Increasing Prevalence of Serotype 6C After Introduction of Pneumococcal Vaccine

By Will Boggs, MD

NEW YORK (Reuters Health) Jan 05 - Introduction of the 7-valent conjugate vaccine (PCV7) has led to increases in invasive pneumococcal disease caused by serotype 6C, but decreases in invasive pneumococcal disease caused by serotype 6A, according to a new report.

"Pneumococci are evading the pneumococcal conjugate vaccine, which was introduced in 2000," Dr. Moon H. Nahm from University of Alabama at Birmingham told Reuters Health. "In view of this, there is an increasing need to reformulate the 7-valent conjugate vaccine."

Dr. Nahm and colleagues investigated the prevalence of the 6A and 6C serotypes among invasive pneumococcal disease isolates and the capacity of serum specimens from vaccines to opsonize 6A and 6C serotypes in vitro.

Among children under age 5 years, the incidence of serotype 6A invasive pneumococcal disease decreased by 91% from 2003 to 2006, the authors report in the December 15th issue of The Journal of Infectious Diseases, while there was no consistent trend in the incidence of serotype 6C invasive pneumococcal disease in this age group.

In older children and adults, the incidence of serotype 6A invasive pneumococcal disease decreased by 58% between the introduction of PCV7 and 2006, while the rate of invasive pneumococcal disease caused by serotype 6C increased by 158%.

Susceptibility to penicillin was higher among 6C isolates (71%) than among 6A isolates (51%), as was susceptibility to erythromycin (70% versus 50%, respectively).

The ability of serum from vaccinated children to opsonize serotype 6A and serotype 6B (which is included in the vaccine) was significantly higher than that for serotype 6C, whose median opsonization index was 10-100 fold less than those for serotypes 6A and 6B.

"These results suggest that the two currently available pneumococcal vaccines, both of which contain the serotype 6B capsule, elicit significant cross-opsonic antibodies to 6A but not to 6C," the investigators say.

"Until we discovered 6C in 2007, 6C was 'mis-typed' as 6A," Dr. Nahm explained. "This is surprising because 6A (and probably 6C) serotype is common and has been known to scientists for more than 80 years."

"This further emphasizes how diverse pneumococci are, how little we know about them," and how difficult it is to control pneumococcal infections, Dr. Nahm concluded.

J Infect Dis 2008;198:1818-1822.