Changing serotypes causing childhood invasive pneumococcal disease: Massachusetts, 2001-2007

Heptavalent pneumococcal conjugate vaccine (PCV7) was licensed in the United States in February 2000 and distributed in Massachusetts, starting in July 2000 for universal administration to children <2 years of age and selected use in children 2 to 5 years of age. Statewide surveillance was begun in October 2001 to monitor incidence of invasive disease, serotypes causing disease, antimicrobial susceptibility, and risk features associated with ongoing childhood invasive pneumococcal disease (IPD).

Massachusetts pediatric IPD cases were identified via enhanced passive surveillance of microbiology laboratory reports of pneumococcal isolates from sterile body sites of children <18 years. Serotyping and antimicrobial susceptibility testing were performed on isolates of Streptococcus pneumoniae from normally sterile body fluid. Demographic and clinical data, were collected via follow-up telephone interviews with primary care providers. Incidence rates were derived using Census 2000 denominators.

A total of 586 IP cases were reported between October 2001 and September 2007. Among 433 (74%) cases with isolates available for serotyping, 366 (85%) were caused by non-PCV7 serotypes and 67 (15%) were caused by PCV7 serotypes. 19A was the most common cause of any serotype identified episode of IPD (28%). IPD incidence was stable during the 6 study years because, although IPD cases due to PCV7-serotypes decreased, the incidence of non-PCV7 serotype IPD increased from 3.0 cases/100,000 children less than 18 years to a high of 5.3 cases/100,000 during 2005 to 06. Since 2005, ceftriaxone non-susceptible isolates comprised approximately 20% of isolates. There were 8 (1.4%) fatalities from IPD; 5 deaths occurred in children <1 year of age.

Non-PCV7 serotype IPD, especially serotype 19A disease, increased during the 2001 to 2007 surveillance period in Massachusetts. The proportion of ceftriaxone non susceptible isolates also increased, particularly since 2005. Ongoing surveillance will be necessary to detect future increases in IPD incidence or antibiotic resistance in Massachusetts children, changes which have important implications for introduction of second generation pneumococcal conjugate vaccines and presumptive antibiotic choices in critically ill children.