Seasonal variation in rates of emergency room visits and acute admissions following recommended infant vaccinations in Ontario, Canada: a self-controlled case series analysis

Objectives — To determine if birth month has an effect on the incidence of adverse events following the 2- and 12-month recommended vaccinations.

Study design — Using health administrative databases, the authors conducted a population-based retrospective cohort study and employed a self-controlled case series analysis approach. The authors included children born in Ontario, Canada between April 1st 2002 and March 31st 2010 who received the diphtheria, tetanus, pertussis, inactivated poliovirus and Haemophilus influenzae type b (DTaP-IPV-Hib) vaccine recommended at 2 months and/or the measles, mumps, and rubella vaccine recommended at 12 months. We calculated the relative incidence (RI) of hospitalizations and emergency room visits within a pre-specified risk period compared to a control period following vaccination. We measured the effect of birth month using relative incidence ratios (RIRs) to compare the RI for infants born in each month to that for the month having the lowest RI.

Results — For the 2-month vaccination, we observed the lowest and highest RIs for infants born in October and April, respectively. The RIR (95% CI) for April compared to October was 2.06 (1.59–2.67, p < 0.0001), consistent with a strong seasonal effect. For the 12-month vaccination, November births had the lowest RI, whereas August births had the highest. The RIR (95% CI) for August compared to November was 1.52 (1.30–1.77, p < 0.0001).

Conclusions — Our findings suggest a seasonal effect on susceptibility to adverse events following vaccination exists. Further study will be important to elucidate potential biological and/or behavioral explanations for the seasonal effect we observed.