As the novel coronavirus
continues to spread worldwide,
one common theory is that the pandemic
will die down when warmer weather arrives.
Donald Trump: Looks like by April, you know,
in theory, when it gets a little warmer,
it miraculously goes away.
I hope that's true.
Narrator: But the answer
is a little more complicated than that.
So, why do so many people think that summer
will stop the COVID-19 spread?
Seasonal patterns are commonplace
for many infectious diseases.
Take the flu, for example.
Jeffrey Shaman: We get these big surges of seasonal flu
that peak in January and February
in the Northern Hemisphere
and then the opposite side of the year,
in July and August, in the Southern Hemisphere,
places like Australia, New Zealand, South Africa.
So the question has been why,
and there are three groups of hypotheses
that have tried to explain this.
Narrator: The first of these hypotheses
is human behavior.
The flu spreads more in colder seasons,
when people are more likely to gather in enclosed spaces,
and lowers once people venture outside.
However, COVID-19 appears to spread
much more efficiently than the flu,
which is why strict physical-distancing laws
have been enforced,
but also why you may have heard experts
warn of a rise in cases
once people go back to their offices, schools
and normal life.
The second hypothesis has to do with your immune system.
Immune systems have been found to be better
at fighting respiratory viruses
during longer summer days,
which also increase the production
of the immunity booster vitamin D.
So when summer rolls around,
people might have a better chance of fighting off
the coronavirus as well.
And also why you may have seen suggestions
during this pandemic to spend time safely outdoors,
for example, on a balcony or in your garden,
or even to take vitamin D supplements.
But people have focused on the flu's
third seasonality hypothesis, the environment.
That's because both COVID-19 and the flu
are respiratory diseases
that can be spread through droplets.
Once these droplets have been
coughed, sneezed, or spat out,
they're in the hands of the environment
until they reach a new victim.
Studies have found that the transmission levels
decrease when the influenza virus encounters
a humid and hot environment.
That's the opposite of what we see during winter,
where cold temperatures go hand in hand with dry air.
One study found that when humidity levels
were at 23% after 60 minutes,
70% to 77% of the flu virus survived.
But when humidity increased to 43%,
only 14% to 22% of the flu virus particles
survived in the same amount of time.
Shaman: This humidity change seems to be
modulating the survival of the virus,
such that it doesn't last as long in the summertime
and it lasts a much longer time
when it's drier in the wintertime.
And that may encourage and allow the transmission,
and hence we see these surges in the wintertime.
Narrator: And scientists have a few answers
as to why this is the case.
The influenza virus is a simple genetic code
surrounded by a protein layer and a fatty envelope,
similar to SARS-CoV-2,
the virus that caused COVID-19.
But once it leaves the body,
a layer of water encapsulates it as well.
Some scientists believe that in high-humidity environments,
the virus picks up more water particles
and becomes heavier,
which forces it to drop out of the air,
lowering the chance of transmission.
In dry conditions, like in winter,
those same droplets evaporate,
making the virus much lighter and airborne for longer.
But that evaporation has another
positive transmission effect.
The water droplet and virus are not alone.
They are also joined by salts,
and one study found that as the water droplets
evaporate in the dry air,
those salts are removed,
increasing the virus' stability
and probability of transmission.
But it's not just humidity that has an impact
on the influenza virus.
Researchers have found the virus to be
much more robust in colder temperatures too,
lasting far longer than when exposed to higher temperatures.
So, by now, you may be thinking the flu and COVID-19
are both respiratory infections
with somewhat similar structures,
so surely they will follow
similar seasonal patterns as well.
But let's be clear:
They're not the same.
Not only in transmission rates, symptoms, and death rates,
but also in the length of time that they have existed
in human populations.
This is pretty important to consider,
especially when you look at
how virus transmissions slow down.
Micaela Martinez: At the beginning of the high season,
you have some big susceptible pool of individuals,
and then as that epidemic rose and cases climbed,
that susceptible pool's getting smaller and smaller
and smaller and smaller,
so when you get up here at the top,
you have fewer people that can get infected.
Narrator: You can think of this like kindling for a fire.
The longer the fire burns,
the more fuel is used,
so the less kindling you have,
and the fire will eventually begin to burn out.
Both the smaller susceptible pool
and the onset of the low season,
such as a rise in humidity,
dictate when a disease's high season
begins to turn around.
Martinez: That's very different from
a pandemic situation,
where you very well may have
seasonal changes in transmission,
but then that's having to work essentially all by itself
if there's not a shrinking of the susceptible pool
that comes along with it.
Narrator: For a new viral pandemic like the current one,
the susceptible pool is huge.
Populations haven't developed immunity to it
like they have against, say, the flu.
And when you throw in a lack of vaccines and treatments,
the fuel for the fire is almost endless.
This isn't to say that COVID-19 won't be hindered
at all by seasonality.
One initial study from China showed decreases
in SARS-CoV-2 transmission
along with increases in humidity.
However, another study
wasn't able to establish that connection.
And both should be taken with a grain of salt
in the first place,
as neither has been produced in a lab setting yet.
What could give a better insight
are the ways in which
the four known common human coronaviruses,
which are partly responsible for the common cold,
react to seasons.
Shaman: So, there's evidence from other coronaviruses,
and all four of them do this,
that they have this pronounced seasonality.
So there is this enticing possibility
that has to be considered seriously
and not dismissed
that this novel coronavirus may also have this property
because it's related to them.
Narrator: But notoriously humid places,
such as Hong Kong and Louisiana,
have seen prominent transmissions.
And experts warn that when large groups
begin to gather again,
we could actually see another spike in cases,
which brings us back to the fact
that we may not see seasonal patterns just yet.
But this doesn't mean
that we won't see one in the future.
If and when COVID-19 becomes
a more regular presence in populations,
there is a strong possibility of it being seasonal.
As immunity, vaccines, and treatments
become more readily available,
the susceptible pool will decrease,
and the spread will likely increase
only when the conditions are more favorable
and therefore seasonal.
But, again, this is a new disease,
and these are only predictions.
Current methods of stopping the outbreak
such as self-isolation are far more efficient
than waiting for it to stop naturally with the weather.