Serotype replacement and multiple resistance in Streptococcus pneumoniae after the introduction of the conjugate pneumococcal vaccine.

BACKGROUND

The pneumococcal conjugate vaccine (PCV7), introduced in February 2000, covered 82% of the U.S. pediatric population in 2005. Changes over time in serogroup prevalence and multidrug-resistance (MR) to antimicrobials were evaluated using the U.S. SENTRY surveillance program.

METHODS

The study included 704 U.S. isolates, with equal numbers before (1998-1999) and after the introduction of the vaccine (2003-2004). Demographic data, serotype, and resistance profiles for five antimicrobial classes were analyzed. Strains displaying resistance to >or=2 classes were considered MR. Statistical analysis was performed using logistic regression.

RESULTS

Prevalence of PCV7 serotypes was 68.5% in the prevaccine years, dropping to 29.3% in the postvaccine period. Among PCV7 serotypes, only 19F persisted, with nonvaccine (NV) serotype 19nonF strains increasing from 3% to 20% of total p<0.001. NV serotypes were 1.9 times (95% confidence interval [CI] 1.1-3.1) more likely to acquire MR over time. Although PCV7 serotypes constituted 84% of all MR isolates in the prevaccine era, MR was unchanged in the postvaccine period due to increased prevalence and acquisition of resistance by NV serotypes. MR among invasive isolates did not change over time, but increased among noninvasive NV isolates by 17% (95% CI 12-22%).

CONCLUSIONS

The complete switch in prevalence of PCV7 by NV serotypes has been aided by a herd effect in adults and older children. NV serotypes have acquired MR at a rate that is proportional to the replacement process.