As of now, it is safe to say that Threema and WhatsApp are equally secure. They both use end-to-end (E2E) encryption with strong cryptography to protect the contents of your messages. No one can read them except the intended recipient. However, the big difference is privacy. Threema collects as little data as possible. WhatsApp collects a lot more data and shares it with the parent company (Facebook) and their partners.