Streptococcus pneumoniae (Spn) is a lancet-shaped, Gram-positive, facultative anaerobic bacterium first identified in 1881¹. It is a major opportunistic pathogen that commonly colonizes the human nasopharynx, with colonization rates ranging from 27% to 65% in children and less than 10% in adults ^1,2. Generally, it does not cause disease, depending on host characteristics and changes in the colonization environment¹.

Spn is widely distributed in nature, with humans as its only host. It spreads from person to person, typically through respiratory droplets or by colonizing the upper respiratory tract, with infections more common in winter and early spring¹. Although the infectious period of pneumococcal diseases is not well-defined, it is presumed that transmission can occur as long as the pathogen is present in respiratory secretions³.

Pneumococcal disease (PD) refers to a group of infections caused by Spn, categorized into invasive pneumococcal disease (IPD) and non-invasive pneumococcal disease (NIPD) based on the site of infection⁴. NIPD is more common but often undiagnosed due to the lack of pathogen testing, whereas IPD, though less frequent, presents more severe symptoms and higher mortality risk⁵.

The migration of Spn to sterile tissues and organs is the main cause of all pneumococcal diseases⁶. Spn colonizing the nasopharynx can cause invasive infections if not cleared by the immune system⁷, leading to conditions like pneumonia, bacteremia, and meningitis.

Risk factors for IPD include immunosuppression due to disease or medication, functional or anatomical asplenia, chronic heart diseases, lung diseases (including asthma), liver or kidney diseases, smoking, alcoholism, cerebrospinal fluid leaks, cochlear implants, and attending day-care or other preschool childcare settings⁸.

Nearly 100 serotypes of Spn are known, most capable of causing disease. Approximately 62% of global IPD cases are attributed to the 10 most common serotypes⁶. The prevalent serotypes among carriers are also the primary causes of infections. While colonization does not always lead to disease, it is a critical prerequisite for PD⁹. Different serotypes may preferentially cause various diseases or conditions, such as asymptomatic colonization, meningitis, or pneumonia.

The distribution of pathogenic Spn serotypes varies by geography, age, and clinical presentation. Globally, the seven most common serotypes in children under five are 1, 5, 6A, 6B, 14, 19F, and 23F, accounting for over half of IPD cases¹⁰. A systematic review published in 2010 described the serotype distribution among children under five with IPD worldwide from 1980 to 2007 (Figure 2) ¹⁰. In Southeast Asia, the most common serogroups and serotypes among reported IPD cases are serogroup 19 (16.2%), serogroup 6 (14.6%), serogroup 23 (12.5%), serotype 14 (9.2%), serotype 1 (5.7%), and serotype 3 (4.0%)¹¹. In China, a literature review from 1980 to 2008 indicated that serotypes 14, 19A, and 19F were most prevalent among pneumonia and meningitis cases in children under five¹². Additionally, studies have found that the common serotypes causing IPD in children in mainland China include 19A, 19F, 14, 23F, and 6B¹³.

Currently available pneumococcal vaccines include the pneumococcal polysaccharide vaccine (PPV) and the pneumococcal conjugate vaccine (PCV), which cover the most common serotypes causing PD. The serotypes covered vary among different vaccines (Figure 3).

Content Editor: Tianyi Deng, Ziqi Liu

Page Editor: Ziqi Liu

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References

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