Has WHO said it is dangerous to mix and what the dangers are OR have they said they don’t know enough yet so I’m their opinion it is the unknown that is dangerous.
No. Go look at the media briefing itself - ignore the headlines and people like Phil who can't manage context.
The link I put up jumps right to the part the media went nuts with, but you might want to go back a few minutes to the first person's answer to the question as well.
Someone asked about booster shots. The WHO doesn't think Booster shots need to be worried about at this time, although obviously people should be researching it. The concern right now is that adequate first coverage is given world wide, and obsessing over booster shots in rich countries is going to slow that down. Then the bit that resulted in all the headlines happens. As an add on, Dr Swaminathan says how they are getting questions from people about going back and just mixing and matching extra shots for things. She thinks this is a bad idea because they don't have evidence. They have it for some with Astra Zeneca, but citizens shouldn't just decide for themselves to go and mix shots. It may be fine, but studies haven't been done yet.
So why did this come up now? Because people are talking about it now. There are people online saying "Oh, I got J&J and maybe Delta is bad so I should go and get Moderna as a booster".
This isn't about secret dangers of mix and matching or hiding info. This is about people in rich countries just deciding to go and get boosters of different shots because "maybe why not" and using up supplies that can be donated elsewhere for dubious gain.
Even now, though, this is getting reported as "The WHO issued a statement mixing is bad" which isn't what happened.