That is very unclear. Some studies suggest it isn't, but a recent metastudy reached very different conclusions [1]:

> Bicycle helmet use was associated with reduced odds of head injury, serious head injury, facial injury and fatal head injury. The reduction was greater for serious or fatal head injury. Neck injury was rare and not associated with helmet use.

[1] http://ije.oxfordjournals.org/content/early/2016/09/06/ije.d...