Summarised from various sources18 30-32
- Superspreading events: the virus is often transmitted at mass events
from one or a few people to many people.33 34
- Long-range transmission: the virus spreads in shared air among people
who have never physically met or touched any common
surface.35
- Asymptomatic and presymptomatic transmission: a high proportion of
people who pass on the virus have no symptoms at the
time.36-38
- Indoor dominance: transmission is many times greater indoors than
outdoors,39 and ventilation reduces
transmission.40
- Nosocomial infections occur despite strict contact-and-droplet
precautions, and reduce when airborne precautions are
added.41
- Whilst SARS-CoV-2 is difficult to isolate from
air,42 viable SARS-CoV-2 has been detected in the
laboratory43 and in real-world settings where
infected people had been.44 45
- SARS-CoV-2 has been detected in air filters in building ducts (could
only have got there via airborne route).46
- Transmission between animals has occurred when their cages were
connected via air ducts.47
- The virus exhibits overdispersion (one person with Covid-19 may infect
no-one; another may infect dozens).48
- Empirical evidence supporting droplet or fomite transmission is
surprisingly sparse.11 12
Countries such as Japan,21 where aerosol scientists
were part of the inside track with the ear of
government,13 had introduced airborne precautions
early in the pandemic (Box 1). But in most Western countries, the
aerosol narrative—at least initially—fell on deaf policy ears.
By July 2020, aerosol scientists were alarmed that official advice was
based on over-simplistic and incorrect models of transmission (which had
perpetuated for decades in the infection control
literature49), and wrote an open letter to the World
Health Organisation offering to help.40
“Covid is ‘situationally’ airborne”
The World Health Organisation’s early guidance on protecting healthcare
workers from Covid-19 recommended a standard level of protection for
most activities but a higher level for so-called “aerosol-generating”
ones,50 reflecting a long-established (but perhaps
flawed) research tradition.51 Its Infection Prevention
and Control Research and Development Expert Group for COVID-19
(IPCRDEG-C19) did not include any aerosol scientists and did not welcome
the open offer of help. A new scientific brief was quickly published,
reiterating the dominance of droplet transmission in most circumstances
but acknowledging airborne transmission in certain
situations—aerosol-generating medical procedures and crowded,
poorly-ventilated indoor settings.52
This “situationally airborne” narrative has persisted despite evidence
against it (next section), and has far-reaching implications. If
aerosols transmit only when certain procedures are being performed, only
a small fraction of healthcare staff need higher-grade protection, and
only when performing particular procedures. If that assumption is
incorrect, staff (especially non-medical and less senior ones) and
patients in most healthcare facilities are
under-protected.53
“Everyone generates aerosols; everyone is
vulnerable”
A systematic review revealed wide disagreement among guideline panels
about which procedures and activities should count as “aerosol
generating” (and hence earn respirator-grade protection for the person
doing them).54 Many procedures (e.g. taking a
nasopharyngeal swab) were inconsistently classified, some
aerosol-generating acts (e.g. coughing) were not procedures, and several
procedures were classified as aerosol-generating only because they
induced coughing.54
A detailed review of the physiology and aerodynamics of respiratory acts
concluded that coughing, sneezing, breathing (especially if laboured),
speaking (especially loudly) and singing generated significant amounts
of aerosol; well-documented super-spreader events for Covid-19 involved
a critical triad of poor ventilation, crowding and loud
vocalisation.34
These findings raise some paradigm-challenging questions. Should
respirator-grade protection be worn by everyone—including other
patients—whenever patients are coughing ? Should more attention
be paid to measures higher up the hierarchy of controls, such as
ventilation or filtration of air, or ensuring that fewer people share
air and for shorter periods5?
In the sections which follow, we consider some dramatic consequences of
the government’s decision to deny, dismiss or downplay the importance of
airborne transmission of SARS-CoV-2.