Secular trends in invasive meningococcal disease, Massachusetts, 1988-2011: what happened to invasive disease?

Invasive meningococcal disease (IMD) reported to the Massachusetts Department of Public Health from 1988 to 2011 was reviewed. The average annual incidence of IMD/100 000 decreased from 1·57 [95% confidence interval (CI) 1·42-1·73] for 1988-1991 to 0·22 (95% CI 0·17-0·29) for 2008-2011. The pattern of decreasing incidence over time differed by age group. There was a decrease in IMD/100 000 in the 0-4 years age group after 1991 from 10·92 (95% CI 8·08-14·70) in 1991 to 5·76 (95% CI 3·78-8·72) in 1992. Incidence in the 0-4 years age group remained below 5/100 000 per year on average thereafter. A substantial reduction in incidence in all age groups was observed between 2000 and 2009, which began before the introduction of conjugate meningococcal vaccine in 2005. Marked reductions in incidence of IMD in Massachusetts, and elsewhere, deserve further investigation with respect to potential factors that go beyond the introduction and deployment of improved meningococcal vaccines.