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Go to Brilliant.org/SciShow to check out their course Joy of Problem Solving. [ ♪INTRO ]. You may have heard conflicting messages about whether COVID-19, the disease caused by the coronavirus SARS-CoV-2, will become less severe or go away in the summer months.

And that's because we're just not sure yet if this virus will follow a seasonal pattern. Based on what we know about the flu, as well as other coronaviruses, researchers think it's possible that COVID-19 is seasonal. The problem is, even if that's the case, it probably won't make much of a difference for a while.

Because it turns out pandemic viruses don't spread quite the same way as their more established cousins. Here's the thing about a novel pandemic virus like this one: It is rare for there to be any built-in immunity within the population, so it can pretty much spread to anyone. So there's a lot of ways to model disease spread, but when it comes to the impact of seasons on viruses, researchers are interested in how the virus spreads -- and the weather.

Specifically, the humidity. While cold temperatures themselves don't make you sick, it is true that some diseases, like the seasonal flu, are much more common during the colder months of the year. We tend to be packed together inside more, increasing our average rate of person-to-person contact -- or so the theory goes.

Along with susceptibility, close contact is probably the biggest contributor to a seasonal virus' spread. But it's not the only thing going on in winter that plays a part. See, winter isn't just cold: In many places, especially temperate regions of the planet, it's less humid.

And studies have consistently found that some viruses have an easier time surviving when the humidity is lower, which makes them easier to pass between people. Researchers don't fully understand how higher humidity limits the spread of certain viruses. But we know that certain viruses, like influenza and coronaviruses, can spread by hitching a ride on the little droplets of moisture that leave your lungs when you breathe or cough -- often, on the outer surface of those droplets.

When it's more humid, the air can support larger droplets of moisture, with more surface area for the viruses to hang out. But being at the surface of the droplets is actually bad for these viruses. Influenza and coronaviruses are both enveloped viruses, meaning they're surrounded by a layer of oily lipid molecules.

At the droplet's surface, that envelope can get disrupted pretty easily -- which can destroy the virus. So, there are reasons to think COVID-19 could be sensitive to humidity, and therefore could be seasonal -- eventually. At least one paper has found that higher humidity levels may have reduced the transmission rates of COVID-19 in some areas of China.

However, it is a preprint that hasn't passed peer review yet. A lot of people have also pointed out that there seem to be fewer cases in warmer, more humid climates, but there are flaws in that argument as well — in Singapore, for example, the virus has spread plenty. Which brings us to the main problem with the question of whether this virus isn't a fan of humidity: For now, the answer... doesn't really matter.

Because pandemic viruses, even those that turn out to be sensitive to changes in humidity, aren't seasonal when they're new. Remember, no one -- or almost no one -- is immune to a brand new virus. Which means too many people are vulnerable to the infection for humidity to make a meaningful impact in how much it spreads.

Over the years, we've seen this play out with flu pandemics. One key difference between seasonal and pandemic flu is that question of how many people are susceptible. The 2009 swine flu pandemic, for example, didn't follow the pattern of established seasonal flu strain — even though influenza is usually affected by humidity, it had no problem continuing through the spring and summer in many places.

In a 2010 paper, researchers started from actual New York City swine flu statistics to model how a flu pandemic could spread, if different percentages of people were susceptible. They found that even in August, when humidity levels should in theory reduce transmission the most, the pandemic could still spread if eighty percent [80%] of people were susceptible to infection. For May and June, even sixty percent [60%] susceptibility was enough.

For reference, at the time we're making this video, only a very small fraction of people in the US are confirmed to have gotten sick with COVID-19. The real total here is likely to be higher -- maybe much higher -- but even according to the biggest estimates, less than 5% of the population are likely to have been infected so far. So way more than 80% of the population is still susceptible to it.

So, yes — although it will be a while before we really know, there's reason to think. COVID-19 could become a seasonal affliction, just like the seasonal flu. But even if it is, that likely won't give us the reprieve that people are hoping for.

There's just too much of the population that's still susceptible to the virus. Our goal is to eventually get the number of people who are susceptible to the disease down without them getting the disease. Right now limiting contact with others is the best way to limit the spread of the virus.

And if we're lucky, that will buy researchers enough time to develop a vaccine -- and that is how we decrease the number of people who are susceptible, by making them immune. We hope that once people have gotten sick and recovered, they are immune too -- but we don't actually know that yet. And if they are, we don't know for how long.

But one way or another, eventually, our efforts will decrease the number of people who are susceptible. And hopefully soon, we will see COVID-19 fade in the background as a virus with occasional, possibly seasonal outbreaks, instead of the world-disrupting pandemic it is now. Whatever the humidity outside.

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