[Focus on evaluating the pneumococcal vaccine response]

Streptococcus pneumoniae is an encapsulated bacterium responsible for invasive infections, particularly in the very young and very old, where morbidity and mortality rates are high. Its polysaccharide capsule, which defines the serotype and is the main virulence factor, also serves as an antigenic target for pneumococcal vaccines. There are two main categories of vaccines: the non-conjugated polysaccharide vaccine PPV-23, covering 23 serotypes but with limited and short-lived efficacy in children, and the more recent conjugate vaccines (PCV), offering longer protection, even in the pediatric population. Their serotype coverage has gradually expanded to include the latest generation PCV-21. According to the WHO, the evaluation of the vaccine response must be based both on the quantification of specific IgG to a given serotype by ELISA and on the qualitative study of antibody functionality via the opsonophagocytosis (OPA) test. ELISA thresholds have been defined to assess protection: 1 μg/mL in adults and 0.35 μg/mL in children. A patient is considered to have protective vaccine immunity if these levels are achieved for at least 70% of the serotypes tested (50% in children). For OPA, each laboratory defines limits of quantification (LOQ) for every serotype tested, and the response is considered protective if the opsonization index exceeds the LOQ in at least 70% of cases. Introduction of new vaccines is changing the epidemiology of these invasive infections, making it necessary to regularly update vaccination recommendations and expand the evaluation of vaccine response in order to include these new serotypes.