Abstract It is now well-established that COVID-19 in patients with cancer carries a higher morbidity and mortality, especially in amongst patients with hematologic malignancies1, 2. Effective vaccines have been developed and authorized by the FDA to combat this pandemic3-5. However, emerging data suggests that despite these vaccines inducing high levels of immunity in the general population, patients with hematologic malignancies have lower rates of seroconversion for the SARS-CoV-2 Spike antibody6-9. Indeed, our prior data had suggested excellent immunogenicity of the currently FDA authorized Covid-19 vaccines for most patients with a malignant diagnosis with exception of unique cohorts- patients with hematological malignancies and following highly immune suppressive therapies (SCT, CAR-T, anti-CD20). Our current study plans to address the efficacy of Covid-19 booster vaccinations amongst patients with a cancer diagnosis with a special focus on patients with undetectable or low anti-Spike IgG antibody levels. 250 patients with a diverse range of ethnicities, cancer diagnosis and prior cancer therapies will be enrolled in this study and the response to booster vaccinations will be assessed using standard anti-Spike IgG as well as investigational T cell and Virus Neutralization assays. In addition, in a second cohort we will define if a “mix and match” vaccination strategy might be more successful at achieving detectable immunity in patients with persistently negative/low antibody levels despite booster vaccinations. In this cohort, 100 patients will be accrued who by anti-spike IgG assays have not developed detectable or have low levels of humoral immunity. These patients will be randomized to receive an additional dose of an mRNA based vaccine (BNT162b2) versus an adenoviral vaccine (AdCov2.S).Both studies will assess safety and efficacy of these vaccination schemes and will provide long-term data with 24 months of follow up planned. These studies should provide critical information to best address proper protective strategies for uniquely vulnerable patient populations with a cancer diagnosis.