Must all env variables be prefixed with REACT_APP for use with yarn?

Using create-react-app, my environment variables are loaded from .env as long as they're prefixed with REACT_APP_, but not otherwise:

// .env
REACT_APP_API_KEY=1234
MY_FUNNY_API_KEY=5678

// index.js
console.log(process.env.REACT_APP_API_KEY) // 1234
console.log(process.env.MY_FUNNY_API_KEY)  // undefined

Running yarn run env (from create-react-app) showsMY_FUNNY_API_KEY, but it doesn't seem to be available from the app. Is there some way to access general environment variables without the REACT_APP_ prefix?