Patients with haematological malignancies are at increased risk of severe disease and death from COVID-19 and are less likely to mount humoral immune responses to COVID-19 vaccination, with the B cell malignancies a particularly high-risk group. Our COV-VACC study is evaluating the immune response to COVID-19 vaccination in patients with B cell malignancies. Eligible patients were either receiving active treatment or had received treatment within the last 24 months. Patients were vaccinated with either the BNT162b2 (Pfizer-BioNTech) (n=41) or ChAdOx1 nCoV-19 (Oxford-AstraZeneca) (n=14) vaccines. The median age of participants was 60 years (range: 27-82) and 50% were receiving systemic anti-cancer therapy (SACT) at the time of vaccination. This interim analysis from the first 55 participants describes anti-S seropositivity rates, neutralising antibody activity and association with peripheral lymphocyte subsets. After the first vaccine dose, 36% overall had detectable anti-S antibodies rising to 42% after the second dose. Sera from seropositive patients was assessed for neutralisation activity in vitro. Of the seropositive patients after first dose (n=17), only 41% were able to neutralise SARS-CoV-2 pseudotyped virus with a 50% inhibitory dilution factor (ID50) of >1:50. After two doses (n=21) 57% of the seropositive patients had detectable neutralisation activity (median ID50 of 1:469, range 1:70 - 1:3056). Total blood lymphocyte, CD19, CD4 and CD56 counts were significantly associated with seropositivity. Patients vaccinated more than 6 months after completing therapy were significantly more likely to develop antibodies than those within 6 months of treatment or on active treatment; OR: 5.93 (1.29 - 27.28). Our data has important implications for patients with B cell malignancies as we demonstrate a disconnect between anti-S seropositivity and virus neutralisation in vitro following vaccination against COVID-19. Urgent consideration should be given to revaccinating patients with B-cell malignancies after completion of anti-cancer treatment as large numbers currently remain at high risk of infection with the increasing transmission of SARS-CoV-2 in many countries.