Non-PCV7 serotypes show increasing penicillin resistance

The percentage of non-pneumococcal conjugate vaccine serotypes 15A, 23A, 35B and 6C that are nonsusceptible to penicillin rose significantly by 2007 when compared with data from 1999, according to results from a recent study.

CDC researchers obtained information from the CDC’s Active Bacterial Core surveillance on cases of invasive pneumococcal disease (IPD) during 2007 and compared it with reports from 1999.

Of the 3,511 IPD isolates reported in 2007, 898 were penicillin nonsusceptible and 2,619 were from non-PCV7 (Prevnar, Wyeth) serotypes. Non-PCV7 serotypes — excluding serotype 19A — were responsible for 56% of IPD cases among children aged younger than 5 years and 77% in those aged 5 and older, demonstrating significant increases from 15% and 41%, respectively, in 1999.

Results also indicated that serotype 19A accounted for 53% of penicillin nonsusceptible isolates in 2007, whereas non-PCV7 serotypes accounted for 40%. The researchers noted that, excluding 19A, types 15A (11%), 23A (8%), 35B (8%) and 6C (5%) were responsible for most penicillin nonsusceptible-related IPD. The sharpest increase between 1999 and 2007 occurred in serotypes 15A (25% vs. 83.9%; P=.001) and 23A (26.3% vs. 67.6%; P=.002).

Further analysis revealed that higher incidence of IPD and increased penicillin resistance in these serotypes was unrelated to recent capsular switching events, according to the researchers.

“A factor that has potentially led to the modestly increased disease rates for theses strains in the post-PCV7 era is that decreased carriage of PCV7-serotype strains has provided a biologic niche for pre-existing nonvaccine-serotype strains, especially those that are nonsusceptible to antibiotics,” the researchers wrote. “These serotypes are not included in the soon-to-be-introduced 13-valent conjugate vaccine, which makes their continued monitoring important.”

Gertz RE. J Infect Dis. 2010; 201:770-775.