Impact of pneumococcal conjugate vaccination on pneumococcal nasopharyngeal carriage in the Gambia: Population-based cross-sectional surveys

Background: The introduction of pneumococcal conjugate vaccines (PCV) has reduced carriage of vaccine -type (VT) pneumococci in many settings. We determined the impact of The Gambia's national PCV programme on carriage of VT pneumococci in the population. Methods: Seven-valent PCV (PCV7) was introduced in August 2009 without catch-up and with doses scheduled at 2, 3, 4 months of age; it was replaced by PCV13 in May 2011. We did cross-sectional carriage surveys in 2009, 2015, and 2017 in age -stratified, population -based samples. Nasopharyngeal specimens were collected and processed according to WHO guidelines. We calculated observed and adjusted prevalence ratios (PR) of VT carriage before and after PCV introduction. Findings: We enrolled 2988, 3162, and 2709 participants in 2009, 2015, and 2017 respectively. The baseline (2009) prevalence of VT pneumococcal carriage among children aged 0-4 years was 42.6 %, which declined to 14.9 % and 17.5 % in 2015 and 2017 respectively (adjPR 0.32 [95 % CI 0.27, 0.38] and 0.38 [0.31, 0.46] respectively). VT prevalence among children aged 5-14 years was 16.6 %, 15.1 %, and 15.8 % in the three surveys (2017 vs 2009, adjPR 0.70 [0.58, 0.83]). VT prevalence among 15-44 year -olds was 6.4 %, 5.7 %, and 7.1 % in the three surveys (2017 vs 2009, adjPR 0.59 [0.46, 0.75]), while in those aged >= 45 years it was 4.5 %, 6.5 %, and 4.5 % respectively. Non -VT carriage increased in all age -groups. Prevalent residual serotypes were 34 and 15B (age 0-4 years), 3 and 34 (age 5-14 years), and 3 and 16F (age >= 15 years). Conclusions: Introduction of PCV was associated with reduced VT pneumococcal carriage in young, and older children, although with substantial residual prevalence. Persisting VT, and non -VT, carriage indicate significant, persistent transmission of pneumococci in the population.