Imagine someone had, like, the worst body odor ever, like,
so, so bad.
Now imagine that that was contagious,
like, if you smelled it, there was a chance that you would
get infected by that body odor, and you'd have to live with it,
like, forever.
Sounds horrifying, right?
Nobody would want that.
Now imagine that there is a 25% chance
that if you caught that odor, you yourself couldn't smell it.
And you could still spread it.
You'd still affect others with it.
But you'd have no idea why everyone
was running the other direction when they smelled you coming.
You pop over to a store.
You go to work.
You know, a few people there contract the worst body
odor imaginable because you had no idea you were spreading it
around, even with the visual cue of people
running the other way.
Now imagine that no one can smell the odor until, say,
an hour after you've left the room.
Imagine how many people would catch it
and how hard it would be to pin down
the culprit of that contagious odor.
That is the world of asymptomatic transmission
and why it is just so scary.
If you turn on the news right now,
you're going to hear the word asymptomatic over and over
and over again.
Of asymptomatic infections.
You may be asymptomatic.
People who are asymptomatic.
What we call asymptomatic infection.
But it's jargon, and I hate jargon.
Do we really know what that means?
Why do some people not have symptoms?
Do asymptomatic people have viruses in them?
Am I asymptomatic?
Is there a way to know?
There is.
So let's find out.
[TECHNO MUSIC]
Hey there, germ bags.
Trace here.
Thank you so much for stopping by.
COVID-19 is the latest mutant in the coronavirus family.
One of the things that scientists have recently
discovered about this novel or new coronavirus
is that it is possible that it might be asymptomatic.
Asymptomatic means that you have a disease without symptoms,
like asymmetrical, you have something without symmetry,
or atheist, you have someone without a god or gods.
Most of us associate being sick with symptoms,
a cough, a sneeze, fever, chills,
you know, that kind of thing.
But you can be absolutely 100% as sick
as someone who is experiencing that
and have no symptoms at all.
That is called asymptomatic.
According to a study from the British Medical Journal,
78% of the new COVID-19 cases reported over one day in China
in April 2020 they said were asymptomatic.
Now that number, you should take with a grain
of salt. It is still way too early to know all the facts.
It was just one day of looking and one place,
so it's not generalizable.
But that's a big deal.
If even a few of those are right, it's a big deal.
Dr. Maria Van Kerkhove at the World Health Organization
told ProPublica they found, quote, "few truly
asymptomatic cases," and instead,
many people considered asymptomatic
went on to develop symptoms.
And yet, asymptomatic disease is real and why epidemiologists
need to track and test people repeatedly to be sure and safe.
More research is definitely needed.
Epidemiologists study the pattern
of the spread of disease, and to them, asymptomatic carriers
are a huge challenge.
Carriers can spread the virus far and wide
and never know that they have it.
This might call to mind something like HIV or the flu,
but that's a little different, because that's presymptomatic.
Presymptomatic means that you've got the virus,
you're probably contagious, but you're not
feeling the symptoms yet.
Still dangerous, but not quite asymptomatic,
because asymptomatic means you'll never
have an indicator of disease.
Both are dangerous, but one even more so.
So let's use herpes as an example.
16% of Americans have the herpes simplex type 2 virus
in their bodies right now.
I might have it.
You might have it.
A lot of people have it.
Of those people, 90% of us have no symptoms.
You can get herpes from someone who's never and will never
have a breakout, and that is how so many of us have it.
It lives in our bodies without us ever even knowing,
and the only way to find out if you have HSV-2
is to get tested for it.
But without symptoms, why would you ever do that?
People who are asymptomatic have the viruses in their systems
and thus the antibodies for those viruses.
They are still contagious or what
experts call shedding virus.
Asymptomatic carriers of herpes are shedding viruses
about one of every 10 days.
That's about half as often as people who are symptomatic.
But when they are shedding, it's just the same amount of virus.
So they're just as contagious, but unlike symptomatic people,
they don't have a herpes breakout
to signal to everyone and themselves
that they're infected.
People who don't know that they have an infection
are often found to be a major source of the spread
of that infection.
Let me tell you a story from American history.
I love history, OK?
Mary Mallon was an Irish immigrant
in the US in the late 19th century.
She worked as a domestic servant and a cook
for several wealthy American families.
Of the eight houses she worked in, seven experienced typhoid,
and some who got typhoid died.
There was no cure for it at the time.
A researcher was studying the illness in New York
and eventually realized the common denominator here
was Mary.
She was infected, as it turned out, with salmonella typhi
and carried it for the rest of her life
without showing symptoms.
She was the first case of a healthy carrier of salmonella
typhi, an asymptomatic person.
It's a deadly typhoid infection, and it spread it
to at least 122 people.
Many died, and she was in denial about it for a long time.
And she didn't spread the disease on purpose,
but she did spread it all the same.
And now her name is famous, Typhoid Mary.
In epidemiology, they have the 80-20 rule.
80% of disease transmission events in epidemics
are caused by 20% of people.
It's well-established, and it's why people are so worried
about presymptomatic and asymptomatic carriers
of infections like COVID-19.
This is why physical distancing is so important.
We're keeping those 20% of us at home to slow the spread
and reduce people's chances of coming
into contact with an infection.
But because people don't have symptoms,
it's tough to nail down who they are and how many there are.
Today, we know that with typhoid,
for example, maybe one to six out of 100 people
become asymptomatic.
The seasonal flu might be five to 35 out of 100.
Ebola, which is highly infectious,
that could be 27 to 71 out of 100.
COVID-19 is so early to tell, but maybe 18 out of 100,
maybe 78 out of 100, maybe just 33 out of 100.
Why some people don't show symptoms
is because of how our immune system works.
It's not a perfected, honed machine.
And in fact, frankly, it's a bit of a mess.
But in the case of salmonella typhi,
for example, a great study was done by Stanford.
And they looked at how the bacteria survived the fight
with our immune system.
Put simply, the immune system starts fighting the infection.
Typhoid keeps up the good fight and causes
a lot of inflammation around the body.
The body doesn't like that after a while.
So eventually, in some people, the immune system
will change tack and start to secrete
anti-inflammatory factors.
Those factors, aside from decreasing inflammation,
tell the white blood cells to chill out and stop actively
fighting the typhoid.
Some strains of salmonella could trick some bodies
into releasing those factors right away,
letting the salmonella live inside the body
without ever triggering symptoms.
So in some people with some infections,
our immune system knows that the bug is bad,
but like a corrupt cop, it just lets
the infection do its thing.
As long as it doesn't kill us, it just
lives there, making more of itself, eating,
you know, partying, whatever.
Up here in the head, we are none the wiser.
The thing is, every infection is different.
There are so many known asymptomatic disorders,
like early-stage high blood pressure, glaucoma,
tuberculosis, MRSA, primary hyperparathyroidism,
endometriosis, hypertension, diabetes, and now
maybe COVID-19, too, and there are so many more.
And this is where we come back to COVID-19.
We need to test people, even if they aren't sick,
to know if asymptomatic people are out there.
And we need to do it again and again and again,
because people could get this and not know they have it
and still spread it.
We should be testing millions of Americans,
and we're not for some reason.
At this point, you could probably
get why you should care.
But let's go back to the body odor example.
You have debilitating body odor.
You couldn't smell it.
That would be bad.
If you had that body odor, and it was contagious,
and everyone knew that you had it, that's terrible, too.
But if you had it, and you were asymptomatic,
meaning no one could smell your stench, including you,
then you'd be spreading it everywhere
you go to new people in every place you visit.
Before you run away from this video thinking everyone
could have COVID, you should stop for a second,
because I'm not saying that.
I'm actually saying the opposite.
We don't know that.
It's too soon, and more research is needed.
But it's better instead to act as if you have
it because of that 80-20 rule.
What if you're in the 20?
Pretend you have it and act as if.
But don't take my word for it.
This is Sam.
She is neck deep in COVID research every day.
So I asked her to weigh in here.
Closely related to asymptomatic transmission
is presymptomatic transmission or transmission just
in the early stages of feeling symptoms.
Now this has been pretty robustly reported
amongst COVID-19 patients.
We're in the middle of a pandemic.
People are really closely monitoring
how they feel, one, 'cause they're home,
and two, because we're anxious.
I mean, I'm anxious.
Are you anxious?
I'm anxious.
People are more closely monitoring how they feel,
so they might start noticing they're unwell a little earlier
than they typically might.
And it's been suggested that people
in this presymptomatic, really early stage, just when they
start feeling COVID-19 symptoms, they're
actually shedding the most amount of virus.
And so we think that that might be their most contagious
period, the first few days after they
start feeling those initial symptoms.
But with COVID-19, transmission is mostly
happening via droplets that are produced, typically when
you're coughing or sneezing.
Of course, you can produce some droplets
when you're speaking or breathing.
But you're producing the most with coughing or sneezing.
And it's yet to be shown whether asymptomatic and presymptomatic
individuals produce enough droplets
with enough virus in them to actually get other people sick.
I'm sure we'll find the answers to these questions soon.
But as of yet, we just don't know.
Sam is a fantastic resource.
You can follow her @science.sam on Instagram
or heysciencesam on Twitter.
Highly recommend, five stars.
You can also find information on her website.
We need a lot more testing, like, a lot more, like, oh,
so much more.
And we need people to wear masks to stop us infecting others.
We need to wash our hands.
We need to avoid touching our face holes
so we don't wipe pathogens on boxes at the store or wine
bottles at the store or door handles.
We still don't know a lot about what
happens when we talk or sing or breath or sneeze or cough
or spit, because that saliva could come out of our mouth,
and it could carry infection with it.
And if you're asymptomatic, again,
you could be the 20 in that 80-20 rule.
Ideally, a vaccine would help the 80% fight
off the infection.
But that's not possible right now.
Right now, physical distancing, wearing masks
over the noses and mouths can stop us spraying those droplets
onto shelves and bus railings and into the breeze.
Look, let's be honest.
It's not very American to go to the doctor
when you're feeling ill.
We tend to put things off.
We all know why.
It's expensive to get sick in the United States.
There are political and societal ideas floating around
to try and fix that problem.
But regardless, at the moment, many Americans
are trained to not trust or see doctors,
which is a problem, because asymptomatic people either
won't show symptoms of disease, and presymptomatic people
will not show them until later.
And that means epidemiologists usually discover them
when they're looking for something else, which
is, again, why we need people testing our humans constantly.
And this doesn't just apply to COVID.
But, you know, kind of right now, it does.
The term asymptomatic was coined in the 19th century.
And even though humans had been diagnosing
each other with illnesses and infections for centuries,
for some reason, it had never really occurred to us
that we could have a disease and not show it in any way,
but come to find out, it's fairly common.
The A in front of another word thing
was how the ancient Greeks were, like, nah, so, like, asymmetry,
or without a, aTrace, meaning without a Trace,
so you can keep me around all the time, right?
So, like, consider subscribing, maybe, or sharing this
with a friend who doesn't know what asymptomatic means.
Not asymptotic, though.
That means approaching a value or curve arbitrarily closely.
It's math, not medicine.
Anyway, thanks again.
I'm Trace.
Thanks for watching.
I'll see you in the future.