Reduction of carriage and transmission of Streptococcus pneumoniae: the beneficial "side effect" of pneumococcal conjugate vaccine.

Received 30 June 2008; accepted 1 July 2008; electronically published 9 September 2008. Reprints or correspondence: Prof. Chiara Azzari, Dept. of Pediatrics, Anna Meyer Children’s Hospital, Viale Pieraccini 24, 50139 Firenze, Italy (chiara.azzari@unifi.it). Clinical Infectious Diseases 2008; 47:997–9 2008 by the Infectious Diseases Society of America. All rights reserved. 1058-4838/2008/4708-0002$15.00 DOI: 10.1086/591967 In 2000, the 7-valent polysaccharide pneumococcal conjugate vaccine (PCV7) was recommended in the United States for routine use in all infants and children aged ! 2 years and in high-risk children aged 4 years [1]. Several controlled clinical trials have shown that the vaccine is highly efficacious in preventing invasive pneumococcal disease (IPD) in vaccinated children. The incidence of vaccine-type IPD among children aged ! 5 years decreased 94% from 1998 to 2003 [2] and plateaued in 2002–2005 [3]. Use of the vaccine also reduced the incidence of IPD among unvaccinated populations. In fact, from 1998 to 2003, the incidence of vaccine-type IPD decreased up to 60% among persons aged 15 years [2–4], and a statistically significant decrease of ∼40% among infants too young to be vaccinated was also found [5]. These data suggest a key role for indirect effects of PCV7, and it has been suggested that both reduction in nasopharyngeal carriage and reduction in transmission of vaccine-type pneumococci could be possible mechanisms of herd immunity to Streptococcus pneumoniae. Nasopharyngeal carriage at every age is well documented, but it is particularly frequent among children aged ! 2 years, children attending child day care centers [6], children living with tobacco-smoking parents, and children whose families have a higher number of preschool-attending children. However, the true role of nasopharyngeal carriage in herd immunity still needs to be completely clarified. In this issue of Clinical Infectious Diseases, Millar et al. [7] report data on the indirect effect of PCV7 on carriage among unvaccinated adults and children living with children who have been vaccinated. In their study, conducted at the Navajo and White Mountain Apache reservations from 1997 through 2000, they evaluated both vaccinated and unvaccinated children and their household members (children aged ! 18 years and adults aged 18 years) for a study of carriage. They collected single nasopharyngeal swab specimens from the children and both nasopharyngeal and oropharyngeal swab specimens from the adults. Isolates, obtained on sheep-blood agar plates, were serotyped by the Quellung reaction. Pneumococcal serotypes were divided into vaccine-type serotypes (4, 6B, 9V, 14, 18C, 19F, and 23F), vaccine-associated serotypes (serotypes in serogroups 6, 9, 18, 19 or 23 that are not vaccine types), and non–vaccine type/non–vaccine associated (NVT/NVA) serotypes (all others). The results by Millar et al. [7] demonstrate that adults and unvaccinated children aged ! 5 years who were living in households with a PCV7 vaccinee were less likely to be colonized with vaccinetype pneumococci than were those living in a household with unvaccinated children. These results are consistent with those obtained by Hammitt et al. [8] in a community-based, observational study of adult carriage, and a similar conclusion was reached, that the protective effect in unvaccinated populations is attributable to reduced exposure and reduced transmission when exposure occurs. Millar et al. [7] also observed that there was no difference in carriage for children aged 5– 17 years and comment that the degree of indirect protection may be smaller for this age group because the older children attend school and thus have much less exposure to the youngest children than do the adults. Which mechanisms are involved in this indirect effect? The hypotheses suggest first the involvement of reduced acquisition of vaccine-type serotype pneumo-