Pneumococcal bacteraemia in Belgium (1994-2004): the pre-conjugate vaccine era

Objectives: To analyse the evolution of antibiotic resistance and serotype distribution in pneumococcal bacteraemia before the introduction of the 7-valent pneumococcal conjugate vaccine (7PCV). Methods: Serotyping and susceptibility testing for penicillin and erythromycin were performed on 11 163 blood isolates of Streptococcus pneumoniae collected between 1994 and 2004. Results: Penicillin resistance rose from 4.7% in 1994 to 15.2% (P = 0.001) in 2000 and decreased thereafter to 9.7% (P = 0.001) in 2004. Erythromycin resistance rose from 20.4% in 1994 to 34.4% (P = 0.001 in 2001) and stabilized thereafter. Paediatric serogroups/serotypes (SGTs) (SGTs 6, 9, 14, 19 and 23; 47.4% of bacteraemic isolates), characterized by decreasing penicillin and stable erythromycin resistance, decreased by the end of the study period. Non-paediatric SGTs (SGTs 1, 5 and 7; 20.5% of bacteraemic isolates), characterized by temporal fluctuations, the absence of penicillin resistance and rising erythromycin resistance, increased significantly by the end of the study period. The age group 5-59 years was most affected by these changes. Compared with the age group < 5 years, the age group >= 60 years has a relative risk of 7.6 (CI: 4-11.6; P = 0.001) of having a pneumococcal bacteraemia with SGT 3. The overall coverage rate of bacteraemic SGTs offered by the 7PCV is 81.9% in the < 5 years age group with an additional coverage of 11.6% offered by the 13-valent pneumococcal conjugate vaccine (13PCV) in this age group (P = 0.001). The coverage of bacteraemic isolates offered by the 13PCV and 23-valent pneumococcal polysaccharide vaccine (23PPV) in the >= 60 years age group is 78.7% and 95%, respectively. Conclusions: Although the 7PCV was not used in Belgium during the study period, the overall prevalence in paediatric SGTs decreased significantly. This may be linked to secular trends in SGTs not included in the 7PCV and/or herd effects at the international level. Overall penicillin resistance decreased as well and this may be due to a shift towards susceptible serotypes and/or a decrease in antibiotic use in our country. Antibiotic resistance and trends in SGT distribution will need further surveillance in order to assess 7PCV effects on pneumococcal epidemiology, to adapt future vaccine formulations and to target the population at risk.