Outbreak of invasive meningococcal disease caused by a meningococcus serogroup B expressing a rare porA genosubtype (19-54, 15), Spain, March to April 2024.

In Spain during March-April 2024, an outbreak of invasive meningococcal disease (IMD) occurred in four young adults, exhibiting high case fatality with two deaths. Cases 1 and 4 were confirmed by isolation of Neisseria meningitidis from blood samples, while Cases 2 and 3 were PCR-confirmed from cerebrospinal fluid (CSF). Serogroup B meningococcus with identical characterisation (B: 19-54, 15: F5-1: ST-34, cc32) was identified for all cases; the outbreak strain genosubtype PorA_VR1: 19-54 had not been previously described. Potential coverage of the outbreak strain by available MenB vaccines could not be predicted by molecular tools, so bactericidal response to the 4CMenB vaccine against the outbreak strain was measured by human serum bactericidal antibody assay (hSBA), defining the strain as covered by the vaccine. Two different social events were involved in transmission of the outbreak strain. According to the national meningococcal disease surveillance protocol, an active search for close contacts of the cases was conducted by public health authorities and timely chemoprophylaxis and/or vaccination with 4CMenB vaccine was recommended to over 200 contacts. The evolution of meningococcal strains with genosubtype 19-54 should be closely monitored, as it might confer a greater transmission capacity.