マスクでコロナウイルスが防げるわけではない

Follow CDC’s recommendations for using a facemask.

CDC does not recommend that people who are well wear a facemask to protect themselves from respiratory diseases, including COVID-19.

Facemasks should be used by people who show symptoms of COVID-19 to help prevent the spread of the disease to others. The use of facemasks is also crucial for health workers and people who are taking care of someone in close settings (at home or in a health care facility).

Prevention, Treatment of Coronavirus Disease 2019 (COVID-19) | CDC

CCD(アメリカ疾病予防管理センター) より via Stop buying masks, it does not prevent coronavirus - US DoH - (GizChina)

症状がある人が感染を広げない為には有効
もちろん医療従事者には必要

そもそもマスクが手に入らないからどうしようもない