A nurse is caring for several clients who are receiving well-child check-ups. The nurse should identify that the initial diphtheria, tetanus, and pertussis (DTaP) vaccine is indicated for which of the following clients?

A. A 2-month-old infant: The DTaP vaccine series begins at 2 months of age according to the routine immunization schedule. Administering the first dose at this age provides early protection against diphtheria, tetanus, and pertussis, which can be severe in young infants. Subsequent doses are given at 4 months, 6 months, and booster doses at later intervals. B. A 4-month-old infant: At 4 months, the infant is due for the second DTaP dose, not the initial dose. Administering the vaccine at this age without a prior first dose would be incomplete, as the series is designed to build immunity progressively from 2 months onward. C. A 6-month-old infant: The 6-month visit typically coincides with the third DTaP dose in the primary series. The initial dose must have been administered at 2 months, making this age inappropriate for starting the series. D. A 15-month-old toddler: By 15 months, the primary DTaP series should already be completed with a fourth dose administered between 15–18 months. Initiating the series at this age would require catch-up immunization protocols rather than the standard initial vaccination schedule.