There are two types of pneumococcal vaccines for Strep pneumoniae.
The pneumococcal conjugate vaccine (PCV13) is given to children under 5. It is also given to adults age >=65 who are immunocompromised (PPSV23 is also given afterwards).
The polysaccharide vaccine PPSV23 is given to adults, the indications for which I won't go into.
Since this patient is 2-years old he should have received the conjugate vaccine per CDC guidelines.
ihTs oby ahs ienisingmt udaesc by Sptre inmoepaeun, teh tmso ncomom seacu of fnitiecous tginiiensm ni en.laegr hTe aieccvn rfo perSt oeumnp is a repdacilhsyaco pternoi tgjuenoac aeccvi.n eTh treho joram betrcaai wtih a eicnavc ielk htsi si .H iueenflzna.
PCV13 for strep pneumo is polysaccharide conjugated to diptheria toxin-like protein (FA20-105). This is needed for a T-cell dependent response which allows for class switching and production of IgG Ab. This is a much more robust immune response. This provides memory and is why the vaccine is given at an early date.
On the contrary, antigens lacking a protein (such as the capsular polysaccharide) such as PPSV23 vaccine are not presented to T-cells, and therefore have a much weaker response + do not class switch.