Meningococcal invasive disease by serogroup W and use of ACWY conjugate vaccines as control strategy in Chile

Background: Serogroup causing invasive meningococcal disease (IMD) can change abruptly, as it occurred in Chile when serogroup predominance switched from MenB to MenW in 2012. As a response, a national vaccination strategy was implemented since 2012 using tetravalent meningococcal-conjugate vaccines (MCV–ACWY) in children 9 months through 4 years of age. The aim of this study was to describe IMD cases by MenW in Chile 2009–2016, and to analyse its trend after the introduction of MCV-ACWY. Methods: Descriptive study of IMD cases in Chile, period 2009–2016. Cumulative incidence and mortality rate per 100,000 inhabitants, and case fatality rate (CRF) were used for descriptive analysis. Linear regression was used for post-intervention trend analysis. Results: In 2012, MenW, mainly ST-11 cc, became predominant. MenW incidence rose from 0.01/100,000 inhabitants in 2009 to a maximum of 0.6/100,000 in 2015. Infants and adults 80 years of age and older were mostly affected, with an incidence peak of 9.7/100,000 and 1.6/100,000, respectively, in 2015. In the group of children from 1 to 4 years of age MenW incidence declined from 1.3/100,000 in 2012 to 0.1/100,000 in 2016, a 92.3% reduction after vaccination implementation. In the same period and age-cohort, CFR decreased from 23% to 0%. High mortality rates concentrated in infants and adults 80 years of age and over. Conclusion: MenW became predominant in Chile since 2012. IMD cases increased steadily from 2009 to 2016, with higher incidence, CFR and mortality concentrating in infants and people 80 years of age and older. MCV–ACWY provided direct protection against MenW, reducing its incidence after mass meningococcal vaccine implementation. Indirect effects of vaccination are not yet observed.