Table 1. Characteristics of participants at the beginning of the study, the Netherlands, October 2011 (n = 239 participants).

Characteristics	Overall	10 year-olds	12 year-olds	15 year-olds
Number of participantsa	239	81	82	76
Mean age in years in October 2011 (SD)	NA	9.9 (0.3)	12.0 (0.3)	15.0 (0.3)
Number of participants of male sex (%)	120 (50)	34 (42)	42 (51)	44 (58)

NA: not applicable; SD: standard deviation.

^a All participants were primed with a whole-cell pertussis combination vaccine four times in the first year of life (3 + 1 schedule). Ten and 12 year-old participants received an acellular pertussis booster vaccine at 4 years of age.