So this is the "How Effective is the Vaccine?" section from Meningitis.org. It says basically what the JCVI statement above does, that we don't yet know how the immunogenicity relates to actual Men B cases prevented.
"The effectiveness of a vaccine is determined by many things, including how strong an immune response it produces (its ‘immunogenicity’), and how widely it covers disease-causing strains circulating in the country. Results from the vaccine trials are very encouraging, showing that the vaccine triggers a strong immune response in infants, toddlers and adolescents[1-3]. Studies looking at how well circulating MenB strains match the vaccine have predicted that it will cover approximately 88% of MenB circulating in the UK[4], and 78% of MenB in Europe over all[5]. The actual proportion of cases prevented will depend on other things too, including how widely the vaccine is offered and taken up, whether it prevents the bacteria from being carried and passed on as well as protecting from disease, how long protection lasts, and whether it works sufficiently well in all age groups."
The safety data they cite is slightly more encouraging than was used at the time of Bexsero roll out here, but is still only a few 10,000s doses (with annotated safety data, as I read it).