On this subject, I like to quote Pavel Durov, the founder of Telegram:
"Since the creation of WhatsApp, there's hardly been a moment in which it was secure: every few months researchers uncover a new security issue in the app. I wrote about this in detail 2 years ago (read here if you missed it). Nothing has changed since then.
It would be hard to believe that the technical team of WhatsApp is so consistently incompetent. Telegram, a far more sophisticated app, has never had security issues of such severity."