By what?
Well by anything.People are injured or killed by many things. If the scheme is going to be used for millions of journeys then people will inevitably be injured or killed in time.
It may be that you are right and the lack of helmets and the number of people who are injured/killed is disproportionate to general cycling or other methods of transport. So far it doesn't seem that way but there haven't really been enough journeys to say for sure.
I remember seeing a breakdown of accident stats for various modes of transport which showed walking as more dangerous than cycling. I can't find it now though so take that with a pinch of salt. The point was that people get run over every day, it's possible that if pedestrians wore a helmet then the number of fatalities of pedestrians would fall. But in the grand scheme of things not that many pedestrians are killed and we don't bother wearing helmets as a result. I personally feel the same way about cycling, I don't wear a helmet because I don't think it's worth the hassle even though it would make me more likely to survive an accident.