Directly unlocking Whatsapp with a fingerprint, is something that iOS users have been doing for months. Android users, on the other hand, are just starting to get access to the update that will enable this feature.
There is a stable Whatsapp APK beta version available for download that lets you use your fingerprint to keep your conversations locked. A Fingerprint Lock option is available in version 2.19.221, and you can find it in the Settings>Privacy option. If you don’t find this enabled, you may have to delete and reinstall the app. After that, you will just have to decide whether you want the app to lock as soon as you close it, or if you want it to wait for 30 seconds or 1 minute. If you want to try it out, just follow this link.