Serotype replacement means the replacement of the serotypes in the vaccine with serotypes not included in the vaccine. In a vaccinated population the replacement usually happenes reasonably soon after the beginning of the vaccination program. Because of serotype replacement, vaccine effectiveness against invasive disease has been milder than expected. Nurhonen and Auranen have built a model [1], that describes the mechanism of replacement in a vaccinated population. Using the model it is possible to calculate predicted effects of different serotype combinations in a conjugate vaccine against pneumococcal disease in a population and find the serotypes that would be most beneficial additions to current vaccine compositions to prevent invasive pneumococcus disease.
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Pneumococcus carriage (on x-axis) and incidences per carriage (on y-axis) for serotypes in vaccine (VT) and serotypes the vaccine does not affect (NVT) before the vaccination program (chart A) and after (chart B). Incidences (DNVT and DVT) are a result of multiplying the values on the axes and are displayed as rectangular areas. The carriage of serotypes in vaccines has been divided into three sections according to the effect of carriage in a population: (i) Carriage removed by vaccine changed into another carriage (VT*pq); (ii) Carriage removed by vaccine and not replaced by new carriage (VT*(1-p)q); (iii) Carriage not removed by vaccine (VT*(1-q)). In this chart q is carriage removed by vacccine and p is is carriage removed by vaccine and replaced by new carriage not included in the vaccine's serotypes. The incindences decrease because of the difference in virulency of the the removed and replaced carriage. The disappearence of incidences is shown as a blue rectangle (chart B).
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