When Dominic regressed at age 3, he was a late regression. And he was late because we delayed his vaccines - we didn't give him a single shot until he turned two. We did that because our niece nearly died from one of her vaccines. When Dominic regressed, he and I were regular active members in a local moms group that met at a park once a week. I distinctly remember how well he interacted with the other children and talk to and in general was a perfectly typical little boy.
Everyone of those moms in the playgroup watched his regression right along with me. And everyone of them took note of what happened. Most of that group of women has moved on. Which is typical for lots of reasons: it's very hard to watch a child regress who is a friend of your child's and to know that you managed to dodge a bullet. And frankly it's really hard to have a non-typical child in with typical kids - it just doesn't work well.
Recently, I got this message from one of those moms from those play date days. we still keep in touch - I don't think I laid eyes on her in 3 years but that's what Facebook is for.