Exclusion criteria included previous vaccination with VA-MENGOC-BC®, use of antibiotics, documented immunodeficiency, chronic debilitating illness or any past episode of meningitis. Following informed consent, the cohort received three doses of VA-MENGOC-BC®, applied with a 6–8-week interval and a booster dose applied 6–7 months after the primary immunisation. Vaccine was administered by intramuscular injection into the non-dominant deltoid muscle.

Blood was taken before and 3, 7 and 14 days after each injection of vaccine during the primary immunisation schedule and 6–7 months (pre-booster sample) after the third dose. After the booster dose, blood was collected at days 3, 7, 14 and 28. A maximum volume of 10 ml heparinised blood was available for the separation of peripheral blood mononuclear cells (PBMC). PBMCs were separated by density-gradient centrifugation check details over Histopaque® (Sigma, St. Louis, USA). Plasma was collected and frozen at −20 °C. The Cuban vaccine strain (Cu385/83) of serotype:serosubtype:immunotype 4,7:P1.19,15:L3,7,9 was used for the preparation of outer membrane vesicles (OMV) to be used as the coat antigen for ELISPOT and as a target strain for the bactericidal assay. H355/75 (B:15:P1.19,15:L3,7,9,8) and

its variants PorA− and Opa− were also used for the opsonic and bactericidal antibody assays. The origin of these strains was previously described [14]. PBMCs prepared form peripheral blood were washed in

RPMI 1640 (HyClone, Utah, USA) supplemented with 10% fetal bovine serum (HyClone), 5 × 10−5 β-mercaptoethanol (Sigma, St. Louis, USA) and JAK activation antibiotics (10,000 U/ml penicillin (Sigma) and 10 mg/ml streptomycin (Proquímios, Rio de Janeiro, Brazil)) and re-suspended to a final concentration of 1 × 105 PBMC/well. Cells were then quantified by ELISPOT technique as previously described [15]. Briefly, 96-well Maxisorp plates (Nunc, Rochester, USA) were coated either with 10 μg/ml of anti-human isothipendyl IgG monoclonal antibody (Kirkegaard & Perry Laboratories, Maryland, USA), or 4 μg/ml of OMV (Cu385 strain) in 0.05 M Tris buffer, pH 9.5, overnight at 4 °C. After washing with phosphate buffer saline (PBS) 0,01 M, pH 7.2–7.4, plates were blocked for 1 h with RPMI supplemented with 1% fetal bovine serum and antibiotics (150 μl/well). Then 100 μl/well of the cells suspension was added to pre-coated ELISPOT plates, and incubated for 16 h at 37 °C in 5% CO2 and then washed with PBS/1% Tween 20 (T20). Secreted IgG was detected with anti-human IgG alkaline phosphatase-conjugated mAb (Kirkegaard & Perry Laboratories, Maryland, USA) at a dilution of 1:5000 in PBS/1% BSA/0.1% T20. ELISPOTs were developed with 1 mg/ml 5-bromo-4-chloro-3-indolylphosphate (BCIP; Sigma) dissolved in amino-methyl-propanol buffer (Sigma). Spots were counted after 2 h by stereoscopic microscopy. Mean values of spots were calculated from six replicates.