. BE-PCV14 cleared both hurdles for all 14 serotypes, a result that, in the words of the trial team, establishes the vaccine as a credible alternative within existing infant immunization schedules rather than an entirely new paradigm requiring altered dosing strategies.

Perhaps the most consequential finding involves serotypes 22F and 33F, the two additional strains incorporated into BE-PCV14 that are absent from PCV13. The vaccine induced strong antibody responses to both, with booster-induced increases in antibody concentration ranging from 2.4-fold to 5.3-fold across serotypes when compared with pre-booster levels. This matters because pneumococcal serotype replacement is a well-documented phenomenon: when a vaccine eliminates the strains it targets, other strains that were previously less common can expand to fill the ecological vacuum. Serotypes 22F and 33F have been increasingly recognized as causes of invasive pneumococcal disease in countries that have deployed PCV13 at scale, and their inclusion in a new vaccine represents a forward-looking strategy intended to stay ahead of shifting disease epidemiology rather than merely replicate existing coverage.

Safety data from the trial were equally reassuring. Adverse events occurred at similarly low frequencies in both vaccine groups and were predominantly mild local reactions, such as injection-site tenderness or redness, or transient fever. Only two serious adverse events were recorded among BE-PCV14 recipients, and the investigators judged neither to be related to vaccination. The safety profile through 28 days after the booster dose, therefore, mirrors the well-characterized tolerability of licensed pneumococcal conjugate vaccines, an essential consideration for a product intended for universal infant immunization in resource-constrained settings where health systems may have limited capacity to manage vaccine-associated complications.

The trial, registered prospectively with the Clinical Trials Registry of India under identifier CTRI/2023/09/057894 and approved by India’s Central Drugs Standard Control Organisation, was conducted according to the Declaration of Helsinki and Good Clinical Practice guidelines, with written informed consent obtained from the parents or legally authorized representatives of all participants. Ethics committees overseeing each of the 31 participating sites granted approval, including the Institutional Ethics Committee of the All India Institute of Medical Sciences. The study, funded entirely by Biological E Limited, carries the inherent limitation that many of the authors are or were employees of the sponsoring company, though the trial’s active-controlled design against a licensed comparator and its prespecified statistical criteria provide meaningful safeguards against bias in the immunogenicity conclusions.

The public health significance of this work extends well beyond India’s borders. Streptococcus pneumoniae is responsible for hundreds of thousands of deaths among children under five each year, with the overwhelming majority occurring in low- and middle-income countries where access to effective vaccines has historically lagged behind wealthy nations. Pneumococcal conjugate vaccines have traditionally been among the most expensive components of childhood immunization schedules, and the introduction of a WHO-prequalified vaccine manufactured in a middle-income country carries the potential to lower prices, expand supply, and reduce dependence on a small number of multinational manufacturers. For India, which runs one of the largest immunization programs in the world through its Universal Immunization Programme, a domestically produced 14-valent vaccine that slots into the standard 3-plus-1 schedule offers both strategic and epidemiological advantages.

The trial’s findings also speak to a broader evolution in pneumococcal vaccine science. The 0.35 micrograms per milliliter IgG threshold used in this study is a population-level benchmark derived from efficacy trials of earlier conjugate vaccines, and while it does not guarantee individual protection, it remains the standard immunological bridge for licensure and comparison of new formulations. The demonstrated booster response, with antibody concentrations rising severalfold after the toddler dose, indicates that BE-PCV14 successfully exploits the immunological memory established during the primary series, an attribute that underpins the durability of protection in the second year of life, when the burden of invasive pneumococcal disease and pneumonia peaks in unvaccinated populations.

As pneumococcal disease patterns continue to shift under the selective pressure of global vaccination, the arrival of a well-tolerated, immunogenic 14-valent conjugate vaccine that extends coverage to emerging serotypes while matching the performance of the established 13-valent standard represents a meaningful advance. The authors conclude that PNEUBEVAX 14, when used as a booster in a 3-plus-1 schedule, is well tolerated and elicits robust responses comparable to PCV13 for shared serotypes while effectively broadening coverage to 15 serotypes when cross-protection against 6A is counted. For immunization programs weighing how to future-proof their pneumococcal strategies, this trial provides a substantial and encouraging body of evidence that broader serotype coverage need not come at the cost of safety or immunogenicity.

Subject of Research: Safety and immunogenicity of a 14-valent pneumococcal conjugate vaccine in Indian infants and toddlers

Article Title: Safety and immunogenicity of a 14-valent pneumococcal conjugate vaccine (BE-PCV14) administered in a 3p + 1 schedule to healthy Indian infants and toddlers: a prospective, multicenter, active-controlled phase IV trial

Article References: Thuluva, S., Gunneri, S., Ningaiah, S., Yerroju, V., Mogulla, R. R., Dhar, C., Thammireddy, K., Desai, S., Paliwal, P., Loka, C., Esanakarra, R., Narayandas, S., Chakravarthy, B. S., Narayan, J. P., Mahantshetti, N. S., Narang, M., Karayar, R. A., Verma, S., Thakkar, P. A., & Prabhakar, J. P. (2026). Safety and immunogenicity of a 14-valent pneumococcal conjugate vaccine (BE-PCV14) administered in a 3p + 1 schedule to healthy Indian infants and toddlers: a prospective, multicenter, active-controlled phase IV trial. BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14412-1

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14412-1

Keywords: pneumococcal conjugate vaccine, PNEUBEVAX 14, BE-PCV14, PCV13, infant immunization, serotype 22F, serotype 33F, invasive pneumococcal disease, India, phase IV trial, booster vaccination, Streptococcus pneumoniae