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July 9, 2020

The World Health Organization has updated its position on how the new coronavirus is transmitted between people, and what is new is that microscopic droplets that are released during activities such as medical procedures, choral singing, rapid breathing in groups in gyms can be contagious, or from conversations in a restaurant.

This is a transfer with the so-called "aerosols", which was warned a few days ago by nearly 240 scientists from 32 countries. Until now, the WHO has adopted the position of March 29 - that larger drops released by sneezing, coughing or talking can be contagious. But they are heavier and fall quickly, so it is necessary to distance people and maintain cleanliness of surfaces on which these drops could fall.

Aerosols are able to stay longer in the air, to travel a greater distance and thus increase the time and space of the infectious area.

Airborne transmission

Airborne transmission is defined as the spread of an infectious agent caused by the dissemination of droplet nuclei (aerosols) that remain infectious when suspended in air over long distances and time.Airborne transmission of SARS-CoV-2 can occur  during medical procedures  that  generate  aerosols  (“aerosol  generating  procedures”).  WHO,  together  with  the scientific  community, has  been  actively  discussing  and evaluating  whether  SARS-CoV-2  may  also  spread  through  aerosols  in  the  absence  of  aerosol  generating procedures, particularly in indoor settings with poor ventilation. The physics of exhaled air and flow physics have generated hypotheses about possible mechanisms of SARS-CoV-2 transmissionthrough aerosols. These theories suggest that

1) a number of respiratory droplets generate microscopic aerosols (<5 μm) by
 evaporating, and 

2) normal breathing and talking results in exhaled aerosols.

Thus, a susceptible person could inhale aerosols, and could become infected if the aerosols contain the virus in sufficient quantity to cause infection within the recipient. However, the proportion of exhaled droplet nuclei or of respiratory droplets that evaporate to generate aerosols, and the infectious dose of viableSARS-CoV-2  required to cause infection in another person are not known, but it has been studied for other respiratory viruses.One experimental study quantified the amount of droplets of various sizes that remain airborne during normal speech. However, the authors acknowledge that this relies on the independent action hypothesis, which has not been validated for humans and SARS-CoV-2.Another recent experimental model found that healthy individuals can produce aerosols through coughing and talking,  and  another  model suggested  high  variability  between  individuals  in  terms  of  particle  emission  rates  during  speech,  with  increased  rates  correlated  with  increased  amplitude  of  vocalization.  To  date,  transmission  of  SARS-CoV-2  by  this  type  of  aerosol route has not been demonstrated; much more research is needed given the possible implications of such route of transmission.Experimental studies have generated aerosols of infectious samples using high-powered jet nebulizers under controlled laboratory conditions. These studies found SARS-CoV-2 virus RNA in air samples within aerosols for up to 3 hours in one study and 16 hours  in  another,  which  also  found  viable  replication-competent  virus.  These  findings  were  from  experimentally  induced  aerosols that do not reflect normal human cough conditions.Some studies conducted in health care settings where symptomatic COVID-19 patients were cared for, but where aerosol generating procedures  were  not  performed,  reported  the  presence of  SARS-CoV-2  RNA  in  air  samples,  while  other  similar  investigations in both health care and non-health care settings found no presence of SARS-CoV-2  RNA;   no studies have found viable  virus  in  air  samples.Within  samples  where  SARS-CoV-2  RNA was  found,  the  quantity  of  RNA  detected was in extremely low numbers in large volumes of air and one study that found SARS-CoV-2 RNA in air samples reported inability to identify viable virus.  The detection of RNA using reverse transcription polymerase chain reaction (RT-PCR)-based assays is not necessarily indicative of replication- and infection-competent (viable) virus that could be transmissible and capable of causinginfection.Recent clinical reports of health workers exposed to COVID-19 index cases, not in the presence of aerosol-generating procedures, found no nosocomial transmission when contact and droplet precautions were appropriately used, including the wearing of medical masks as a component of the personal protective equipment (PPE).   These observations  suggest that aerosol transmission did  not  occur  in  this  context. Further  studies  are  needed  to  determine  whetherit  is  possible  to  detect  viable  SARS-CoV-2  in  air  samples from settings where no procedures that generate aerosols are performed and what role aerosols might play in transmission.Outside of medical facilities, some outbreak reports related to indoor crowded spaces  have suggested the possibility of aerosol transmission, combined with droplet transmission, for example, during choir practice , in restaurantsor in fitness classes.In  these  events,  short-range  aerosol  transmission,  particularly  in  specific  indoor  locations,  such  as  crowded  and  inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out. However, the detailed investigations of these clusters suggest that droplet and fomite transmission could also explain human-to-human transmission within these clusters. Further, the close contact environments of these clusters may have facilitated transmission from a small number of cases to many other people (e.g., superspreading event), especially if hand hygiene was not performed and masks were not used when physical distancing was not maintained.

Main findings

 •Understanding  how,  when  and  in  what  types  of  settings  SARS-CoV-2  spreads  between  people  is  critical  to  develop  effective public health and infection prevention measures to break chains of transmission.

•Current evidence suggests that transmission of SARS-CoV-2 occurs primarily between people through direct, indirect, or close contact with infected people through infected secretions such as saliva and respiratory secretions, or through their respiratory droplets, which are expelled when an infected person coughs, sneezes, talks or sings.

•Airborne  transmission  of  the  virus  can  occur  in  health  care  settings  where  specific  medical  procedures,  called  aerosol  generating  procedures,  generate  very  small  droplets  called  aerosols.  Some  outbreak  reports  related  to  indoor  crowded  spaces  have  suggested  the  possibility  of  aerosol  transmission,  combined  with  droplet  transmission,  for  example,  during  choir practice, in restaurants or in fitness classes.

•Respiratory  droplets  from  infected  individuals  can  also  land  on  objects,  creating  fomites  (contaminated  surfaces).  As  environmental contamination has been documented by many reports, it is likely that people can also be infected by touching these surfaces and touching their eyes, nose or mouth before cleaning their hands.

•Based  on  what  we  currently  know,  transmission  of  COVID-19  is  primarily  occurring  from  people  when  they  have  symptoms, and can also occur just before they develop symptoms, when they are in close proximity to others for prolonged periods of time. While someone who never develops symptoms can also pass the virus to others, it is still not clear to what extent this occurs and more research is needed in this area.

•Urgent  high-quality research  is  needed  to  elucidate  the  relative  importance  of  different  transmission  routes;  the  role  of  airborne transmission in the absence of aerosol generating procedures; the dose of virus required for transmission to occur; the settings and risk factors for superspreading events; and the extent of asymptomatic and pre-symptomatic transmission.

Citations from original article are published under CC BY-NC-SA 3.0 IGO license.



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