The Routes of Transmission of SARS-Cov-2

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SARS- CoV is spread by multiple routes

Droplets and fomite transmission.

The first route of transmission is the direct route in which respiratory droplets are generated by an infected person and inhaled by a healthy person. The second route is an indirect route in which an infected person touches a common object or emits respiratory droplets on the common object. A healthy person touching the object and then her nose or mouth and then become infected.

Effective counter measures  to prevent SARS-Cov 2 infection in healthcare setting or in public settings need to take into account both transmission routes. Face coverings are the primary method for breaking the primary transmission route. Hand sanitation immediately prior to and after touching potential fomites as a primary effective method for breaking the indirect transmission route.  Although masks may reduce the amount of respiratory droplets emitted by infected people.

There is some confusion about the utility of masks in reducing SARS-Cov 2 transmission. Face masks can refer to both surgical masks and cup shaped N95 respirators. Published guidelines from CDC state that surgical masks provide some protection against large droplets. Intended purpose is that it provides fluid resistant protection against large droplets, splashes or sprays of bodily or other hazardous fluids, and protects patients from the wearer’s respiratory emissions.

They are loose fitting and with regards to their filtration they do not provide the wearer with a reliable level of protection from inhaling smaller airborne particles and is not considered respiratory protection. 

N95 respirators are rated to block 95% of aerosols of 200 nm size or larger. It is evaluated, tested and approved by NIOSH as per the requirement in 42 CFR part 84.

Both surgical masks and N95 respirators use a a polypropylene  meshes of filter  layer, referred to as melt-blown layer.  It is melt-blown electrostatic polypropylene fibers.  It catches most virus particles by physical block and by electrostatic attraction while allowing through air.

The main difference between surgical masks and respirators is that the former is loose fitting so air can still be inhaled around the mask . This is why they are believed to be less effective than N95 respirators. However we can still get an idea of their effectiveness of blocking SARS-Cov 2 by studying the studies on flu transmission. SARS-Cov 2 is similar in size to influenza virus.

Indeed three controlled studies in Canada and Australia showed that N95 respirators appeared slightly more effective than surgical masks in preventing acquisition of respiratory disease by healthcare workers but the differences were within statistical error.  In addition a study in Australia showed that wearing surgical masks by adults cut the risk of acquiring flu from their children by 60-80% .  Thus the available evidence shows that the use of surgical masks by the general public is merited, however high exposure environment such as healthcare settings using N95 respirators for maximal protection would be preferred.

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