WhatsApp is reportedly working on a fingerprint authentication feature to protect chats from non-authorized users. According to WABetaInfo, the feature is current under development as seen in the code for WhatsApp Beta for Android 2.19.3 version.
The fingerprint authentication feature will be available within the app under a new section. Once you enable the fingerprint feature, your WhatsApp will be completely protected. You will need to use your fingerprint to unlock the app after tapping the app icon, or a new notification or any other link that leads to WhatsApp. Moreover, this is an app-level lock and you can’t just use it to lock specific conversations.