WEBVTT

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PROFESSOR: So besides
our physical expectation

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that a virus such as
SARS-CoV-2, coronavirus,

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could be transmitted through
respiratory droplets,

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especially aerosol droplets
through the airborne route

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of transmission, there
is substantial evidence--

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both epidemiological evidence
and some physical evidence--

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to support this hypothesis.

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So first, let's
go through some of

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the epidemiological evidence.

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This is a very small fraction
of what is available to date.

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So one of the first
incidents that

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gave a sense there might
be airborne transmission

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was a religious event that took
place at the Tiantong Temple

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in Ningbo China, where
there were hundreds

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of people in attendance.

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But in particular,
there were two buses

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that brought worshippers
in one-hour bus rides

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to this location.

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And on one of the
buses was what was

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known to be the first
infected person with COVID-19,

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entering this region
after having had contact

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with others from Wuhan
China, the initial source

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of the outbreak.

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And on the bus where
the infected person was,

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23 out of 68 passengers became
infected during that ride.

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And despite the fact
that there was also

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contact in the larger
temple building

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with many other people, very
few there were infected.

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And on the second bus, where
they kept the same seating,

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there were no infections.

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So that gave a sense that
on public transportation,

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there could be substantial
superspreading going on.

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So shortly afterwards, there
were a number of incidents,

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including a case in a
restaurant in Guangzhou, China,

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where an infected person
was sitting at a table

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having dinner with
a party there.

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And there was a
documented transmission

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to a far corner of
the room with a person

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who had not been within a
short distance such as six

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feet of the infected
person, had not touched

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anything that could have
led to contact transmission.

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And it was concluded that
it could only be explained

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by airborne transmission.

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Then there was a well-known
case involving a cruise ship--

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one of several such cases--

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of the Diamond Princess
cruise ship, where

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a few infections were detected.

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And on February
3, 2020, the ship

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was quarantined in the
harbor of Yokohama, Japan.

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And during that quarantine,
out of 3,011 passengers

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and crew on the ship, there were
a total, at the end of 12 days,

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of 354 infected
persons, starting

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from an initial estimate of--
well, several known cases.

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And by our own analysis,
we will be estimating

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there were perhaps around
20 initial cases, so

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a dramatic increase in the
number of infected persons

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in 12 days, despite the fact
that the passengers were mostly

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confined to their cabins with
very little movement between.

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And also, later analysis
showed that there

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was very little statistical
correlation between contact

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with an infected person
even being in the same room

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and getting a transmission.

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In other words,
transmission was happening

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between people in
different rooms,

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even on different
floors, presumably

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through the air handling system.

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And among many other
events that followed,

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another famous case was
one of the initial sources

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of spreading in South Korea was
the Shincheonji Church, which

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held services over the
period of February 16 to 25.

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And a significant fraction
of thousands of churchgoers

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were infected, that led to
the initial outbreak in Korea,

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again, coming from contact of a
large number of people sharing

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an indoor space, where
it's not possible for each

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of those people to have
been touching each other

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or within 3 or 6 feet.

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But rather, the only
plausible explanation

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is transmission through the air.

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Another such
example in Korea had

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to do with a call
center, where it

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was hundreds of people working
in a building with many floors.

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And there was an infected
person in one large room of one

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of the floors.

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And a significant
fraction of the co-workers

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were infected in that call
center and yet a relatively

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small number in other
parts of the same floor

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or on other floors,
again pointing

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to airborne transmission
after subsequent analysis.

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And then one of the first cases
in the United States famously

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occurred in the
Skagit Valley Chorale,

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which was holding a
choir practice in Mount

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Vernon, Washington, USA.

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We will be analyzing
this case in more detail.

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But it very dramatically
showed the evidence

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for airborne transmission,
because in a 2 and 1/2 hour

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choir practice, one infected
person managed to infect 53 out

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of 61 others, two
of whom later died,

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when it could be
documented that there

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was no direct contact,
short-range contact,

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or touching between
all of those people.

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But rather, they simply
shared the same indoor space.

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Also there was a
hint that respiration

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and the type of
respiration was important,

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in that these
people were singing.

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And that led to a
dramatic increase

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in the rate of
transmission compared

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to other airborne events.

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There are many other examples
one could go through.

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There have also been,
recently, some meta analyses

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of large numbers of
cases, which further

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point towards indoor airborne
transmission of COVID-19.

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One recent such study
looked at 7,300 cases

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of initial spreading
of COVID-19 out

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of the epicenter of the outbreak
in Hubei Province, China.

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So they looked at 320
cities, to the first known

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cases in those cities
outside of Hubei Province.

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And they identified
all the clusters

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of two or more transmissions.

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And of those clusters--

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there were 72 of them.

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And they all occurred indoors.

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And out of those, 80% were at
home in people's apartments.

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And 34% also included some
public transportation.

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And out of all those
clusters of transmission,

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only one was documented
to be occurring

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outdoors, consistent with a
wide range of other evidence.

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Moreover, there has
been a cataloging

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of superspreading events such
as the ones I've listed here.

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And that list now
numbers well over 1,000.

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And out of all the
superspreading events

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that have been
documented, all of them

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have occurred indoors and
involving large enough numbers

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of people that airborne
transmission is

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the most plausible explanation.

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So besides the overwhelming
epidemiological evidence

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for airborne
transmission of COVID-19,

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there is also growing
physical evidence.

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So first of all, other
diseases that we've

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discussed in this course,
such as tuberculosis,

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a bacterial disease, measles
and the original SARS-CoV

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coronavirus, which
are viral diseases,

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have all been
established and believed

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to be transmitted through
respiratory aerosols.

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SARS-CoV-2 is a very similar
coronavirus to SARS-CoV-1.

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And so it's plausible that
its mechanism of transmission

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would be the same,
if not similar.

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And indeed, there has been
recent work demonstrating

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that infectious
aerosol droplets could

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be isolated from infected
patients with COVID-19.

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And in particular, the
most infectious droplets

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observed were in
the aerosol range

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with radii less than 2 microns.

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So the evidence is
growing and frankly

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becoming overwhelming that the
airborne route of transmission

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is important, if not
dominant, for COVID-19.

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So we will continue
now by analyzing

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how to mitigate and
model the transmission

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of a respiratory
pathogen indoors.