Infants too young to receive the vaccination to prevent invasive pneumococcal disease nevertheless have experienced a decrease in the rate of the disease, possibly because they were less likely to contract it from others who were vaccinated, according to a study in the April 12 issue of JAMA.
The bacteria streptococcus pneumoniae causes invasive diseases such as pneumonia, meningitis and bacteremia in children. A heptavalent pneumococcal conjugate vaccine (PCV7) was licensed in February 2000 and recommended for all children aged two to 23 months in the
Katherine A. Poehling, MD, MPH, of Vanderbilt University School of Medicine in Nashville, and colleagues conducted a study to determine the rates of IPD among neonates and young infants before and after PCV7 was incorporated into the childhood immunization schedule in June 2000. The study included infants aged 0 to 90 days who resided in areas in eight U.S. states with active laboratory surveillance for invasive S pneumoniae infections from July 1, 1997, to June 30, 2004.
For this study group there were 146 IPD cases, 89 in the three years before PCV7 was introduced and 57 in the three years after PCV7 introduction. Isolated bacteremia occurred in 94 cases (64 percent), pneumonia in 27 (18 percent), meningitis in 22 (15 percent), and septic arthritis and/or osteomyelitis (an inflammation of bone and bone marrow) in three (2 percent). Among all the infants aged 0 to 90 days, the average rate of IPD decreased significantly from 11.8 per 100,000 live births in the pre-PCV7 years to 7.2 per 100,000 live births in the post-PCV7 years. The average rate of IPD decreased by 39 percent, 45 percent, and 32 percent for infants aged 0 to 30 days, 31 to 60 days, and 61 to 90 days, respectively.
Among black infants, average IPD rates decreased significantly from 17.1 per 100,000 live births during the pre-PCV7 years to 5.3 per 100,000 live births during the post-PCV7 years. Among white infants, average IPD rates decreased, though not significantly, from 9.6 to 6.8 per 100,000 live births, respectively. With the significant decrease in rates among black infants, the racial difference in IPD rates for black and white infants in the pre-PCV7 years was eliminated in the post-PCV7 years.
The 42 percent decrease in IPD rates among infants aged 0 to 60 days is similar to decreases reported for children older than 5 years and adults, who are not specific targets of PCV vaccination recommendations. These data are the first to suggest that neonates and infants too young to receive PCV7 are benefiting from herd immunity. Herd immunity occurs when vaccinated persons in a population indirectly protect unvaccinated members by impeding the transmission of the infectious agent in the population.
Although the exact mechanism of herd immunity is uncertain, one hypothesis is that vaccinated children are less likely to have nasal carriage of pneumococcus, and hence have less pneumococcal transmission to their contacts, the authors write.
Continued surveillance of IPD in this and other age groups is important to determine if this trend continues or if serotypes not included in PCV7 will emerge as an important cause of IPD in neonates and young infants, the researchers conclude.
Reference: JAMA. 2006;295:1668-1674.
Source: American Medical Association