However, any effect may have been obscured by the healthy vaccinee effect and when we examined the more reactogenic whole cell pertussis vaccine, an elevation in events was evident in the first 24 h [8]. We have also identified a significant elevation in incidence of hospital admissions or emergency room visits from days 4 to 12 post 12-month (MMR) vaccination compared to a control period (Relative Incidence (95% CI) = 1.33

(1.29 to 1.38) [10]. This risk period is consistent with the biologically expected period and previous studies and our estimate of febrile seizures was also consistent with previous estimates [11], [12], [13] and [14]. Using our existing analytic infrastructure, we sought to examine the association

between sex and health services utilization following standard pediatric Y-27632 clinical trial immunizations, defined as emergency room (ER) visits ABT 199 or hospitalizations, during a pre-specified ‘at risk’ period after vaccination. We conducted this study using VISION (Vaccine and Immunization Surveillance in Ontario), an analysis infrastructure that was created using linked health administrative data to monitor vaccine safety and efficacy in Ontario [7]. Using this infrastructure, we examined the effect of sex on rates of ER visits and/or hospital admissions within pre-defined risk periods following standard pediatric immunizations administered at 2, 4, 6 and 12 months in infants born between April 1st, 2002 and March 31, 2009. In Ontario, Canada, standard pediatric vaccines administered at 2, 4 and 6 months of age during our study period included those against diphtheria, pertussis, tetanus, polio, haemophilus influenzae type b (Hib) as one vaccination, and pneumococcus as a separate vaccination. Recommended immunizations at 12 months of age consisted of a vaccine against measles, mumps and rubella (MMR vaccine) throughout the entire study period and in addition, as of September 2004,

a vaccine against meningococcal disease (type C) was added to the schedule of recommended vaccinations at 12 months of age. Our study included all children born in Ontario between April nearly 1st, 2002 and March 31st, 2009, who were present in the Institute for Clinical Evaluative Sciences’ Registered Persons Database. We ascertained vaccination events for our study cohort at 2, 4, 6 and 12 months of age using general billing codes for vaccination in the Ontario Health Insurance Plan Database, including vaccines administered on the exact due dates, as well as those which were administered up to 14 days before or 40 days after the due dates. We identified hospital admissions for our study cohort using the Canadian Institute for Health Information’s Discharge Abstract Database and ER visits using the National Ambulatory Care Registration System. We assessed the relative severity of ER visits by comparing the mean Canadian Triage and Acuity Scale (CTAS) scores between sexes [15].