Routine use of seven-valent pneumococcal conjugate vaccine in young children has dramatically reduced the incidence of vaccine-type and overall invasive pneumococcal disease in children and adults, the Centers for Disease Control and Prevention reported.
The most substantial decline in the rate of vaccine-type disease has been in the target population of children less than 5 years old, according to an analysis comparing disease rates in 2003 with those in 1998–1999, when the Prevnar vaccine was not available.
In this age group, vaccine-type invasive pneumococcal disease (IPD) decreased 94% from 80 cases per 100,000 population to 4.6 cases (MMWR 2005;54:893–7).
Incidence rates of vaccine-type IPD also declined substantially among individuals outside the target population, with the largest absolute rate reduction occurring in those 65 years and older.
The routine use of the vaccine prevented 29,599 expected vaccine-type IPD cases in 2003, according to the analysis conducted by the Active Bacterial Core surveillance of the Emerging Infections Program Network in cooperation with the CDC.
An estimated 9,140 cases of vaccine-type IPD were directly prevented by vaccinating children less than 5 years old. An additional 20,459 cases (69%) were prevented through indirect effects of the vaccine across all ages.
“This is not the first vaccine to be shown to have a herd immunity effect, but the magnitude of the effect being so large is what makes this vaccine important,” Deron C. Burton, M.D., of the CDC's division of bacterial and mycotic diseases, said in an interview.
Among children less than 5 years old and adults aged 40 years or older, the reduction in vaccine-type IPD was offset by an increase in disease caused by pneumococcal serotypes not included in the seven-valent vaccine. There were a total of 4,721 projected additional cases of nonvaccine-type IPD across all age groups in 2003, compared with baseline.
Dr. Burton said ongoing surveillance will be required to determine if the replacement disease effect stays small or if it increases in magnitude and begins to erode some of the benefits of the vaccine.