Background: Herd protection and serotype replacement disease following introduction of pneumococcal conjugate vaccine (PCV) are attributable to the vaccine's impact on colonization. Prior to vaccine introduction in Kenya, we did an epidemiological study to estimate the rate of pneumococcal acquisition, by serotype, in an uncolonized population.
Methods: Nasopharyngeal swab specimens were taken from newborns aged ≤ 7 days and weekly thereafter for 13 weeks. Parents, and siblings aged <10 years, were swabbed at monthly intervals. Swabs were transported in skim milk-tryptone-glucose-glycerin and cultured on gentamicin blood agar. Pneumococci were serotyped by the Quellung reaction. We used survival analysis and Cox regression analysis to examine serotype-specific acquisition rates and risk factors and calculated transmission probabilities from the pattern of acquisitions within the family.
Results: Of 1404 infants recruited, 887 were colonized by 3 months of age, with the earliest acquisition detected on the first day of life. The median time to acquisition was 38.5 days. The pneumococcal acquisition rate was 0.0189 acquisitions/day (95% confidence interval, .0177-.0202 acquisitions/day). Serotype-specific acquisition rates varied from 0.00002-0.0025 acquisitions/day among 49 different serotypes. Season, coryza, and exposure to cigarettes, cooking fumes, and other children in the home were each significant risk factors for acquisition. The transmission probability per 30-day duration of contact with a carrier was 0.23 (95% CI, .20-.26).
Conclusions: Newborn infants in Kilifi have high rates of nasopharyngeal acquisition of pneumococci. Half of these acquisitions involve serotypes not included in any current vaccine. Several risk factors are modifiable through intervention. Newborns represent a consistent population of pneumococcus-naive individuals in which to estimate the impact of PCV on transmission.