And just 270 cases and no deaths? Well, if you can use Vietnam as an argument for the effectiveness of masks then you must also consider other factors. One of which is that the median age in Vietnam is 32.5 years. But you must also disallow other factors, such as perhaps some acquired immunity. Or maybe it's the fish soup. Or survival of the fittest from the war. Since according to this report from one living there the sanitary practices were not very good.
Actually of those 270 cases, I believe 50 are foreigners, who were here when found to be infected.
I believe they were older.
But you are right, that could be a contributing factor.
I believe Hydroxychloroquine may also be a contributing factor.
Not certain of that, though.