Community-acquired acute bacterial meningitis (BM) in adults has a high mortality rate and poor outcome [1,2]. The main causative pathogens vary by country, with Streptococcus pneumoniae being the most common, followed by Neisseria meningitidis, which together account for 85% [[2], [3], [4], [5]]. Nationwide or large-scale surveillance has been conducted to assess the prognosis and sequelae of patients with pneumococcal meningitis (PnM), and the pathogen epidemiology, including serotypes, in the United States [6], United Kingdom [7], and the European Union [[8], [9], [10], [11], [12], [13], [14], [15]].

The prognosis and mortality rates of PnM are significantly poorer in adults than in children [16,17]. In addition to old age, underlying medical conditions, use of immunosuppressive chemotherapy, and chronic diseases are reportedly strong prognostic factors for PnM [2,13,14,18,19]. Currently, the number of countries recommending the pneumococcal conjugate vaccine (PCV) for high-risk adults is increasing.

S. pneumoniae has several virulence factors [[20], [21], [22], [23]]. The polysaccharide capsule is the main factor influencing invasion of the sterile site, blood, and cerebrospinal fluid (CSF); 100 capsule types or serotypes have been identified [24]. Serotype identification in the isolates from patients with PnM revealed a diversity of predominant serotypes. Notably, introduction of the 7-valent pneumococcal conjugate vaccine (PCV7), PCV10, or PCV13 have significantly decreased pneumococcal infections in children, indirectly resulting in a decrease in invasive pneumococcal disease (IPD) [[25], [26], [27], [28], [29], [30]] and meningitis [[31], [32], [33], [34]] in adults.

In Japan, the introduction of PCV7 for children started in November 2010, which was switched to PCV13 in November 2013. The PCV13 vaccination coverage has consistently exceeded 90%, resulting in drastic decrease in childhood IPD [35,36], meningitis [37,38], pneumonia [39], and acute otitis media [40]. The 23-valent pneumococcal polysaccharide vaccine (PPSV23) was introduced into the national immunization program for individuals aged ≥65 years (y) in October 2014, and in May 2020, PCV13 vaccination was approved for at-risk individuals; however, the vaccination rate remains low.

As a result, studies on PnM in adults conducted in Japan are rare. Before introducing PCV7 for children, Ishihara et al. [41] analyzed the prognostic factors and causative BM pathogens in 71 patients. S. pneumoniae and Staphylococcus aureus were the commonest pathogens (21%). Chang et al. [42] compared the epidemiological findings of PnM in adults with those IPD after the introduction of PCV13 for children, but did not identify risk factors affecting outcome.

In this study, we aimed to analyze the data of adult PnM collected by the pneumococcal meningitis surveillance group. To describe the risk factors associated with unfavorable outcomes in adults, we analyzed the patients’ data: i) age and underlying diseases/comorbidities, ii) blood and CSF examination data, and iii) serotypes, resistance genotypes, and antibiotic susceptibilities of the isolates.