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Humans, especially young children, are considered the only reservoir of Streptococcus pneumoniae. Pneumococcal nasopharyngeal colonisation is very common and usually asymptomatic, but it also represents the primordial step in the development of pneumococcal diseases.1 Nasopharyngeal colonisation is very prevalent among young children but appears to be much less common among adults. Yet, adults, especially older adults, are disproportionally affected by pneumococcal diseases. Close interaction with young children is a recognised source of exposure to pneumococcus among adults, and previous studies have documented the associations with colonisation and also disease among exposed adults.2–4
The chain of transmission of pneumococcal colonisation and its connection with disease has been drastically transformed with the introduction of the pneumococcal conjugate vaccines. Unlike non-conjugated polysaccharide vaccines, pneumococcal conjugate vaccines elicit a strong immune response in vaccinated young children and provide protection against colonisation with serotypes included in the vaccine. This leads not only to protection of vaccinated children, but by targeting the main reservoir of transmission, vaccination also prevents transmission of pneumococcus to others in contact with vaccinated children. Indeed, widespread vaccination of infants with conjugate vaccines has led first to reductions and ultimately to virtual elimination of the circulation of serotypes included in the vaccines.5–8
Routine vaccination of infants with pneumococcal conjugate vaccines has led to profound changes in the ecology of S. pneumoniae and the distribution of related infections. As the circulation of vaccine serotypes was reduced due to the powerful biological pressure of the vaccine, disease caused by those retreating vaccine serotypes has declined in both vaccinated and unvaccinated …
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