Both Phonak and Signia, two of the world's leading hearing aid suppliers released statements on hacking hearing aids. See their comments below:
Phonak: "Medical device manufacturers like Signia are committed to removing the risk of hacking. Through secure wireless technology and encryptions, you can be confident your Signia hearing aids work as intended, have a high level of cyber security, and remain out of hackers’ reach."
Signia: "The hearing aids do not use WiFi, so hacking using this connection is not possible. The hearing aids are discoverable via Bluetooth for 3 minutes after switch on, so it is possible for others to connect to them during this period if within physical range and if they (the hearing aid wearer) isn't actively connected to the device."