If you have decided upon this one, I would wait the 1.5 months to just have to do the one shot. Keep in mind, when my dd was small this vax was new, and was marketed as the "ear infection vax". I think now it is more targeted towards people as the meningitis vax.
We actually switched peds during the time my dd should have gotten her last couple shots so when we went in I asked about her getting it and the NP said not to worry with it as she was over 2 years old by that time. Looking back, now that we no longer vax dd and haven't vaxed ds, I find that info very interesting.
But try not to worry yourself to death over this one. Risk of meningitis drops signifigantly with age and in theory the shot at 18 mos. should give her immunity during those last higher risk years (up til 5).
On the prevnar site: "In clinical studies, the most frequently reported adverse events included injection site reactions, fever (38ºC/100.4ºF), irritability, drowsiness, restless sleep, decreased appetite, vomiting, diarrhea, and rash."
HTH, and good luck with your decision