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 In its latest transmission guidance, the WHO acknowledged that some
outbreak reports related to indoor crowded spaces have suggested the
possibility of aerosol transmission, such as during choir practice,
in restaurants or in fitness classes.
But the WHO said more research is "urgently needed to investigate
such instances and assess their significance for transmission of
COVID-19."
The report follows an open letter from scientists who specialize in
the spread of disease in the air - so-called aerobiologists - that
urged the global body to update its guidance on how the respiratory
disease spreads to include aerosol transmission.
Based on its review of the evidence, the WHO said the coronavirus
that causes COVID-19 spreads through contact with contaminated
surfaces or close contact with infected people who spread the virus
through saliva, respiratory secretions or droplets released when an
infected person coughs, sneezes, speaks or sings.
The new guidelines do, however, suggest people should avoid crowds
and ensure good ventilation in buildings, in addition to social
distancing, and encourage masks when physical distancing is not
possible.
"This is a move in the right direction, albeit a small one. It is
becoming clear that the pandemic is driven by super-spreading
events, and that the best explanation for many of those events is
aerosol transmission," said Jose Jimenez, a chemist at the
University of Colorado who signed the letter, which was published on
Monday in the journal Clinical Infectious Diseases.
In a press briefing on Thursday, Dr. Anthony Fauci, director of the
U.S. National Institute of Allergy and Infectious Diseases, said
there is not a lot of solid evidence yet on airborne transmission of
SARS-CoV-2, but added: "I think it's a reasonable assumption that it
does occur."
Although incomplete, Fauci said the evidence so far is "the
fundamental basis for why we are now so intent on getting people -
particularly people without symptoms - to wear masks. To be able to
see if we can mitigate against that," he said.
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Only a very small number of diseases are believed to be spread via aerosols, or
tiny floating particles. These include measles and tuberculosis - two highly
contagious pathogens that can linger in the air for hours and require extreme
precautions to prevent exposure.
Prior WHO guidance only acknowledged airborne transmission of the novel
coronavirus during specific medical procedures. Linsey Marr, an aerosol expert
at Virginia Tech who contributed to the WHO letter, said in an email that she is
encouraged the agency is now acknowledging that airborne transmission may occur.
But she said the WHO is using an "outdated definition of droplets and aerosols"
and is too focused on the size of the droplets and the distance they travel.
WHO defines aerosols as being under 5 microns because only particles that small
could float in the air long enough to be inhaled. However, Jimenez and Marr said
a much larger range of particle size has been shown to contribute to infection.
Rather than size, they said the differences between droplets and aerosols should
be based on how the infection occurs: If a person inhales the virus and becomes
infected, it's an aerosol. If the infection occurs by contact, they are
droplets.
Although WHO has been focused on airborne transmission at long distances, Marr
said breathing in aerosols "is of greater concern at close contact and when
people are in the same room."
(Reporting by Julie Steenhuysen in Chicago, Michael Erman in New York, Stephanie
Stephanie Nebehay in Geneva and Ankur Banerjee in Bengaluru; Editing by Shounak
Dasgupta and Dan Grebler)
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