MPs have been told an increasing number of well-educated middle- class families are not immunising their children.
The Health Select Committee is holding an inquiry into how to improve completion rates of childhood immunisation.
The Prime Minister's chief science adviser, Sir Peter Gluckman, says at-risk families remain the primary target of immunisation strategies.
But over the last 10 to 15 years, he says, well-educated middle-class families are becoming another emerging group.
Sir Peter says the issue is that the parents think if everyone else is immunised, then they or their children don't need to be.