There are many serotypes of Streptococcus pneumoniae and two types of vaccine currently available. A 23-valent polysacharide vaccine is indicated for high-risk groups of adult and older children, where as a 7-valent (soon to be 10-valent and 13-valent) polysacharide-protein conjugate vaccine is used in children under 9 years of age.
The 23-valent unconjugated polysaccharide vaccine
The 23-valent vaccine contains a purified capsular polysaccharide antigen from 23 serotypes of pneumococcus that cause 88% of invasive pneumococcal disease, is approved for use in adults and children older than two years of age.
The vaccine is recommended for adults 65 years or older and persons two years and older with chronic illness, anatomic or functional asplenia, HIV infection (especially if HIV viral load < 100 000 copies/ml) or other immunosuppressive conditions. This vaccine is not recommended for routine use in children.
Pneumococcal conjugate vaccines
The 7-valent conjugate vaccine (PCV-7) consists of pneumococcal polysaccharide conjugated to a diphtheria protein (CRM197) and seven serotypes of the pneumococcus (4, 6B, 9V,14, 18C, 19F, and 23F). These serotypes cause 86% of bacteraemia and 83% of meningitis among children younger than 6 years of age in the United States.
The results of the first PCV-7 trial, reported in 2000, showed a protective efficacy of 93.9% against invasive pneumococcal disease caused by vaccine serotypes among children who received at least one dose of the vaccine. Following the trial (which was conducted in the United States), PCV-7 was introduced into the United States national immunisation programme in 2000. Five years after the introduction, disease surveillance results showed a 77% decrease in invasive pneumococcal disease among children aged 5 years and younger. The incidence among people aged above 5 years who did not receive the vaccine, also reduced substantially, probably as a result of decreased transmission from younger vaccinated children (i.e. "indirect protection").
In low and middle-income countries, pneomococcal vaccine trials have been conducted in The Gambia and South Africa. A randomised controlled trial in the Gambia found that the efficacy of 3 doses of PCV-9 against vaccine-type invasive pneumococcal disease was 77%. A similar study in South Africa found 83% protective efficacy against vaccine-type invasive pneumococcal disease in HIV-negative children and 65% efficacy in HIV-positive children.
PCV-7 is well tolerated and has a good safety profile. It induces a T-cell dependent immune response characterised by immune memory as well as a booster antibody response on subsequent challenge with the pneumococcal polysaccharides included in the vaccine. It also stimulates mucosal immunity, resulting in reduced nasopharyngeal carriage. The indrect protection effect observed with this vaccine is most likely the result of reduced transmission of vaccine-type pneumococci in the community as a result of decreased carriage in young children and reduced transmission in the community. PCV-7 is highly immunogenic in all age groups, but it is currently licensed for use only in children aged less than 9 years. In young children, protection against invasive pneumococcal disease caused by vaccine serotypes may exceed 90%; however, corresponding protection against acute otitis media due to vaccine-serotype is 55%.
Two new vaccines (a 10-valent and a 13-valent pneumococcal conjugate vaccine) have now replaced the 7-valent vaccine.