If they say "Masks work" when it only works 70% of the time then a lot of people will get sick despite wearing masks, there'll be a big public outcry, and people won't trust the WHO in the future.
Maybe. Or they can just lose all trust instantly by claiming like the CDC, that they don't work. Now people who think will never trust the CDC ever again. Not sure what the WHO's wording was, but any organization not encouraging mass mask use cannot be trusted by anyone with a brain. Just because something isn't 100% effective, doesn't mean it's not effective. Only children and idiots think so. Unfortunately, we're surrounded by idiots.
It would certainly be a failure if it is communicated that mask wearing prevents you from getting infected. They reduce the risk of infecting others more than they reduce the risk of getting infected.
A 20% reduction in infection risk is already substantial and worth it. If there is a 50% reduction in infecting others, mask use alone could replace most social distancing measures.
