The article says the girl was also be checked for a sore throat, and the fact is nobody knows what caused the death at this point in time.
A sore throat could be bacterial, and bacterial infections can kill quite quickly. A staph infection, followed by toxic shock, maybe.
It would be interesting to find out the cause of death, but I doubt it will be publicized. Anti-vaxxers love to try to scare people about vaccines--a story like this that they can use to insinuate that the girl somehow died of the vaccine is made-to-order for them.