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RT may affect the immune system by suppressing or enhancing the immune response: thus, the immunogenicity of the SARSCoV-2 vaccine may potentially be different in this setting. Data on immunoresponse for patients treated with systemic therapy are available after the second-third dose, but they are very scarce on patients who received RT. We recently reported findings from a study evaluating the acute tolerance of mRNA-1273 vaccine in patients treated with RT, in absence of systemic treatment. Herein, we describe the results of a prospective trial assessing immunogenicity after the second and third dose of mRNA vaccine.

Data regarding humoral and cellular immune response of patients treated with RT alone were prospectively collected after the second and third dose of mRNA vaccines. To all patients vaccinated in our Institution was offered the chance to participate if deemed eligible. Inclusion criteria were: age≥18 years; no systemic treatment in the six months before the first dose; absence of previous documented SARS-CoV-2 infection. Exclusion criteria were: pregnancy or breastfeeding; inability to give informed consent. With the aim of evaluating the immunogenicity following the second dose, blood collection was performed at the end of September 2021. In February 2022, patients who had been enrolled in the study, were recruited for a new blood sample assessing the immune response of the third vaccine dose. Cases of documented infection were classified according to the presence and severity of symptoms.

Ninety-two patients were enrolled. With a median of 147 days after the second dose, the median SARS-CoV-2 IgG titer was 300 BAU/mL: six patients were seronegative (Spike IgG titer <40 BAU/mL ), whereas 24, 46 and 16 were poor responders (Spike IgG titer:41-200 BAU/mL), responders (Spike IgG titer:201-800 BAU/mL) and ultraresponders (Spike IgG titer>800 BAU/mL), respectively. Among seronegative patients, two patients were negative also for cell mediated response, as tested with IFN-γ release Assay (IGRA) test. With a median of 85 days after the third dose, the median SARS-CoV-2 IgG titer was 1632 BAU/mL in 81 patients: only two patients were seronegative, whereas 16 and 63 patients were responders and ultraresponders, respectively. Among the 2 persistently seronegative patients, IGRA test was negative in one who had previously received anti-CD20 therapy. Documented paucisymptomatic (n=3) or asymptomatic (n=4) infection occurred after the third dose, during the Omicron wave.

Our experience showed that, in patients treated with exclusive RT, even during the Omicron breakthrough, robust humoral response and clinical protection from severe SARS-CoV-2 disease are achievable with three doses of mRNA vaccine. This study reinforces the importance of a full course of vaccination for patients with cancer, who are at high risk for morbidity and mortality from COVID-19.