"R0 and Incubation Periods- How Other Coronavirus Outbreaks Were Stopped"
I’ve talked about the emergence of other deadly
coronavirus outbreaks like SARS and MERS.
How were we able to get them under control?
MERS could be stopped because of its relatively low
“basic reproduction number,” abbreviated as R with a little subscript zero.
(That’s the R naught you may have heard about.)
The reason they call it the reproduction number is because the concept
goes back to the study of human population growth,
like the number of daughters on average each woman had.
But in infectious disease, it represents the number of people
a single infected individual is expected to pass the disease
along to in a susceptible population, so R naught
is a measure of how contagious a new pathogen is.
For the MERS coronavirus, the Middle East Respiratory Syndrome virus,
the R0 was only about 1, so each MERS patient tended
to transmit the disease to only one other person.
You can imagine how much easier a disease like that can be stopped
compared to a virus with the potential to spread exponentially,
viruses like the SARS or COVID-19 coronaviruses,
with an R0 of 2 or greater.
In the case of a virus with an Ro of 2, for example, unless stopped,
one infected person could become two, then four, then eight, and so on.
The coronavirus that causes COVID-19 may indeed be able
to latch onto receptors in the human respiratory tract
better than the coronavirus that causes SARS, and also replicate
better in the upper airways than SARS, but the primary reason:
there were more COVID-19 cases in the first month after it was reported
than SARS ever caused, revolves less around how contagious it is
and more around when it is contagious.
The three characteristics of microbes most likely to cause pandemics are
novelty, a new pathogen, so there’s no pre-existing immunity,
respiratory spread (I mean pneumonia is humanity’s fourth
leading killer even outside a pandemic),
and the third characteristic for optimal pandemic potential..
transmission before symptom onset.
The last four pandemics of respiratory disease were caused
by new flu viruses, originating from bird flu and swine flu viruses,
each of which fit all three of those criteria.
SARS, however, was not considered a pandemic,
despite spreading to 29 countries and regions.
Why did the World Health Organization only consider SARS
a “Public Health Emergency of International Concern,”
and how were we able to stop it within just a few months
at only around 8,000 cases and 800 deaths?
It was a brand new virus, spread via respiratory droplets,
but SARS lacked the third necessary characteristic:
significant spread before symptoms arise.
For SARS, the average incubation period—
the time between first becoming unwittingly infected
after exposure to the virus and first coming down
with symptoms—was around five days, similar to COVID-19.
But it took another six to eleven days, however, for SARS viral loads
to fully ramp up in upper respiratory tract secretions coughed
or sneezed from one person to the next.
So, even after falling ill, patients with SARS weren’t very infectious
in the first five days or so of the illness.
Since viral loads peaked about ten days after people
started feeling sick, after they knew they had it,
you can see how human-to-human transmission could be stopped
if patients could be isolated within the first few days
after the onset of symptoms.
And that’s exactly what happened.
A massive international effort spearheaded by the WHO
was able to identify all of the cases by their symptoms,
isolate the patients, and trace all their contacts.
That’s how we were able to effectively eradicate smallpox
from the planet too with a vaccine in the same way.
Smallpox was also only contagious after you knew who had it.
So fever screening at airports helped stop
the global spread of SARS in its tracks.
You didn’t become particularly infectious until after symptoms started
and, 100% of SARS patients developed a fever.
In a way, SARS was a disease designed to be stopped.
With MERS, 98% of the patients became febrile, meaning you got a fever.
In the case of COVID-19, though, as many as 36% — more than 1 in 3—
do not present with fever at the onset of symptoms,
and, more seriously, patients may be infectious during
the incubation period without any symptoms at all.
In fact, the viral load in an asymptomatic patient with COVID-19
was found to similar to that of symptomatic patients,
as many as 15 million viral copies within every quarter teaspoon of snot.
Same amount of virus in symptomatic snot—taken from someone
who’s just turned sick— compared to asymptomatic snot.
Spewing just as much virus right before you get sick.
With COVID-19 you can feel perfectly fine, no symptoms at all,
but actually have the disease and spread the disease
before the first cough, before you get a fever or any symptoms.
It’s the same thing with the flu.
That’s how new flu viruses can trigger pandemics too.
Like the flu, you can potentially spread COVID-19
before you know you have it, even while you’re feeling perfectly fine.
That’s a disease that’s hard to stop.
To slow the spread of that kind of disease, where you don’t know
who’s infectious and who’s not, you have to try isolating everyone.
That’s where social distancing measures are required.