Humoral and cellular immune response after mRNA SARS-CoV-2 vaccine in children on treatment for cancer: A pilot observational study.

Immunocompromised children are at risk of developing severe COVID-19 infection. We conducted a pilot prospective study to evaluate the impact of cancer treatment and stem cell transplantation on immunogenicity of two doses of BNT162b2 vaccine in pediatric patients. Humoral, B- and T-cell responses to the BNT162b2 vaccine were assessed before, after the first and the second dose in patients aged 5-12 years (n = 35) and in a group of healthy donors (HD, n = 12). Patients were divided in three groups: solid tumors (ST, n = 11), hematological malignancies (HM, n = 14) and Hematopoietic Stem Cell Transplantation (HSCT) recipients (n = 10). After two vaccine doses, the seroconversion rate was 79.3 % (72.7 % in ST, 66.7 % in HM and 100 % in HSCT). The antibodies production was not associated to the presence of memory B and T-cells. Memory B-cells were measurable in 45.5 % ST, 66.6 % HSCT and in 22.0 % HM. The specific T-cell response was observed in most ST (81.8 %) and HSCT (85.7 %) patients and at lesser extent in those with HM (55.5 %). The combination of all immunological parameters (antibodies, memory B and T cells) showed that a significant fraction of HM (33.3 %) and ST (18.2 %) patients completely failed to respond to vaccination. Although able to produce antibodies, 11.1 % of HM and 27.3 % of ST had no B- and T-cell memory. HSCT subgroup showed the best immune function, with 80 % complete response and optimal T-cell function. Combination of anti-RBD antibody, and specific memory B- and T-cell responses represents a reliable read-out of vaccine immune efficacy in frail patients.