Undersøgelsen var meget omfattende, fordi den skulle se nærmere på, om vaccinen forårsagede en meget sjælden, alvorlig bivirkning, der kaldes intussusception.
The study was very large because it was designed to see if the vaccine caused a very rare, serious side effect, intussusception, a condition in which part of the bowel becomes enfolded within another part of the bowel, leading to a blockage.