PROFESSOR: So besides our physical expectation
that a virus such as SARS-CoV-2, coronavirus,
could be transmitted through respiratory droplets,
especially aerosol droplets through the airborne route
of transmission, there is substantial evidence--
both epidemiological evidence and some physical evidence--
to support this hypothesis.
So first, let's go through some of
the epidemiological evidence.
This is a very small fraction of what is available to date.
So one of the first incidents that
gave a sense there might be airborne transmission
was a religious event that took place at the Tiantong Temple
in Ningbo China, where there were hundreds
of people in attendance.
But in particular, there were two buses
that brought worshippers in one-hour bus rides
to this location.
And on one of the buses was what was
known to be the first infected person with COVID-19,
entering this region after having had contact
with others from Wuhan China, the initial source
of the outbreak.
And on the bus where the infected person was,
23 out of 68 passengers became infected during that ride.
And despite the fact that there was also
contact in the larger temple building
with many other people, very few there were infected.
And on the second bus, where they kept the same seating,
there were no infections.
So that gave a sense that on public transportation,
there could be substantial superspreading going on.
So shortly afterwards, there were a number of incidents,
including a case in a restaurant in Guangzhou, China,
where an infected person was sitting at a table
having dinner with a party there.
And there was a documented transmission
to a far corner of the room with a person
who had not been within a short distance such as six
feet of the infected person, had not touched
anything that could have led to contact transmission.
And it was concluded that it could only be explained
by airborne transmission.
Then there was a well-known case involving a cruise ship--
one of several such cases--
of the Diamond Princess cruise ship, where
a few infections were detected.
And on February 3, 2020, the ship
was quarantined in the harbor of Yokohama, Japan.
And during that quarantine, out of 3,011 passengers
and crew on the ship, there were a total, at the end of 12 days,
of 354 infected persons, starting
from an initial estimate of-- well, several known cases.
And by our own analysis, we will be estimating
there were perhaps around 20 initial cases, so
a dramatic increase in the number of infected persons
in 12 days, despite the fact that the passengers were mostly
confined to their cabins with very little movement between.
And also, later analysis showed that there
was very little statistical correlation between contact
with an infected person even being in the same room
and getting a transmission.
In other words, transmission was happening
between people in different rooms,
even on different floors, presumably
through the air handling system.
And among many other events that followed,
another famous case was one of the initial sources
of spreading in South Korea was the Shincheonji Church, which
held services over the period of February 16 to 25.
And a significant fraction of thousands of churchgoers
were infected, that led to the initial outbreak in Korea,
again, coming from contact of a large number of people sharing
an indoor space, where it's not possible for each
of those people to have been touching each other
or within 3 or 6 feet.
But rather, the only plausible explanation
is transmission through the air.
Another such example in Korea had
to do with a call center, where it
was hundreds of people working in a building with many floors.
And there was an infected person in one large room of one
of the floors.
And a significant fraction of the co-workers
were infected in that call center and yet a relatively
small number in other parts of the same floor
or on other floors, again pointing
to airborne transmission after subsequent analysis.
And then one of the first cases in the United States famously
occurred in the Skagit Valley Chorale,
which was holding a choir practice in Mount
Vernon, Washington, USA.
We will be analyzing this case in more detail.
But it very dramatically showed the evidence
for airborne transmission, because in a 2 and 1/2 hour
choir practice, one infected person managed to infect 53 out
of 61 others, two of whom later died,
when it could be documented that there
was no direct contact, short-range contact,
or touching between all of those people.
But rather, they simply shared the same indoor space.
Also there was a hint that respiration
and the type of respiration was important,
in that these people were singing.
And that led to a dramatic increase
in the rate of transmission compared
to other airborne events.
There are many other examples one could go through.
There have also been, recently, some meta analyses
of large numbers of cases, which further
point towards indoor airborne transmission of COVID-19.
One recent such study looked at 7,300 cases
of initial spreading of COVID-19 out
of the epicenter of the outbreak in Hubei Province, China.
So they looked at 320 cities, to the first known
cases in those cities outside of Hubei Province.
And they identified all the clusters
of two or more transmissions.
And of those clusters--
there were 72 of them.
And they all occurred indoors.
And out of those, 80% were at home in people's apartments.
And 34% also included some public transportation.
And out of all those clusters of transmission,
only one was documented to be occurring
outdoors, consistent with a wide range of other evidence.
Moreover, there has been a cataloging
of superspreading events such as the ones I've listed here.
And that list now numbers well over 1,000.
And out of all the superspreading events
that have been documented, all of them
have occurred indoors and involving large enough numbers
of people that airborne transmission is
the most plausible explanation.
So besides the overwhelming epidemiological evidence
for airborne transmission of COVID-19,
there is also growing physical evidence.
So first of all, other diseases that we've
discussed in this course, such as tuberculosis,
a bacterial disease, measles and the original SARS-CoV
coronavirus, which are viral diseases,
have all been established and believed
to be transmitted through respiratory aerosols.
SARS-CoV-2 is a very similar coronavirus to SARS-CoV-1.
And so it's plausible that its mechanism of transmission
would be the same, if not similar.
And indeed, there has been recent work demonstrating
that infectious aerosol droplets could
be isolated from infected patients with COVID-19.
And in particular, the most infectious droplets
observed were in the aerosol range
with radii less than 2 microns.
So the evidence is growing and frankly
becoming overwhelming that the airborne route of transmission
is important, if not dominant, for COVID-19.
So we will continue now by analyzing
how to mitigate and model the transmission
of a respiratory pathogen indoors.