Hi, my partner and I transited through GRU from the US on the way to CPT via JNB in late March '14. We did not get the yellow fever vacinations. When we arrived in GRU and checked in with the SAA gate agent, at first she said we could not board because we did not have evidence of vacination. However, like others indicate on this board, I had done my homework ahead of time so I politely challenged her. She called over a supervisor, and sure enough he agreed that if we remain airside and transited less than 12 hours we were good to go. I took the chance that irrops would not cause an extended delay. I suppose if the delay extended transit beyond 12 hours there could be an issue. Interestingly, when going through immigration in JNB, the agent did not even blink an eye when I presented my landing card with origin at GRU. So, you should be okay. I will also note that if irrops occur and, as a US citizen you do not have a Brazilian visa your options will be very limited since you must remian airside. We did not get a visa because of the cost and hassle. We flew TAM from MIA, and upon checking in the agent also challenged us since we lacked the visa. Again, I had done my homework ahead of time and printed the visa rules from the Star Alliance website (this was just before TAM joined OW). So, with the help of a supervisor we were fine. And, btw, GRU is no picnic for a long layover. Very antiquated facilities. We spent 10 hours on the return at the airport (using FastSleep to catch some rest). But it was very uncomfortable at best.