Flowchart of inclusion in the study. aFourth dose of inactivated vaccine against diphtheria, tetanus, pertussis (acellular), polio, and Haemophilus influenzae type b (DTaP-IPV-Hib) (N = 3077; 70.3%), second dose of vaccination against measles, mumps, and rubella (MMR) (N = 605; 13.8%), not recommended combination of vaccines (N = 358; 8.2%), DTaP-IPV or Hib alone (N = 286; 6.5%), booster dose against different combinations of diphtheria, tetanus, pertussis (acellular), and polio (N = 32; 0.7%), or oral polio vaccine (OPV)2 (N = 20; 0.5%). bSome children had missing information on more than 1 variable. The number of children with missing information on each variable and in parentheses the percentage among the total number of children with missing information was as follows: 9732 (65.3%) with missing information on maternal smoking during pregnancy, 4315 (29.0%) with missing information on educational level for the female adult in the household, 1775 (11.9%) with missing information on birth weight, 955 (6.4%) with missing information on gestational age, 795 (5.3%) with missing information on household income, 276 (1.9%) with missing information on adult composition of the household, 107 (0.7%) with uncertain vaccine allocation for twins or triplets, 102 (0.7%) with missing information on parental place of birth, 80 (0.5%) with missing information on caesarean section, 12 (0.1%) with missing information on population density, and 6 (0.0%) with missing information on maternal age at birth of the child. (Note: Infectious disease admissions are counted from 24 months of age and until date of censoring for the children included in the study or until 36 months of age for the children excluded from the study, because these children did not have a censoring date.)