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The Science Behind the Use of Face Shields and Face Masks

Decreasing the probability of the transmission of COVID-19 requires widespread and layered mitigation strategies.  Vanderbilt University will be employing a cohesive collection of comprehensive efforts that offers a best-in-class solution for the creation of a healthy, equitable, and safe environment within classrooms.

Key elements of these comprehensive classroom efforts for the 2020 academic year include:

  1. Six feet minimum physical social distancing will be arranged and required for students seated within instructional spaces.
  2. All students will be required to wear a face mask/covering within buildings and during class.
  3. All areas from where faculty will instruct, typically at the front of the classroom, will be identified and marked so that the minimum distance between the instructor and the first row of students is not less than six feet – and more if possible.
  4. Most classrooms will have clear mobile boards for use by the instructor as an additional barrier between them and the first row of students.
  5. Resulting from all of the above, and in light of recent data noted below, faculty will wear either face masks or face shields while teaching class.

Face coverings are a critical element of this strategy, contributing to the mitigation of transmission of airborne viruses. Other than respirators reserved primarily for health care and emergency responder personnel, all other face coverings offer varying levels of protection against the possibility of transmitting airborne viruses.  There are no definitive positions or documentation of the ability of any face covering to prevent the transmission of airborne viruses.  What they do, in combination with other efforts (building engineering controls and increased ventilation, physical distancing, and glass dividers), is add an important layer of defense towards the reduction of transmittable airborne particulates.

The data for face shields worn without face masks is not totally definitive and the reason why the Vanderbilt University Public Health Advisory Task Force, assembled by then-Interim Chancellor Wente and charged to assess recommendations for the return to campus, has recommended that only faculty within classrooms should wear a face shield without a face mask.  A preliminary study by Eli N. Perencevich, MD, MS; Daniel J. Diekema, MD, MS; and, Michael B. Edmond MD, MPH, MPA offers sufficient evidence for the efficacy of face shields in the way Vanderbilt intends to use them, which is coupled with other protective measures (social distancing, less density with classrooms, students wearing face masks/coverings, etc.) .

The referenced study states that “the mode of transmission of respiratory viruses has long been a subject of debate. Evidence to date suggests that SARS-CoV-2 is spread like other respiratory viruses: by infectious droplets emitted in close proximity (ie, within 6 feet) to the eyes, nose, or mouth of a susceptible person, or by direct contact with those droplets (e.g., touching a contaminated surface and then touching the eyes, nose, or mouth).6 Although droplet vs airborne transmission is likely to be a continuum, with smaller droplets able to be propelled further than 3 to 6 feet and remaining airborne longer after certain respiratory emissions,7 the implications of limited aerosol spread are most important in health care settings after aerosol-generating procedures, such as open suctioning of airways and endotracheal intubation or extubation.

Contact investigations for SARS-CoV-2 have confirmed community transmission rates that are consistent with droplet and contact spread (household attack rates of 10%, health care and community attack rates of <1%, and R0 [the effective reproduction number, or average number of new infections caused by an infected individual during their infection] of 2-3),5 and much different than for airborne viral pathogens, such as varicella zoster virus or measles (household attack rates of 85%-90% and R0 of 10-18).

This implies that simple and easy-to-use barriers to respiratory droplets, along with hand hygiene and avoidance of touching the face, could help prevent community transmission when physical distancing and stay-at-home measures are relaxed or no longer possible. The 2 major options for such barriers are face masks and face shields.

To reserve medical masks for health care facilities, the Centers for Disease Control and Prevention has recommended that all persons wear a cloth mask in public for source control. Cloth masks have been shown to be less effective than medical masks for prevention of communicable respiratory illnesses,8 although in vitro testing suggests that cloth masks provide some filtration of virus-sized aerosol particles.9 Face shields may provide a better option.

Face shields come in various forms, but all provide a clear plastic barrier that covers the face. For optimal protection, the shield should extend below the chin anteriorly, to the ears laterally, and there should be no exposed gap between the forehead and the shield’s headpiece. Face shields require no special materials for fabrication and production lines can be repurposed fairly rapidly. Numerous companies, including Apple, Nike, GM, and John Deere, have all started producing face shields. These shields can be made from materials found in craft or office supply stores. Thus, availability of face shields is currently greater than that of medical masks.

Face shields offer a number of advantages. While medical masks have limited durability and little potential for reprocessing, face shields can be reused indefinitely and are easily cleaned with soap and water or common household disinfectants. They are comfortable to wear, protect the portals of viral entry, and reduce the potential for autoinoculation by preventing the wearer from touching their face. People wearing medical masks often have to remove them to communicate with others around them; this is not necessary with face shields. The use of a face shield is also a reminder to maintain social distancing but allows visibility of facial expressions and lip movements for speech perception.

Most important, face shields appear to significantly reduce the amount of inhalation exposure to influenza virus, another droplet-spread respiratory virus. In a simulation study, face shields were shown to reduce immediate viral exposure by 96% when worn by a simulated health care worker within 18 inches of a cough.10 Even after 30 minutes, the protective effect exceeded 80% and face shields blocked 68% of small particle aerosols,10 which are not thought to be a dominant mode of transmission of SARS-CoV-2. When the study was repeated at the currently recommended physical distancing distance of 6 feet, face shields reduced inhaled virus by 92%,10 similar to distancing alone, which reinforces the importance of physical distancing in preventing viral respiratory infections. Of note, no studies have evaluated the effects or potential benefits of face shields on source control, ie, containing a sneeze or cough, when worn by asymptomatic or symptomatic infected persons. However, with efficacy ranges of 68% to 96% for a single face shield, it is likely that adding source control would only improve efficacy.

Major policy recommendations should be evaluated using clinical studies. However, it is unlikely that a randomized trial of face shields could be completed in time to verify efficacy. No clinical trial has been conducted to assess the efficacy of widespread testing and contact tracing, but that approach is based on years of experience. Taken as a bundle, the effectiveness of adding face shields as a community intervention to the currently proposed containment strategies should be evaluated using existing mathematical models. The implicit goal of face shields alone or in combination with other interventions should be to interrupt transmission by reducing the R0to less than 1. Notably, effective control of even the most infectious pathogens, such as measles, does not require a vaccine with 100% efficacy. No burden of 100% efficacy should be placed on face shields or any containment policy because this level of control is both impossible to achieve and unnecessary to drive SARS-CoV-2 infection levels into a manageable range.”1

The other major reasons for this position on face shields for faculty are that (1) instructors may be required to verbally project loudly as they may not always have amplification available in all classrooms and (2) there may be students with hearing and other impairments who rely on being able to see the instructors mouth when speaking.

Resulting from this information and subsequent discussions the task force is comfortable that allowing face shields for faculty is a reasonable solution.

1 Moving Personal Protective Equipment Into the Community, Face Shields and Containment of       

  COVID-19, Eli N. Perencevich, MD, MS; Daniel J. Diekema, MD, MS; Michael B. Edmond MD,  

  MPH, MPA