John Timmer, in his article on ARS Technica, writes about a case study on the face shields done in India and involves individuals working in the healthcare field. The research was done in regards to data collection on workers wearing only face masks and workers wearing a combination of face shields and masks.
Focus group were given gloves, surgical masks, and alcohol hand sanitizer; it took only two weeks for the first workers wearing only masks to develop symptoms – of 62 tested, a dozen had been infected.
When the program started again, a clear plastic face shield has been given in addition to protection used in the first round. The health workers were also given ethanol to clean their face shields. From all workers tested, zero were positive.
As indicated the first, the group wearing only masks showed more symptoms after exposure, where a group of healthcare providers that used both shield and a mask showed a drastically reduced number of cases to zero. Thus, the face shields are likely what provided the critical difference.
The original study has also been published on JAMA Network on August 17, 2020.