Background <p>Idiopathic nephrotic syndrome (INS) is the most common chronic kidney disease in childhood, with steroid sensitive nephrotic syndrome (SSNS) as the predominant form. Children with SSNS are at increased risk for invasive pneumococcal disease due to disease-related and treatment-induced immunosuppression. Evidence on immunogenicity of the 23-valent pneumococcal polysaccharide vaccine (PPV23) in this group is limited. This prospective study is the first to evaluate PPV23 exclusively in children with SSNS during remission, directly compared with healthy controls.</p> Methods <p>Thirty patients with SSNS—responsive or dependent on corticosteroids only—in remission for ≥ 3&#xa0;months, and 24 age-matched healthy controls, all pneumococcal vaccine-naïve, received a PPV23 dose. Anti-pneumococcal IgG was measured at baseline and six weeks post-vaccination by ELISA. Seroconversion was defined as ≥ twofold antibody titer increase. In SSNS, relapse count, cumulative corticosteroid dose, and age at diagnosis/vaccination were recorded and correlated with antibody responses. In controls, only age at vaccination was correlated. Adverse events and relapses were monitored for six months.</p> Results <p>Seroconversion was comparable in SSNS (93.3%) and controls (95.8%, p = 0.690). In SSNS, younger age at diagnosis (ρ = –0.48, p = 0.008) and vaccination (ρ = –0.37, p = 0.041) correlated with greater antibody gains, whereas relapse count and cumulative steroid dose showed no association. No relapses or serious adverse events occurred during follow-up.</p> Conclusions <p>PPV23 is safe and immunogenic in SSNS during remission, eliciting responses equivalent to healthy peers. Importantly, younger age emerged as the only independent determinant of antibody gain—a novel finding in this population—underscoring the importance of timely vaccination in the disease course.</p> Graphical abstract <p></p>