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Vaccination Protection Against Meningococcal W

Meningococcal serogroup W (serotype 11, ST11), associated with a high case-fatality rate, emerged in Côte-d'Or in 2016.A cluster of 3 cases, including 2 deaths, was observed in late 2016 on the University of Dijon campus, leading to the organization of a vaccination campaign. This campaign had a dual objective: to provide individual protection for each vaccinated person and to provide herd immunity by reducing the number of people likely to carry the bacteria.This meningococcal strain originally came from South America and spread to England, where the number of W-serogroup IIM cases rose from 19 in 2008–2009 to 176 in 2014–2015 (accounting for 24% of all IIM cases across all serogroups).In response to this increase in cases, a vaccination program for adolescents against meningococcal serogroups A, C, W, and Y began in August 2015. Public Health England conducted a study(1) that showed the vaccination coverage rate for the first vaccinated cohort was 36.6% and led to a 69% decrease in the number of observed cases compared to the expected number of cases, with none of the cases having been vaccinated.While the absence of cases among the vaccinated was expected, the study authors note that the reduction in the number of cases is significant despite a vaccination coverage rate of only 36.6%. This rate did not lead to the eradication of the disease, as the strain continues to circulate, but it significantly reduced the risk by reducing the number of cases by a factor of just over 3. These results confirm the value of vaccination in the event of increased circulation of this meningococcal W serogroup 11 in a population. The British findings, based on a large vaccinated population, showed that with a vaccination coverage rate of 36.6%, the collective risk is significantly reduced, even though cases persist. Furthermore, individual protection is ensured, with no cases among the vaccinated. These results are the first regarding the herd immunity of vaccination against meningococcal W. They suggest that the vaccination coverage achieved on the Dijon campus—although in a different context and higher than that observed in England—significantly lowered the risk of new cases emerging by increasing herd immunity. However, this does not completely rule out the possibility of a new case emerging.