Background deaths are the normal deaths from all other causes that occur just because we are alive. In a group of that many children over that period of time, it is normal for 50 of them to have died from something other than the vaccine.
To suggest that in that same group, over that same time, only 2 children died is a CLEAR misstatement at best, and an outright lie at worst. Either way, it raises sufficient doubt about the veracity of the report.