The seasonal influence that has been shown for immune-mediated diseases could potentially translate into an effect of month of birth on rates of AEFI during the first year of life. In this study, we addressed this question by assessing the association between month of birth and the relative incidence (RI) of AEFI, defined as hospital admissions or ER visits, following vaccination. Children born in Ontario between April 1st 2002 and March 31st 2010 who were enrolled in the Ontario Health Insurance Plan (OHIP) were eligible for inclusion in the study cohort. OHIP is Ontario’s universal health insurance plan

which covers nearly all Ontario residents. We excluded multiple births, infants born prematurely (<37 weeks signaling pathway gestation) and infants in the bottom decile of birth weight for their gestational age. After these exclusions, infants who were vaccinated at 2 and/or 12 months of age were included in the study cohort. MS275 We excluded children who died, or whose follow-up was otherwise terminated before the end of the required observation period (Supplementary Fig. 1). As part of the publicly funded immunization schedule in Ontario, Canada, vaccinations given at 2, 4 and 6 months of age included those against pertussis, diphtheria, tetanus and polio and Haemophilus influenzae type b (cPDT Polio + Hib until January 2005; DTaP-IPV-Hib thereafter). As of

January 2005, a pneumococcal vaccine was also administered at 2, 4, and 6 months of age (Pneu-C-7 until October 2009; Pneu-C-10 thereafter). The first dose of the measles,

mumps and rubella vaccine (MMR) was given at 12 months of age throughout the entire study period, and as of September 2004, a vaccine against meningococcal disease (type C) was added to the schedule [14]. All study data were linked using unique, encoded identifiers and analyzed at the Institute for Clinical Evaluative Sciences (ICES). We identified vaccinations from no the OHIP database using general vaccination billing codes and methods described previously [1] and [2]. To identify the 2-month vaccinations, we selected those occurring on the exact recommended date (60 days) and up to two weeks before or up to one month after. For the 12-month vaccination, we selected those occurring at 365 days of age, as well as up to 60 days past that date. We ascertained hospital admissions using the Canadian Institute for Health Information’s (CIHI’s) Discharge Abstract Database (DAD), and ER visits using CIHI’s National Ambulatory Care Reporting System (NACRS). The Registered Persons Database was used to ascertain eligibility for OHIP coverage and deaths. We defined our composite primary outcome as all-cause ER visits and admissions, with the a priori exclusion of events having diagnoses that could not reasonably be causally associated with vaccination (Supplementary Table 1).