Patterns of emergency room visits, admissions and death following recommended pediatric vaccinations - a population based study of 969,519 vaccination events

Background: The risk of immediate adverse events due to the inflammation created by a vaccine is a potential concern for pediatric vaccine programs.

Methods: We analyzed data on children born between March 2006 and March 2009 in the province of Ontario. Using the self-controlled case series design, we examined the risk of the combined endpoint of emergency room visit and hospital admission in the immediate 3 days post vaccination to a control period 9-18 days after vaccination. We examined the end points of emergency room visits, hospital admissions and death separately as secondary outcomes.

Results: We examined 969,519 separate vaccination events. The relative incidence of our combined end point was 0.85 (0.80-0.90) for vaccination at age 2 months, 0.74 (0.69-0.79) at age 4 months and 0.68 (0.63-0.72) at age 6 months. The relative incidence was reduced for the individual endpoints of emergency room visits, admissions and death. There were 5 or fewer deaths in the risk interval of all 969,519 vaccination events. In a post hoc analysis we observed a large reduction in events in the immediate 3 days prior to vaccination suggesting a large healthy vaccinee effect.

Conclusion: There was no increased incidence of the combined end point of emergency room visits and hospitalizations in the 3-day period immediately following vaccination, nor for individual endpoints or death. The health vaccinee effect could create the perception of worsening health following vaccines in the absence of any vaccine adverse effect and could also mask an effect in the immediate post-vaccination period.