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Study design for the early and late booster vaccination (A) and catch-up vaccination (B). W, weeks; M, months; syringe, vaccination; BS, blood sampling. Due to a delay in the study start, the age range for receiving the booster dose in the 9- to 12-month group was extended to 9 to 18 months of age, but only the results for the 9- to 12-month subgroup were used for the assessment of the early booster.
