For the 2-month vaccination, the highest relative risk incidence was observed in April births, the same month as the highest RIR. However, one of the lowest relative control incidences was also observed for infants born in April, suggesting that both of these effects were important factors in driving the seasonal pattern observed at the 2-month vaccination (Table 1). For the 12-month vaccination, the birth month with

the highest RIR was July, which corresponded to the month in which the lowest relative control incidence occurred. However, the relative risk incidence peaked earlier, in March. We investigated the impact of month of birth on the relative incidence of AEFI using ER visits and hospital admissions as a proxy. Our study is, to the best of our knowledge, the first to describe a seasonal effect of susceptibility to AEFI. We observed a strong effect of month of birth on the RI of ER visits and admissions. The observed effect was buy PFI-2 strongest at the 2-month vaccination, at which the first dose of the DTaP-IPV-Hib vaccine

is given. For the 2-month vaccination, we observed a greater than two-fold increase in the RI of events for children born in April, compared to children born in October, the month of the lowest RI of events. A clear sinusoidal pattern was observed between the month of birth and RI. One of our sensitivity analyses suggested that an important driver ERK inhibitor datasheet of elevated RI was a decrease in incidence during the control period. This provides evidence that the background burden of seasonal illness may be another contributing factor to the seasonal effect we observed. During months

of higher burden of illness Casein kinase 1 (e.g. fall/winter) the incidence in the control period was higher as compared to the control period in months of lower burden (spring and summer). These fluctuations in the background burden of illness may have contributed to lower RIs in fall/winter and higher RIs in spring/summer either through access to care issues in the fall/winter (e.g. crowded ERs), or by making vaccine reactions less likely when infants are battling many other circulating infections. Another possible explanation is that during the colder months in Ontario Canada, inclement weather and ER waiting rooms crowded with children suffering from influenza and common cold may make it less likely that a parent decides to visit an ER when their child is suffering from a relatively mild post-vaccination reaction. Since the correlation coefficient between birth month and vaccination month was measured to exceed 0.99 for both of the 2- and 12-month vaccinations, due to well established immunization schedules, we performed additional analyses aimed at isolating the effect of month of vaccination as distinct from birth month. We found evidence suggesting that month of vaccination may have contributed to the seasonal variation we observed in our results.