Because booster doses of pneumococcal conjugate vaccine (PCV) may be given at a similar time to yellow fever vaccine (YF), it is important to assess the immune response to YF when co-administered with PCV. This has been investigated during a reduced-dose PCV trial in The Gambia. In this phase 4, parallel-group, cluster-randomized trial, healthy infants aged 0-10weeks were randomly allocated to receive either a two-dose schedule of PCV13 with a booster dose co-administered with YF vaccine at age 9months (1+1 co-administration) or YF vaccine administered separately at age 10months (1+1 separate) or the standard three early doses of PCV13 with YF vaccine at age 9months (3+0 separate). Blood samples were collected 28-35days post-vaccination and YF neutralizing antibody (NA) titres were measured. Proportions with seroprotective YF NA titres≥1:8 were calculated with 95% confidence intervals (CI). Non-inferiority was demonstrated if the lower limit of the CI for the difference in proportions between the co-administration and separate groups was greater than-10%. Forty-eight, 66, and 98 participants enrolled in 3+0 separate, 1+1 co-administration, and 1+1 separate groups respectively had NA results. Per protocol analysis of the 3+0 separate, 1+1 co-administration, 1+1 separate, and the combined 1+1 separate and 3+0 separate groups found that 81%, 85%, 92%, and 88% of participants respectively had YF NA titres ≥1:8. Results were similar with analysis by intention-to-treat. The difference in proportions comparing 1+1 co-administration and 1+1 separate groups was -7% (95% CI, -18% to 3%). The difference between 1+1 co-administration and 3+0 separate groups was 4% (95% CI, -10% to 15%). There was no statistical difference in the YF seroresponse when the YF vaccine was co-administered with PCV or administered separately. No evidence was found of the non-inferiority of the seroresponse to YF vaccine when co-administered with PCV13. The levels of YF NA attaining seroprotection (NT ≥1:8) were high in all groups. PCV13 co-administered with YF vaccine at 9months does not affect seroresponse to YF vaccine. http://www.isrctn.org/ - ISRCTN72821613.
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