3 ways to end a virus
It’s spring 2021.
The Alpha variant of the coronavirus
has spread rapidly,
becoming the dominant variant worldwide.
But another, more transmissible
variant is about to appear—
Delta.
What happens when two variants clash?
Let’s do a thought experiment.
Suppose that the variants reach
a hypothetical isolated city
of 1 million people who are
completely susceptible to both viruses
on the same day.
When a person here is infected with Alpha,
they transmit it to,
on average, 5 close contacts,
then begin to feel sick
and immediately isolate themselves
for the rest of the simulation.
The same thing happens with Delta,
except that an infected person transmits
it to, on average, 7.5 close contacts.
What would you guess happens next?
After six days,
Alpha will have infected 15,625 people.
Delta will have infected
more than 10 times as many.
Just 20 hours later, Delta will have
infected the rest of the population—
all before Alpha could infect 6% of it.
With no one left to infect,
Alpha dies out.
This model is
drastically simplified,
but it accurately reflects one thing
that did happen in real life:
when both variants competed,
Delta drove Alpha
towards extinction in a matter of weeks.
Viruses are wildly successful organisms.
There are about 100 million times
as many virus particles on Earth
as there are stars
in the observable universe.
Even so, viruses can and do go extinct.
There are three main ways that can happen.
First, a virus could run out of hosts.
This might have happened in early 2020
to a flu lineage known as B/Yamagata.
When much of the world shut down,
social distanced, and wore masks
to slow the spread of COVID 19,
that dramatically reduced the number of
hosts available for B/Yamagata to infect.
It’ll take a few more flu seasons
to know for sure
if it’s truly extinct or just
hiding out in an animal reservoir.
Many viruses, as part of their life cycle,
cause diseases severe enough
to kill their hosts.
This can be a problem because
if a virus kills all its hosts,
it could— in theory— run out of hosts
to infect and go extinct.
This almost happened
back in 1950s Australia.
At the time, Australia was overrun by the
European rabbit— an invasive species—
so, in an attempt to control
the population,
scientists released a virus
called myxoma,
which had been previously shown to be
almost 100% lethal to European rabbits.
During the initial outbreak,
as planned, tens, perhaps hundreds,
of millions of European rabbits died.
But as the virus spread,
it evolved a series of mutations
that happened to make it less deadly,
killing rabbits more slowly
and killing fewer rabbits overall.
With more infected hosts hopping around,
this strain of the virus was more likely
to spread than its deadlier cousin.
And of course, rabbits evolved too,
to mount better immune responses.
Overall, instead of killing
every single rabbit,
the virus evolved,
the rabbit population bounced back,
and both survived.
The second way a virus could go extinct
is if humans fight back
with an effective vaccine—
and win.
Vaccination campaigns have driven
two viruses essentially to extinction
since vaccines were invented in the 1800s:
smallpox and rinderpest,
which kills cattle.
More on vaccination later.
The third way a virus can go extinct
is if it’s outcompeted
by another virus or strain,
like we saw earlier with Delta and Alpha.
By the way, viruses don't always
compete with each other.
A viral species can carve
out its own distinct niche—
for example, influenza infects
your respiratory tract,
and norovirus infects cells
in your intestine,
so both of these viruses can co-exist.
A virus’ ecological niche can be tiny:
hepatitis B and hepatitis C viruses
can infect the same cell—
hep B occupies the nucleus,
and hep C occupies the cytoplasm.
In fact, epidemiologists estimate that
2 to 10% of people with hep C
are also infected with hep B.
So, will SARS-CoV-2—
the species of virus that causes COVID 19—
ever go extinct?
Variants within the species
will continue to arise.
Those variants might drive prior
ones to extinction, or not.
Regardless of how the variants compete
(or don’t),
the species itself—
to which all the variants belong—
is pretty firmly established among humans.
If we managed to vaccinate enough people,
could we drive SARS-CoV-2 to extinction?
Our vaccination campaign
against smallpox worked
because the vaccine was highly protective
against infection
and smallpox had no close animal reservoir
in which it could hide.
But SARS-CoV-2 can hide out in animals,
and our current vaccines—
while they provide excellent protection
against severe illness and death—
don't prevent all infections.
So, conceivably there are two ways
that SARS-CoV-2—
the entire species—
could go extinct:
a cataclysmic disaster could kill us all.
Or...
We could invent a universal vaccine
that prevents all SARS-CoV-2 infections—
those caused by all the variants
that currently exist and those that don’t.
Let's work toward that second option.
