Live vaccines have distinct safety profiles, potentially causing systemic reactions one to 2 weeks after administration. In the province of Ontario, Canada, live MMR vaccine is currently recommended at age 12 months and 18 months.
[W]e examined… the relative incidence… of emergency room visits or hospital admissions in consecutive one day intervals following vaccination.
These were compared to a control period 20 to 28 days later*.
[W]e examined the reasons for emergency room visits… during the at-risk period following the 12 month vaccine.
Four to 12 days post 12 month vaccination, children had a 1.33… increased relative incidence of the combined endpoint compared to the control period, or at least one event during the risk interval for every 168 children vaccinated.
The primary reason for increased events was statistically significant elevations in emergency room visits following all vaccinations.
There are significantly elevated risks of primarily emergency room visits approximately one to two weeks following 12 and 18 month vaccination.
[B]ecause it is a live vaccine the MMR vaccine has the potential to cause adverse events one to 2 weeks following vaccination.
[R]eactions to [the MMR vaccine can occur] with fevers occurring in 5 to 15% and rashes in 5%.
More serious reactions… may not be identified during pre-licensure trials.
Post market surveillance has identified an incidence of febrile seizures following the MMR vaccine…
Previous studies have identified that children are at increased risk for systemic reactions at different times from 5–14 days after vaccination.
[We] classified days 20–28 [post-vaccination] as unexposed…*
Deaths after the 12 and 18 month vaccinations were explored in a separate analysis…
[C]hildren who died during the follow-up period were excluded from the SCCS analysis of ER visits and hospitalizations.
Consecutive statistically significant elevations in combined endpoints began on day 4 and continued to day 12. A total of 6462 children experienced at least one of the combined endpoints during the combined 9 day at risk period compared to 4845 during the 9 day control period.
The highest relative incidence during the at-risk period occurred between days 8 and 11 peaking at 2.04 on day 9.
The primary reason for the elevation in the combined endpoint was an increase in ER visits.
The largest relative risk was associated with febrile seizures…
Our analysis demonstrated that the 12 and 18 month vaccinations are not associated with an increase in adverse events immediately following vaccination.
We identified an increase in events occurring between 4 and 12 days post-vaccination…
The majority of these events represented ER visits and at their peak, on day 9 following the 12 month vaccine, were approximately twice the baseline rate.
Although there was an increase in hospital admission in each period, none of these increases were statistically significant. Overall the increase in event rate following the 12 month vaccines accounted for approximately 598 extra children experiencing one or more ER visits during the risk interval per 100,000 vaccinations.
The development of an inflammatory response approximately one week after vaccination is recognized in the literature. For example, the Centres for Disease Control and Prevention list days 7 to 12 post vaccination as the highest risk period for developing fever and possibly a rash.
The estimated 595 additional children experiencing at least one event for every 100 000 vaccinated translates into approximately one child experiencing at least one event per 168 children vaccinated.