A study led by the Medical Research Council in The Gambia in collaboration with the London School of Hygiene & Tropical Medicine and published in this week's PLoS Medicine shows for the first time in Africa, that vaccinating young children against the pneumococcus (a bacterium that can cause fatal infections) causes a herd effect in which the entire community is protected against this infection.

In a randomised, controlled trial involving 21 villages in rural Gambia, the authors showed that vaccination of young children reduced carriage of serotype pneumococci (the type of pneumoccus contained in the vaccine) not only in the vaccinated children but also in vaccinated and non-vaccinated older children and adults. Furthermore, the study showed that vaccinating whole communities did not result in a community wide increase in carriage of nonvaccine serotype pneumococci (other types of pneumococci that are not included in the vaccine) in the two-year period after vaccination.

As an alternative method to long-term observational studies and to anticipate the potential long term effects of the introduction of pneumococcal conjugate vaccination in sub-Saharan Africa, the authors conducted a village-randomized trial in The Gambia in which the whole population of some villages were immunized with pneumococcal conjugate vaccine (PCV-7) (vaccinated villages) and in other villages only children <30 months of age at vaccination or other children born during the trial received PCV-7 while older residents received a control vaccine (control villages).

Before immunization, the overall prevalence of pneumococcal carriage in both groups was high at 71.1% and decreased with age. The overall prevalence of pneumococcal carriage at 6, 12, and 22 months after vaccination was similar between vaccinated and control villages, showing a marked fall. However, the prevalence of carriage of vaccine serotype pneumococci was significantly lower in vaccinated than in control villages in all surveys for all age groups. The authors also found that the overall prevalence of pneumococcal carriage fell markedly after vaccination and reached minimum levels at 12 months after vaccination in both study arms and in all age groups.

The authors, led by Anna Roca from the MRC in The Gambia, say: "We have shown here, to our knowledge for the first time in Africa, a herd effect on carriage of [vaccine serotype] pneumococci, which might be translated into herd protection against [invasive pneumococcal disease] in adults, following routine immunization of infants and ."

They conclude: "Our findings are relevant for other countries in Africa contemplating the introduction of [pneumococcal conjugated vaccines] where the pattern of pneumococcal infection is similar to that in The Gambia."

More information: Roca A, Hill PC, Townend J, Egere U, Antonio M, et al. (2011) Effects of Community-Wide Vaccination with PCV-7 on Pneumococcal Nasopharyngeal Carriage in The Gambia: A Cluster-Randomized Trial. PLoS Med 8(10): e1001107. doi:10.1371/journal.pmed.1001107