The Netherlands vaccination schedule has been advanced by 1 month. The reporting rate of collapse or hypotonic-hyporesponsive episode increased from 5.2 [95% confidence interval (CI): 4.8-5.7] to 8.5 (95% CI: 8.0-9.1) and from 1.2 (95% CI: 1.0-1.4) to 2.5 (95% CI: 2.2-2.8) per 10,000 first and second doses, respectively. At 3 months of age it fell from 5.2 to 2.5. This suggests some protection by the first dose. Different mediators in primary and secondary immune response may play a role.