The study team screened 898 articles and included 6 reports of 5 observational studies from 4 countries (USA, India, Columbia, and United Kingdom) that tested face shields, goggles, and wraparound eyewear on 7567 healthcare workers.
The three before-and-after and one retrospective cohort studies showed statistically significant and substantial reductions in SARS-CoV-2 infections favoring eye protection with odds ratios ranging from 0.04 to 0.6, corresponding to relative risk reductions of 96% to 40%.
These reductions were not explained by changes in the community rates.
However, the one case–control study reported odds ratio favoring no eye protection (OR 1.7, 95% CI 0.99, 3.0). The high heterogeneity between studies precluded any meaningful meta-analysis.
None of the studies adjusted for potential confounders such as other protective behaviors, thus increasing the risk of bias, and decreasing the certainty of evidence to very low.
The study findings were published in the peer reviewed journal: Antimicrobial Resistance & Infection Control. https://aricjournal.biomedcentral.com/articles/10.1186/s13756-021-01025-3
Facial protection – for both wearer and close contacts – has been a crucial and controversial feature of the COVID-19 pandemic. For example, WHO recommends eye protection (goggles or face shield) for health care workers caring for COVID-19 patients but are not currently recommended for those caring for COVID-19 patients at home even when in the same room.
A major uncertainty has been how much protection is provided by different forms and combinations of facial coverings.
We know that respiratory viruses such as SARS-CoV-2 can enter the respiratory tract via the nose, mouth, or eyes and inoculation may occur via air-to-face or hands-to-face. Early in the COVID-19 pandemic the mix of these routes was unclear, but now the air-to-face route is agreed to be an important factor in most cases. Less clear is what are the proportions of inoculation that occurs via the nose versus eyes.
One of the earliest evidence for potential importance of eyes in infection transmission and therefore eye protection came from the Spanish influenza epidemic [2]. Despite evidence from the previous SARS and MERS outbreaks suggesting an impact of eye protection [3], the COVID-19 pandemic has seen much less research focused on eye protection.
A call for face shields to provide eye protection early in the pandemic seemed to be largely ignored in both practice and research [4], despite some promising studies. One early observational study in India of healthcare workers dealing with COVID-19 patients in the community showed a dramatic decline in the numbers of workers getting infected after face shields were made mandatory [5].
However, this study has received relatively little attention. A recent plea in the Lancet Microbe pointed to eye protection as a potential missing key [1].
Therefore, to examine the potential contribution of eye protection, we aimed to identify, appraise, and synthesise all studies that estimated the impact of any form of eye protection including face shields and variants, goggles, glasses, and others on transmission of SARS-CoV-2.
Discussion
Of the five observational studies identified, four showed substantial and statistically significant reductions in COVID-19 infections of health care workers after mandatory eye protection – mainly from face shields – was introduced; one case–control study showed an increase which was partly explained by an increase in community transmission.
All five studies were non-randomized, and did not adjust for potential confounders, so the overall risk of bias was high. Therefore, the evidence summarised here is very low certainty.
One important confounder for the before-after studies is any change in community transmission between the before-after periods; as demonstrated in Fig. 2, the community rates were generally higher in the after period; hence adjustment would only increase the size of the estimated reduction.
However, the higher rates may also mean increases in other protective behaviours which are not reported in any of the studies. Finally, while the studies’ main intervention was face shields, they also allowed the use of some other forms of eye protection such as goggles.
A previous review of observational studies on the effects in of eye protection in the SARS and MERS epidemics found a reduction in transmission of 66% and 76% respectively [3]. These reductions are comparable with the reductions seen in the three before-after studies of this review. Several laboratory studies using mannikins have examined the potential effects of face shields but vary greatly in their design and their application to real-world settings.
One study tested facing mannikins 25 cm apart with the emitter sending an aerosol spray with particles with a range of size from less than 0.3 µm to 10 µm; they found a reduction in particles received of 55% with face shields compared to 22% for a mask, and 97% for both [18].
A study with a mannikin 60 cm from the spray found greater reductions and face shields providing greater protection than masks [19]. However, these studies were of water aerosols not transmission of viral particles and are very incomplete simulations of human interaction. Neither separated eye protection specifically from face protection.
These studies provide suggestive evidence that face shields provide some protective effect, and that this may be substantial. These studies cannot determine how much of the protective effect is due to reduction of transmission from the eyes via nasolacrimal duct to nose.
Furthermore, a face shield – the main protection used – may provide additional inhalation protection as seen in some of the laboratory studies. While goggles also provide eye protection, face shields will likely give substantial protection against inhalation of droplets as well as eye protection and are more comfortable to wear.
Hence face shields – in addition to masks – should be considered for higher risk situations – such as contact tracing, quarantine workers, and some primary care consultations – or when there is substantial Covid spread in the community. Additional protection is likely to be particularly important to health care workers in settings where currently only face masks are being used.
While these observational studies show an interesting potential protection from face shields as add-on to face masks, they do not clarify whether such protection is from reduced inhalation or eye protection. Trials of the incremental value of face shields in addition or instead of face masks and comparative studies of face shields and eye goggles with face masks all seem warranted. Such studies should also measure comfort and adherence of different options used, as correct and sustained usage is also critical to effectiveness.
