All my babies slept on tummies since newborn. I never worried, not even a little bit.
SIDS is a very mysterious disease and no one can really say what causes it. There are a LOT of other risks factors to consider besides just sleep positioning.
Did your baby have resp issues after birth? Was your baby premature? Does your family have a history of asthma or other resp issues? Does anyone in your household smoke? Is your baby sleeping in your bed with you? Is your baby sleeping with soft bedding? Are you breast or bottle feeding? Is your baby a twin or multiple? Are you a teen mom? Is your child male?
If you answer no to all of these questions, your SIDS risk is really low.
SIDS is a lot more complex than just back or tummy sleeping. Unfortunately, SIDS is still really misunderstood. A lot of deaths are MIS-labeled as SIDS. We don't know what actually causes SIDS.
So there are a lot of studies showing correlation, but none of them show CAUSATION. A death caused by a baby suffocating is unfortunate, but it is NOT SIDS. It's a suffocation death, not unexplained.
Here are some links about SIDS:
I personally feel that in a family with no previous history of serious health issues, no asthma or breathing problems, no substance abuse, no smoking, in a healthy infant, born at term with a normal birth weight, SIDS is not really something to worry too much about.