According to the former Road Safety Minister there is no known case where cyclist safety has improved with increasing helmet use.
That, at least, is entirely unambiguous. The question is, why? Many helmet promoters have invested huge amounts of sincere effort into proving that helmets reduce injury, but all they have really done is to prove, over and over again, that voluntary helmet use is a marker for a cyclist less likely to crash badly. In other words, risk-averse cyclists are more likely to wear helmets. Who knew?