see SAGE minutes for 21/4/2020
That date is very significant, because I've read a whole article on how the officially accepted science on masks was wrong at the start of the pandemic. I don't remember the exact date the WHO reversed it's advice, but it it was definitely later than that date, a few months later.
Apparently the WHO guidelines (and what was taugtht in medical schools around the world) are taken from USA guidlines published at the start of the 1960's, and that was in turn based on the work of an American researcher over the previous 20 years. But whoever drew up the USA guidelines had conflated or misunderstood the meaning of the figures in the previous research, and got the guidelines wrong.
Bottom line, scientific advice and medical textbooks from before mid-2020 are wrong, advice has changed since.
The article I read was about an American physicist who sat in on a WHO zoom call in early 2020 where people were saying masks weren't effective, and when she piped up to say that what they were saying was wrong from a physics point of view, she was shut down by senior medical staff quoting medical orthodoxy. She then spent a few months tracing why the medical profession believed what they did, found the source of the historical error, and convinced WHO to change their advice, which they did, in 2020, but long after the April date you mention.