Used everywhere
WhatsApp is in wide use right across the NHS. On issued phones and personal devices. The issue here is a breach because someone outside was added (so - I agree with the address book leak issue).
It's an unfixable problem as far as I can see. But we are a in a better place than when it was by SMS.
The investigation here does nothing to address the issue that WhatsApp is the default messaging solution for both front line teams and and management teams in many hospitals. I know this is the case because I manage data security (as best I can) within a hospital. We know WhatsApp is there and in use, but cannot offer a compelling alternative.