What happened to invasive pneumococcal infections after PCV7 was introduced?

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Researchers monitored the clinical and microbiologic features of invasive Streptococcus pneumoniae infections in children, including antimicrobial susceptibility and serogroup distribution, before and after routine introduction of the 7-valent pneumococcal conjugate vaccine (PCV7). This was a 9-year prospective surveillance study of all invasive pneumococcal infections in children, from January 1, 1994, through December 31, 2002. It included infants and children cared for at eight US children's hospitals whose invasive infections were confirmed by culture.

Compared with the 1994–2000 annual mean, the annual number of invasive pneumococcal infections in children aged 24 months or younger fell by 58% in 2001 and 66% in 2002. For PCV7 serogroups alone, cases in this age group fell by 63% and 77%, respectively. Serogroup 14 isolates showed the greatest decrease. The number of nonvaccine serogroup isolates in this age group rose by 28% in 2001 and 66% in 2002. Nonvaccine serogroups 15 and 33 showed the greatest increases in isolate numbers.

The proportion of all isolates nonsusceptible to penicillin increased each year from 1994 to 2000, plateaued at 45% in 2001, and fell to 33% in 2002. The decrease in penicillin nonsusceptibility occurred entirely among isolates with a penicillin minimum inhibitory concentration of at least 2 microg/mL. Penicillin nonsusceptibility increased slightly among nonvaccine-serotype isolates. Most infections after at least two PCV7 doses were caused by nonvaccine-serotype isolates.

The researchers concluded that, since PCV7 was introduced, invasive infections caused by vaccine-serogroup isolates had decreased by more than 75% among children aged 24 months or younger at the eight hospitals. Penicillin resistance also decreased in 2002 for the first time since their surveillance began in 1993–1994. However, replacement by serogroups 15 and 33 may be developing. Penicillin resistance also seemed to be increasing among nonvaccine serogroups. The researchers said surveillance must continue to detect changes in serotype distribution and antibiotic susceptibility.