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.