Invasive Pneumococcal Disease in Children 5 Years After Conjugate Vaccine:

Editorial Note

The introduction of PCV7 in the United States led to substantial reductions in IPD among the target population (children aged <5 years), and the benefits of vaccination with PCV7 remain evident 5 years later. Overall IPD rates in 2005 were 77% lower for children aged <5 years compared with average rates in 1998-1999. Results of this report indicate that overall IPD rates in 2005 among children aged <5 years (23.2 per 100,000) remain below the Healthy People 2010 objective (14-5a) of 46 per 100,000.† Findings in this report are supported by a growing body of evidence of the beneficial effects of PCV7 introduction on noninvasive pneumococcal disease; recent studies report dramatic declines in all-cause pneumonia and pneumococcal pneumonia in the PCV7 target population[4] and reductions in frequent otitis media.[5]

Results of this analysis also demonstrate that, after reductions in IPD rates among children targeted for vaccination during the first 3 years after PCV7 introduction, further reductions were offset by increases in non-PCV7 serotypes. PCV7-type IPD rates continued to decline, but overall IPD rates leveled off during 2002-2005.

Since the introduction of PCV7, a shift in the distribution of serotypes causing IPD in this age group has occurred; only 7% of cases were caused by PCV7 serotypes in 2005, compared with approximately 80% during 1998-1999. Serotype 19A was the most common serotype causing IPD among children in 2005, and changes in non-PCV7-type IPD were largely driven by increases in IPD caused by this serotype. Even though absolute increases in rates of non-PCV7-type IPD remain modest relative to reductions in PCV7-type IPD, an estimated 1,200 additional cases of IPD not preventable by PCV7 occurred among children aged <5 years in 2005, compared with prevaccine baseline. Increases in non-PCV7-type disease among vaccinated and unvaccinated populations have been reported since PCV7 introduction.[2,6] The results of this analysis indicate that, in the general U.S. population, these increases have been small relative to declines in PCV7-type disease.

The findings in this report are subject to at least one limitation. The relationships between PCV7 coverage or numbers of PCV7 doses received and PCV7 effects could not be explored directly. Vaccination status was not available for persons with IPD, and PCV7 coverage estimates from a different data source were used to estimate PCV7 direct effects. Therefore, the level of PCV7 coverage needed to induce indirect (i.e., herd) effects is unknown. In this analysis, a range of PCV7 coverage estimates (≥3 or ≥1 doses) for each birth cohort was used to obtain a range of estimates for the direct and indirect effects of PCV7.

Initial substantial declines in IPD after PCV7 introduction are strikingly similar to reductions in invasive disease caused by Haemophilus influenzae type b (Hib) after Hib conjugate vaccine introduction in the United States.[7] Increases in disease caused by H. influenzae serotypes other than type b were a concern; however, the experience with Hib conjugate vaccine indicates that non-type b H. influenzae were not as successful as Hib in causing invasive disease.[8] In contrast with the six serotypes of H. influenzae, approximately 90 pneumococcal serotypes have been described. Fortunately, different pneumococcal serotypes also vary in their ability to cause invasive disease.[9]

The findings in this report suggest that expanded-valency conjugate vaccines for children that also provide protection against serotype 19A would be useful to improve prevention of IPD. A 13-valent conjugate vaccine containing type 19A polysaccharide and a 10-valent conjugate vaccine, which might provide cross protection against type 19A,[10] are currently in clinical trials. Continued surveillance for IPD is crucial to provide information on emerging pneumococcal serotypes and the optimal composition of future conjugate vaccines.

sourc: http://www.medscape.com/viewarticle/571344_2

†US Department of Health and Human Services. Healthy people 2010 (conference ed, in 2 vols). Washington, DC: US Department of Health and Human Services; 2000. Available at http://www.healthypeople.gov.