Bacterial isolation and typing

All invasive N. meningitidis strains were isolated from CSF and all carrier strains were isolated from mouthwash samples at the Laboratory of Microbiology, Military Institute of Preventive Medicine, Hanoi as described previously (Tran et al., 2019). Two isolates, NMB_VN2013 and NMB_VN2015, were from the CSF of two confirmed meningitis cases discovered in 2013 and 2015 at Military Unit 2, a camp in the geographical closeness to Military Unit 1, the camp where the previously reported DuyDNT isolate was identified. Both cases were treated at the military hospital and subsequently recovered. Serogroup identification and multi-locus sequence typing (MLST) were either done according to previously described standard methods (World Health Organization, 2011) or by manually extracting the corresponding sequences from WGS data.

DuyDNT isolate was renamed NMB_VN2014 here for convenience and consistency. Available laboratory records of the Laboratory of Microbiology, Military Institute of Preventive Medicine, Hanoi were reviewed and a suitable collection of 25 isolates were chosen based on reported year (before and after 2013–2015), location (Military Unit 1, 2, and two nearby units here named Unit 3 and Unit 4) and availability of molecular characterization data (Table 1). Serogroup data, MLST data, fHbp and porA allelic variants were obtained from laboratory records. Case-related metadata, including year, clinical status, and location were obtained where available.

Notes.