Preventable Diseases - Pneumococcal infectionsPneumococcal infections are an important public health issue throughout the world. Serious diseases caused by the bacteria include pneumonia, meningitis and febrile bacteraemia, while the most common, but less serious, associated diseases are otitis media, sinusitis and bronchitis. It is estimated that almost one million children die each year due to invasive pneumococcal infection (Ref1a p113). Disease Streptococcus pneumoniae or more commonly, pneumococcus, is a Gram-positive bacteria that is transmitted by direct contact with the respiratory secretions of patients with the disease as well as healthy carriers. Almost 90 different serotypes of pneumococcus have so far been identified, and are distinguished on the basis of differences in composition of the outer capsule of the bacteria. In addition, it is this very capsule that protects the bacteria conferring virulence. The serotypes responsible for the most serious infections are 14, 3, 9, 19, 1, 6, 23 and 7 in adults, and 1, 5, 6, 14, 19, 7, 9, 12 and 23 in infants and children (Ref2a p114-115). Isolation of pneumococcus from the nose and pharynx with no associated disease (healthy carriers) is fairly common, involving some 5% of the general population. This level of asymptomatic carriage varies particularly as a function of age and environment. For instance, 27 to 58% of students and boarding school pupils are carriers, while between 30% and 70% of young children are carriers (Ref3a p632). Carriage of the organism is transient and generally lasts longer in children. The relationship between the development of natural immunity and carriage is as yet poorly understood (Ref4a p99). A very small number of serotypes is responsible for the majority of infections seen in man. It is estimated that around 11 of the most common serotypes are in fact responsible for 75% of invasive infections in children, and that the 10 most common forms are responsible for 62% of all infections throughout the world (Ref1b p114). Resistance of pneumococci to antibiotics (penicillin, cephalosporins, trimethoprime /sulfamethoxazole and macrolides) is on the increase throughout the world (Ref 1, c p114 ; Ref 5, a p223). Broadly speaking, pneumococcal infections are considered either invasive or non-invasive. Of the latter, otitis media, sinusitis, bronchitis and non-bacteraemic pneumonia are the most common and less serious forms. In addition, there are 3 main types of so-called invasive infection, namely bacteraemic pneumonia, meningitis and systemic infection or bacteraemia, which are more serious, especially in subjects with immune deficiency, spleen dysfunction, chronic organ failure, and more generally in children aged under 2 years and in elderly subjects aged over 65 years. Pneumococcal pneumonia is the most common form of bacterial pneumonia and is responsible for the majority of cases of community-acquired pneumonia. It may be sporadic but it occurs more frequently in winter. Pneumonia is often preceded by upper airways infection. It is characterised by high fever, shivers and a productive cough associated with pulmonary pain and breathing difficulties. The associated mortality rate is high, 10%, especially in infants aged under 1 year and in elderly subjects and subjects presenting a risk factor. Unsuccessfully treated pneumonia or pneumonia resistant to antibiotics may be complicated by extrapulmonary spread of the infection. Pneumococcal meningitis, like all forms of bacterial meningitis, constitutes a medical emergency, since it may worsen dramatically within space of a few hours. Respiratory symptoms or a simple sore throat frequently precede fever, headaches, vomiting and occasionally stiff neck. In older children and adults, signs may include disturbed consciousness, irritability, confusion or drowsiness, and can even include convulsions followed by coma. In infants, the clinical picture is less predictive. The mortality rate associated with pneumococcal meningitis is 30%, but may be up to 80% in elderly subjects (Ref6a p235). Systemic infection or pneumococcal bacteraemia is in fact a complication of non-invasive ENT infections, pneumonia or meningitis. It involves general spread of the bacteria throughout the body via the bloodstream and often leads to septicaemia. This systemic infection is more serious when it occurs in immunodepressed subjects without any defence against spread of the bacteria. The mortality rate is generally 20% but may be higher in patients at risk (Ref6b p234). 
Epidemiology Pneumococcal infections affect all members of the population, but have higher mortality and morbidity rates and children aged under 2 years and elderly subjects. In the developing countries, invasive pneumococcal infections constitute a considerable problem. For instance, it is estimated that throughout the world 2 million children aged under 5 years die of acute respiratory infections, the majority of which are caused by S. pneumoniae, without counting cases of non-invasive pneumonia or acute otitis media, which although less severe, nevertheless affect millions of children each year. In Europe and the United States, the annual incidence of invasive pneumococcal infections in adults ranges from 10 to 100 per 100 000 with a mortality rate of 10 to 50%; the highest values are seen in elderly subjects aged 65 years or more (Ref7a p189). Historical background to vaccination and vaccinal strategy Antibodies directed against the antigens present in the bacterial capsule confer protection specific for each serotype present in a particular vaccine. First of all, there is a polyoside vaccine containing 23 different serotypes (1, 2, 3, 4, 5, 6B, 7F, 8, 9N, 9V, 10A, 11A, 12F, 14, 15B, 17F, 18C, 19A, 19F, 20, 22F, 23F and 33F) and thus covering 90% of the serotypes incriminated in invasive infections. This vaccine is indicated from the age of 2 years and is particularly recommended for persons at high risk for invasive pneumococcal infection, more particularly subjects aged 65 years or more and those living in institutions. In adults, this vaccine has demonstrated general protective efficacy against invasive infections of between 21 and 81%, with the lowest degree of efficacy being seen in immunodepressed subjects. A single dose of vaccine is required at the initial vaccination (Ref8a p547). For infants aged under 2 years, the only available vaccine is a so-called conjugated vaccine containing 7 serotypes (4, 6B, 9V, 14, 18C, 19F and 23F). These serotypes represent 80% of invasive infections in young children in the USA. The vaccine has been shown to be clinically effective in more than 95% of children receiving the 3 doses required for initial vaccination (Ref9a p592). Conclusion The consequences of pneumococcal infections are considerable, in both the developed and the developing countries. Pneumococcal vaccination remains the most effective public health measure to reduce the incidence and impact of pneumococcal infections. Bibliography
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