Moxie Marlinspike (founder of Signal) [0]implemented the same E2EE algorithm as Signal (Signal Protocol) into WhatsApp, but that was 10 years ago, so who knows if things have changed since then.
Practically speaking, it isn't secure; no closed app can be. It receives regular compulsory updates (old versions refuse to work) and there's nothing at all stopping Zuck from sneaking in backdoors targeted at you personally.
[0] https://en.wikipedia.org/wiki/Moxie_Marlinspike