Preserved SARS-CoV-2 T-cell responses despite impaired humoral immunity in children with profound B-cell lymphopenia.

Publication date: Jul 20, 2026

Defining vaccine-induced protection in children with humoral immunodeficiency is essential to guide SARS-CoV-2 vaccination strategies in this high-risk population. We conducted a longitudinal analysis of SARS-CoV-2 immunity at 1, 6 and 12 months after a primary Pfizer-BioNTech mRNA vaccine series in 27 children aged 5-11 years with primary or secondary antibody deficiencies and 48 matched healthy controls. Functional T-cell responses were quantified by IFN-γ and IL-2 ELISpot, and SARS-CoV-2-specific B-cells and T-cells were assessed by spectral cytometry. Systemic and mucosal antibody responses were measured in serum and saliva, and neutralizing activity against ancestral and Omicron BA. 5 strains was evaluated through microneutralization. Children with humoral immunodeficiency exhibited impaired systemic antibody responses after two mRNA doses, even after SARS-CoV-2 infection. A third dose improved humoral immunity in children with preserved B-cell compartments but did not rescue neutralizing antibody responses in those with severe B-cell lymphopenia. In contrast, preserved, polyfunctional SARS-CoV-2-specific T-cell responses were observed in children with humoral immunodeficiency, including those with severe B-cell lymphopenia, and were higher in asymptomatic immunocompromised children. These findings reveal a dissociation between humoral failure and preserved cellular immunity in B-cell-deficient children, supporting timely vaccination and integration of T-cell responses into vaccine-response assessment when neutralizing antibodies are absent.

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