A face shield on its own is not sufficient protection against coronavirus, and wearing a mask (correctly, covering the mouth and nose) is much safer, research has shown.
The reason for this is that when we breathe or especially when we speak, we release thousands of water droplets which are mostly invisible to the naked eye.
When you are infected with Covid-19, each of these droplets can contain thousands of viral particles that can infect people nearby.
A face shield does not stop the water droplets nearly as effectively as a mask does. A multi layer cloth mask will trap almost all of your water droplets, preventing them from circulating in the surrounding air. It will also form a better barrier to stop you from picking up viral particles that may be circulated by others.
Why Mask Plus Face Shield?
People who work in close proximity to the general public often wear a mask plus face shield. This is because the additional protection of the face shield also helps them to avoid airborne particles from entering their body via the eyes. A face shield is thus an added layer of protection for the wearer, not so much for the people they are dealing with, and is not a substitute for a mask.
HERE is a link to an interesting article that shows the rate of infection with a mask compared to a face shield only.