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Their guidance does not seem to suggest that people should not wear masks in public, nor does it comment on the effectiveness of doing so.

From their site:

  If you are healthy, you only need to wear a mask if you are taking care of a person with suspected 2019-nCoV infection.
  Wear a mask if you are coughing or sneezing.
  Masks are effective only when used in combination with frequent hand-cleaning with alcohol-based hand rub or soap and water.
  If you wear a mask, then you must know how to use it and dispose of it properly.

https://www.who.int/emergencies/diseases/novel-coronavirus-2...