The effect of the 7-valent conjugate vaccine (PCV7) on the incidence of pneumococcal infections world-wide

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Streptococcus pneumoniae (pneumococcus) is responsible for the death of about 1 million children world-wide every year, causing community-acquired pneumonia, or invasive pneumococcal diseases. The highest disease burden is among young children or the elderly. The polysaccharide vaccine (Pneumovax, Merck) contains 23 capsular antigens, but this is ineffective in children less than 2 years old. Therefore a new vaccine was developed (Prevenar, Wyeth), in which seven capsular polysaccharides are conjugated to a carrier protein. Prevenar was first introduced in the USA in 2000 and later in several European countries. Many studies have considered the consequences of the vaccination with it. The major benefit is the general decrease in infections caused by the vaccine types, and because of herd immunity, also in unvaccinated children and adults. Conjugate vaccines are also able to reduce the nasopharyngeal carriage, which is a great problem with pneumococci. Additionally, the vaccine influenced pneumococcal resistance by the suppression of resistant clones. On the contrary, however, an increase in infections due to nonvaccine types has been detected everywhere. More worryingly, often a vaccine-induced serotype switch has been observed, that is the strains change their capsular type to a non-vaccine type, while retaining their genetic background, resistance or virulence.

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