Did the WHO and CDC say “Masks will not help.” or did they say “We don't yet know whether or not masks will help.”?
The conspiracy people say it's because they wanted to protect mask supplies - but most people just blame incompetence.
I can only find one paper by the WHO from 2003 that discusses the effectiveness of masks in reducing transmission of SARS.[1] The paper says that because many different social distancing and hygiene measures were taken at the same time, it is impossible to assess how effective each individual measure was. For example, the abstract states,
> The independent effectiveness of measures to "increase social distance" and wearing masks in public places requires further evaluation.
I found plenty of papers that find that mask use by healthcare workers in hospitals is effective, but that's not the same as what you're claiming.
0 https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-si...