As an additional privacy measure, you can prompt Touch ID or Face ID when you open WhatsApp on your phone. When this is enabled, you'll have to use Touch ID or Face ID to unlock the app.
In iOS 9+, you can enable Touch ID or Face ID by doing the following:
Note: You can still reply to messages from notifications and answer calls if the app is locked. In order to use Touch ID or Face ID, you'll need to have it enabled in iPhone settings. This feature is available on iPhone 5s or later and iOS 9+.