Hypervirulent serogroup W meningococcal strain belonging to the ST-11 clonal complex is associated with higher case fatality rates: nationwide cohort study of invasive meningococcal disease caused by serogroup W in England from 2010-2019.

BACKGROUND: In 2009, England experienced an increase in serogroup W (MenW) invasive meningococcal disease (IMD) caused by the hypervirulent ST-11 clonal complex (MenW:cc11) South American strain sublineage. The 'original UK strain' that emerged in 2009 evolved into the '2013 strain' in 2013 resulting in further increases in case numbers. Here, we compare demographics, co-morbidities, clinical characteristics and outcomes of patients with MenW IMD caused by the original UK, 2013, and other MenW strains during 2010-2019 in England. METHODS: The UK Health Security Agency (UKHSA) conducts enhanced national IMD surveillance in England and provides a national reference laboratory service for diagnosis confirmation and meningococcal characterisation. All confirmed cases are followed-up with general practitioners for additional information, including comorbidities, clinical presentation and outcomes. RESULTS: Between 2009/10 and 2018/2019 epidemiological years, there were 1,126 MenW IMD cases in England, of which 947 (84.1%) were confirmed by culture and 174 (15.9%) by PCR only. Cultured isolates were characterised as the original UK strain (n=361, 38.1%), 2013 strain (n=481, 50.8%), and other MenW strains (n=105, 11.1%). There were no significant differences in age, sex, risk factors, comorbidities, clinical presentation or intensive care requirement between MenW strain groups. For the cases between 2011-2017, clinical data analysis was performed. MenW IMD case fatality rate (CFR) was 12.7% (110/877) overall, but significantly higher for the 2013 strain (15.0%, 51/340) than for the original UK strain (9.8%, 30/305) for the 2013 strain and for other MenW strains (7.2%, 6/83) (p=0.05). In a multivariable logistic regression model, the 2013 strain, age ≥25 years compared to <5 year-olds and clinical presentation with septicaemia compared to other clinical presentations were independently associated with death. Implementation of an emergency adolescent MenACWY immunisation programme in 2015 was associated with large declines in MenW IMD cases and deaths across all age-groups. CONCLUSION: The emergence of the original UK and 2013 MenW strains led to large increases in MenW IMD cases across all age groups and the latter was associated with higher case fatality. This has important implications for countries where the 2013 strain continues to circulate.