Inclusion Criteria:  Age ≥18 years On treatment for cancer during the last 6 months or being treated >6 months ago but being ultravulnerable About to receive "Pfizer-BioNTech COVID-19" vaccine Lymphocyte count≥0.5x10^9/L  Exclusion Criteria:  Subjects who are not eligible for "Pfizer-BioNTech COVID-19" vaccine administration Inability and/or unwillingness to sign written informed consent