|Passengers wearing face masks and face shields queue to ride a bus, amid the coronavirus disease outbreak (REUTERS)|
(livemint) - Face shields are inferior to masks in preventing droplet transmission of SARS-CoV2, the virus that causes covid-19, the World Health Organization (WHO) has said.
In its latest interim guidance—mask use in the context of covid-19—the apex global public health agency has said that at present, face shields are considered to provide a level of eye protection only and should not be considered equivalent to masks in providing respiratory droplet protection and/or source control. Source control refers to the use of masks to cover a person's mouth and nose and to help reduce the spread of large respiratory droplets to others when the person talks, sneezes, or coughs. This can help reduce the spread of SARS-CoV2 by someone infected but does not know it.
Current laboratory testing standards only assess face shields for their ability to provide eye protection from chemical splashes. “In the context of non-availability or difficulties wearing a non-medical mask (in persons with cognitive, respiratory or hearing impairments, for example), face shields may be considered as an alternative, noting that they are inferior to masks with respect to droplet transmission and prevention. If face shields are to be used, ensure proper design to cover the sides of the face and below the chin," the WHO said.
However, the global health agency has also warned that a mask alone, even when it is used correctly, is insufficient to provide adequate protection or source control. Other infection prevention and control (IPC) measures include hand hygiene, physical distancing of at least 1 metre, avoidance of touching one’s face, respiratory etiquette, adequate ventilation in indoor settings, testing, contact tracing, quarantine and isolation. Together these measures are critical to prevent human-to-human transmission of SARS-CoV2, the WHO said. At present there is only limited and inconsistent scientific evidence to support the effectiveness of masking of healthy people in the community to prevent infection with respiratory viruses, including SARS-CoV2, it said.
Further clarifying doubts on usage and efficacy of masks in preventing virus transmission, the WHO has tightened its previous guidelines. The WHO has advised that the general public should wear a non-medical mask indoors such as at shops, shared workplaces, schools or outdoor settings where physical distancing of at least 1 meter cannot be maintained.
If indoors, unless ventilation has been be assessed to be adequate, the global public health agency advised that the general public should wear a non-medical mask, regardless of whether physical distancing of at least 1 meter can be maintained, the WHO's interim guidance said. Since start of the covid-19 pandemic, the WHO has advised the use of masks as part of a comprehensive package of prevention and control measures to limit the spread of SARS-CoV2.
The WHO also issued elaborated guidelines on mask usage among children. “Children aged up to five years should not wear masks for source control. For children between six and 11 years of age, a risk-based approach should be applied to the decision to use a mask; factors to be considered in the risk-based approach include intensity of SARS-CoV2 transmission, child’s capacity to comply with the appropriate use of masks and availability of appropriate adult supervision, local social and cultural environment, and specific settings such as households with elderly relatives, or schools," the interim guidance said, and added that mask use in children and adolescents 12 years or older should follow the same principles as for adults.
As far as quality of masks is concerned, the WHO’s latest guidance said that homemade fabric masks of three-layer structure (based on the fabric used) are advised, with each layer providing a function—an innermost layer of a hydrophilic material, an outermost layer made of hydrophobic material and a middle hydrophobic layer which has been shown to enhance filtration or retain droplets.
“Factory-made fabric masks should meet the minimum thresholds related to three essential parameters: filtration, breathability and fit. Exhalation valves are discouraged because they bypass the filtration function of the fabric mask rendering it unserviceable for source control," the WHO said.
Further, the WHO said that high quality research is required to address the knowledge gaps related to modes of transmission, infectious dose and settings in which transmission can be amplified. Currently, studies are under way to better understand the conditions in which aerosol transmission or superspreading events may occur.
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