A collaborative approach to investigating the risk of thrombocytopenic purpura after measles-mumps-rubella vaccination in England and Denmark.

The assessment of rare adverse events following vaccination may not be possible within a single country due to an insufficiently large denominator population. In 2008 a European consortium (VAESCO) was funded to perform collaborative vaccine safety studies. To help assess the feasibility of multi-country collaboration England and Denmark, who have established vaccine safety research infrastructures, undertook to work to a common protocol and share results and data to estimate the risk of a known true adverse event, thrombocytopenic purpura (TP) following measles-mumps-rubella (MMR) vaccination. TP is a known rare reaction to MMR and therefore provided an opportunity to assess whether two countries would produce similar results when working collaboratively. Despite some initial problems with ensuring data were comparable, the two countries gave very similar estimates of the relative incidence in the 6 weeks after vaccination and a pooled relative incidence estimate of 2.13 (95% confidence interval 1.55-2.94) and attributable risk of 1 in 50,000 doses. Both countries used hospital admissions for TP and the analysis was performed using the self controlled case series method which is particularly suited to collaborative studies because of its implicit control for individual level confounding. The study therefore shows the potential for vaccine safety collaborations across Europe to detect true associations through use of common protocols and sharing of results or data.