Background: Bacterial meningitis continues to be a substantial cause of morbidity and mortality, but the epidemiological trends after adjunctive dexamethasone recommendations are unknown in the USA. We aimed to describe the changing patterns among the most common bacterial causes in the USA after conjugate vaccination and to assess the association between adjunctive dexamethasone and mortality. Methods: For this population-based observational study, we searched information available from hospital discharges about incidence and inpatient mortality for the most important causes of community and nosocomial bacterial meningitis based on International Classification of Diseases coding across all hospitals in the USA between 1997 and 2010 with the HealthCare Cost Utilization Project (HCUP) network database. We calculated incidences according to US Census Bureau data and used a negative binomial regression model to evaluate the significance of changes over time. We assessed mortality from pneumococcus for three periods 1997-2001 (baseline), 2002-04 (transition years), and 2005-08 (after corticosteroid recommendations were available). Findings: Streptococcus pneumoniae incidence fell from 0·8 per 100 000 people in 1997, to 0·3 per 100 000 people by the end of 2010 (RR 0·3737, 95% CI 0·1825-0·7656). Mortality from pneumococcal meningitis decreased between 2005 (0·049 per 100 000 people) and 2008 (0·024 per 100 000 people) compared with between 2002 (0·073 per 100 000 people) and 2004 (0·063 per 100 000 people; RR 0·5720, 95% CI 0·4303-0·7582). The incidence of Neisseria meningitidis infection decreased from 0·721 per 100 000 people in 1997, to 0·123 per 100 000 people in 2010 (RR 0·1386, 95% CI 0·048-0·4284), which has placed this pathogen close to common bacterial causes of nosocomial meningitis such as staphylococcus and Gram-negative bacteria and to Haemophilus influenzae. Interpretation: S pneumoniae continues to be the leading identifiable cause of bacterial meningitis in the USA, but with a significant decrease in incidence and mortality associated with the introduction of conjugated vaccines and a mortality decrease that is associated with the introduction of recommendations for use of adjunctive dexamethasone for pneumococcal meningitis. © 2014 Elsevier Ltd.
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