Table 2.

Summary of different pneumococcal vaccine candidates.

Pneumococcal vaccine candidates	Type	Status	Reference
23-valent pneumococcal polysaccharide vaccine (PPV23)	Polysaccharide	In use	Robbins et al., 1983
Pneumococcal conjugate vaccines (PCV-7, PCV-10, and PCV-13)	Conjugate	In use	Hsu et al., 2010; Berical et al., 2016
Pneumococcal conjugate vaccines (PCV-15 and PCV-20)	Conjugate	In clinical trial	Pichichero, 2017
Pneumococcal protein vaccines (PspA, PhtD, StkP, and pneumolysin)	Protein	In clinical trial	Ginsburg et al., 2012; Lagousi et al., 2019

PspA, pneumococcal surface protein A; PhtD, pneumococcal histidine triad protein D; StkP, serine/threonine kinase protein.