Inclusion Criteria:  Subjects are between 50 and 75 years of age; Have not had known or suspected infection with SARS-CoV-2 at any time;  Subjects must fall into one of the following subsets of cancer patients:  Localized prostate cancer on radiotherapy or have completed radiotherapy in the last three months Prostate cancer on androgen-deprivation therapy for biochemical recurrence Colon cancer on adjuvant chemotherapy for stage III disease CLL on a BTK inhibitor as a first-line single agent for at least 3 months Melanoma patients on adjuvant immunotherapy with PD-1 checkpoint inhibitors for at least 3 months Have received both doses of SARS-CoV-2 mRNA-1273 vaccine with proof of vaccination or are scheduled to do so; Have at least a one-year life expectancy; Prior radiation therapy is allowed, as long as it was completed more than 6 months prior to vaccination  Exclusion Criteria:  Subjects have a known hypersensitivity to a vaccine component; Have had known or suspected infection with SARS-CoV-2 at any time; Are organ transplant recipient on immunosuppression; Had received or are scheduled to receive a live virus vaccine in the period from 4 weeks prior to Dose 1 through 28 days post second dose; Had received or are scheduled to receive an inactivated vaccine in the period ranging from 7 days prior to Dose 1 through 7 days post second dose; Received high-dose corticosteroids at any time after receiving the vaccine; Are unable to give informed consent; Are receiving treatment for CLL with another agent (such as rituximab) in addition to a BTK inhibitor