They say that so we, the dumb animals, don't stockpile them like we do with toilet paper (can't find any for 2 weeks straight in my area of Berlin for example).
Of course masks are useful, but they're more useful in hospitals / on health care workers' face, than on the average citizen which won't be in contact with the virus that much now that most of the world is starting to get locked down anyway
Of course masks are useful, but they're more useful in hospitals / on health care workers' face, than on the average citizen which won't be in contact with the virus that much now that most of the world is starting to get locked down anyway