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Background Activity and safety of the SARS-CoV2 BNT162b2 vaccine in actively treated patients with solid tumors is currently unknown. Methods We conducted a retrospective study of 326 patients with solid tumors treated with anti-cancer medications to determine the proportion of cancer patients with immunogenicity against SARS-CoV2, following two doses of the BNT162b2 vaccine. Control group was comprised of 164 vaccinated healthy adults. Anti-SARS-CoV-2 S IgG (Immunoglobulin G) antibodies (Abs) were measured, using level>50 AU/ml as cutoff for seropositivity. Adverse effects were collected using a questionnaire. All statistical tests were 2-sided. Results Most patients (205, 62.9%) were treated with chemotherapy, either alone or with additional therapy, 55 (16.9%) were treated with immune checkpoint inhibitors (ICI) and 38 (11.7%) with targeted therapy alone, 28 (8.6%) received other combinations. The vaccine was well tolerated and no severe side effects were reported. Among patients with cancer 39 (11.9%) were seronegative, compared to 5 (3.0%) of the control group (P=0.001). Median IgG titers were statistically significant lower among patients with cancer compared to control (931 AU/ml vs. 2817 AU/ml, P=0.003). Seronegativity proportions were higher in the chemotherapy treated group (19, 18.8%) compared to the ICI-treated patients (5, 9.1%) and to those treated with targeted therapy (1, 2.6%) (P=0.02. Titers were also statistically significant different among treatment types (P=0.002). Conclusion The BNT162b2 vaccine is safe and effective in actively treated patients with cancer. The relatively lower antibody titers and lower proportion of seropositive patients, especially among chemotherapy treated patients, call for continuing the use of personal protective measures in these patients, even following vaccination.

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