Impact of pneumococcal conjugate vaccine on infections caused by antibiotic-resistant Streptococcus pneumoniae

Studies have shown that vaccination with seven-valent pneumococcal conjugate vaccine (PCV7) results in a decline in nasopharyngeal carriage of penicillin-resistant Streptococcus pneumoniae, in carriage of vaccine-type pneumococci, and in replacement by non-vaccine serotypes. Vaccines can reduce pneumococcal resistance in vaccinated and unvaccinated populations by reducing the carriage of antibiotic-resistant serotypes, which protects the vaccinated population and prevents spread of disease to others, and by decreasing antibiotic resistance through overall reduction in antibiotic use. However, while reducing the level of vaccine serotypes and drug-resistant serotypes in the nasopharynx, PCV7 also causes non-vaccine pneumococci replacement. The impact of serotype replacement on disease is not clearly understood. Pelton et al. surveyed two communities shortly after the introduction of the PCV7 immunization programme and found that while colonization with vaccine serotypes declined from 22% to 2% from 2000 to 2003, prevalence of non-vaccine serotypes increased from 7% to 16%. Although penicillin-resistant colonizing S. pneumoniae isolates initially declined, penicillin-intermediate isolates increased 2 years following PCV7 introduction. The change was primarily accounted for by an increase in penicillin-intermediate serotype 19A. Serotype 19A is the only serotype not affected by PCV7 that is prevalent worldwide, clinically important, and highly multidrug-resistant. A study by Hicks et al. established serotype 19A as the predominant post-PCV7 cause of invasive pneumococcal disease (IPD) in children and the elderly. An increase in IPD rates caused by antibiotic-resistant serotype 19A isolates can also occur without vaccination; reports indicate increases in regions characterized by extensive antibiotic use, underscoring the importance of strategies to contain antibiotic resistance.