Association between socioeconomic status and adverse events following immunization at 2, 4, 6 and 12 months

Using a population-based self-controlled case series design, the researchers examined data on children born between the years 2002 and 2009 in the province of Ontario, Canada. The researchers specifically examined how socioeconomic status (SES) influences rates of adverse events following immunization (AEFI), defined as emergency room visits and/or hospital admissions. For vaccination at 2, 4 and 6 months combined, the relative incidence of AEFI (95% CI) in the first 72 hours after vaccination was 0.69 (0.67 to 0.71). For all three vaccinations combined, the researchers observed no relationship between the relative incidence of an event and quintile of socioeconomic status (p = 0.1433). For the 12-month vaccination alone, the relative incidence of events (95% CI) on days 4 to 12 following immunization was 1.35 (1.31 to 1.38). The researchers observed a significant relationship between socioeconomic status and vaccination at 12 mo, with lower SES being associated with a higher relative incidence of events (p = 0.0075). When the lowest two quintiles of income combined were compared with the highest 3 quintiles, the relative incidence ratio (95% CI) was 0.94 (0.89 to 0.99, p = 0.02). These results translate to 150 additional adverse events in the lower SES quintiles as compared with the higher SES quintiles for every 100,000 children vaccinated, or 1 additional event for every 666 individuals vaccinated. Future studies should explore for potential explanations for this observation.