Would facemasks reduce virus transmisions in vulnerable populations?

Question

The US surgeon general has asked (tweeted) the public to not buy face masks, because the claim is that healthcare workers need them more.

CDC does recommend:

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).

Is there any reason not to believe that asymptomatic transmissions can be reduced by having all high risk populations (elderly, subway riders, healthcare workers) wear masks to reduce public spread of respiratory droplets?

Given the COVID-19 context, The reason the question is posed: does it make good prophylatic sense to:

• require visitors to nursing homes to wear masks?
• encourage metropolitan subway & bus riders to wear masks and gloves?

It is my understanding that the mask's purpose is to attenuate spread by preventing droplets from entering the environment will not prevent an infection, once the virus is in the environment.

Answer 1

If you want clear scientific evidence on this (effectiveness of masks use in the general public), I think there's no such study out, although there is some data pointing toward a combination of masks and hand hygiene being sometimes effective (the effect size might no be large though given the lack of uniform results even on this "combo").

As far guidelines go, the Lancet has a mini survey of the varying stance of various government and health authorities as of March 20.

There are also some recent developments regarding the stance of the US gov't and CDC:

One internal memo for the Centers for Disease Control and Prevention says that even simple cloth masks could help reduce the risk of virus transmission, the Washington Post reports.

It's also clear the prior CDC stance was substantially influenced by the shortage affecting medical professionals:

Dr Fauci told CNN he would "lean towards" recommending a "much more broad, community-wide use of masks outside of the health care setting" once there was a sufficient supply of masks for healthcare workers.

Prior CDC (2009) [pandemic] recommendations, do/did include respirators for vulnerable populations, when no alternatives exist, like simply them avoiding a "crowded setting" (see table/image below). I'll also emphasize from there that with respect to inhalation, a distinction (of degree of protection) can made between masks and respirators:

The optimal use of respirators requires fit testing, training and medical clearance. Proper use is recommended to maximize effectiveness. The use of facemasks may be considered as an alternative to respirators, although they are not as effective as respirators in preventing inhalation of small particles, which is one potential route of influenza transmission. There is limited evidence available to suggest that use of a respirator without fit-testing may still provide better protection than a facemask against inhalation of small particles. Respirators are not recommended for children or persons who have facial hair.