The analysis was conducted PCI-32765 in vitro using the self-controlled case series (SCCS) design [15] and [16] and the Vaccine and Immunization Surveillance in Ontario (VISION) analytic architecture
[17]. Our general analytical strategy has been described in detail elsewhere [1] and [2]. We were primarily interested in adverse events following first vaccine exposure at two months (cPDT Polio + Hib or DTaP-IPV-Hib), and first exposure to MMR vaccine at 12 months of age. Therefore, we selected observation periods that biologically relate to these exposures. For the 2-month vaccination, we designated the 48 h post-vaccination (days 0–1) as the risk period and days 9–18 as the control period. At 12 months, the risk period included days 8–12 post-vaccination and the control period included days 20–28. These risk periods were modified a priori from our previous studies to include only the time of most intense excess event incidence. In many instances, acute admissions immediately follow an ER visit (i.e. a patient presents to the ER and requires admission). We counted only the first event to occur in a risk or control period, thus avoiding the need to decide Z-VAD-FMK whether events close together in occurrence truly were distinct, or part of the same ‘episode’ of care. We calculated the RI of the primary endpoint in the risk
period compared to the control period using a conditional Poisson regression model, which included terms for exposure period and for identifying each individual child, thereby accounting for intra-individual correlation and allowing each
individual to serve as his/her own control. To illustrate the magnitude of the effect of birth month on the RI of our endpoint, we computed relative incidence ratios (RIRs) by comparing the RI of events in infants born in each month to that for the month having the lowest RI. This was identified post hoc. A test for interaction between risk period and month aminophylline of birth was used to establish statistical significance of differences in RIs between birth month subgroups [16]. To test for the presence of a cyclical seasonal pattern in RIs, we repeated the SCCS analysis at both the 2- and 12-month vaccination with the season effect parameterized using a cosinor modeling approach [18]. Details of the cosinor model implementation are provided in the Supplemental Methods. All p-values were two-sided, and all analyses were conducted using SAS version 9.2 (SAS Institute, Cary, NC). In order to determine whether the effect of season was similar across individual calendar years, we repeated our analysis for each year separately from 2002 to 2010. To determine the impact of using risk periods restricted to days 0 and 1 for 2-month vaccinations and days 8–12 for 12-month vaccinations as compared to risk periods from past studies (days 0–2 and days 4–12, respectively), we conducted our analysis by birth month using both risk period definitions.