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A Virus for Every Season

The fact that many viral infections have seasonal peaks defies simple explanations, making extrapolations for COVID-19 quite difficult.

We have all heard the claim, passed down the generations, that you catch a cold by being exposed to cold weather. That’s what I used to believe when I was younger. Then I graduated to the explanation that respiratory tract infections, like the cold and the flu, peak in the winter months because we spend so much more time indoors. Close contact favours viral transmission, you see. But the reality is a lot more complex than these simplistic accounts, which break down when we contemplate their logic. The barely understood interactions that give rise to the seasonality of viruses make it hard to predict how COVID-19 will behave when winter comes.

The hypothesis that the flu is a winter illness because we spend more time indoors is appealing (especially as we are told to avoid indoor crowds during our coronavirus pandemic) and may have held sway prior to the Industrial Revolutions, but now? Many of us are spending on average 90% of our time in enclosed spaces regardless of the season, living, sleeping and working in insulated buildings with air conditioning and central heating. In fact, based on data from the United States’ Environmental Protection Agency, Americans only spend on average one to two hours more indoors in cold weather compared to warm weather. Is that enough to send flu cases spiking in the winter? Also, indoor crowding is particularly important in schools, so why don’t we see cases of the flu skyrocketing in the fall and spring, when children spend much of their time inside school buildings? We can also look at the problem the other way: the Southwestern states in the U.S. get very hot, forcing people to spend time indoors during heat waves... but their flu season is still in the winter.

When we zoom out from exclusively considering the flu, we realize that not all viruses peak during the winter months. The journal Science put together a nice infographic showing the seasonality of some infectious diseases pre-vaccination. Chickenpox used to peak in the spring. Polio was a summer disease, so much so that swimming was branded as a dangerous activity. And some viruses, like adenoviruses and rhinoviruses responsible for many cases of the common cold, are actually year-round. There cannot be a single explanation or else every viral infection would be in sync. Winter would be the season of illnesses and summers would feel miraculous. Scientists have sweated over trying to identify the many factors that play a role in virus seasonality. One important factor? Humidity.

A number of studies are suggesting that, at least when it comes to influenza, there are two kinds of meteorological conditions that are great for transmission of the virus: cold-dry (seen in winter in temperate regions) and humid-rainy (the wet season in the tropics). The idea is that the virus can survive really well when it’s very humid and also when it’s very dry, but not in the Goldilocks zone in the middle. (One proposed explanation I saw is rather complicated. High humidity means heavy droplets fall on surfaces and that’s how people get sick. Low humidity means salt in the droplet crystallizes out of solution, leaving the virus intact. Between 40 and 60% relative humidity, the salt crystallizes in solution and destabilizes the virus.) But scientists too often rely on relative humidity to study this phenomenon, while some are arguing the key is in fact absolute humidity. In a temperate zone like most of Canada and the U.S., 50% relative humidity in the summer and winter are very different things. Relative humidity denotes how close the air is to being unable to hold onto any more water molecules, but this absolute threshold goes down as it gets colder. However, even if humidity plays a key role, its effect differs from virus to virus, with enveloped viruses (like influenza and the new coronavirus) being more fragile to changes in the weather and non-enveloped viruses (like many cold viruses that infect us year-round) more resilient.

Viral infections that rely on an animal, like West Nile fever and Zika which are transmitted to us via mosquitoes, can follow seasonal patterns because the animal’s life cycle is itself influenced by changes in the weather. And then there’s the question of how seasons affect the human body.

Does our immune system change with the seasons? It might, and this hypothesis is being studied. Scientists have hypothesized that these changes could be driven by the diminishing sunlight in the winter and thus affect our production of melatonin, the so-called sleep hormone. But if our immunity is “restructured” in some way every few months, we would again expect all viral infections to peak at the same time, and if melatonin has such sway over our immune system, our use of artificial lighting would probably even things out. Other scientists have studied our mucus production and how it changes depending on the temperature and humidity. And there’s the impact of breathing in dry air in the winter on the skin cells inside our nostrils: they detach and we lose some of them. Is that enough to make a difference? Finally, there’s the hypothesis that lower vitamin D levels during the winter predispose us to infection, although the literature on the subject is mixed. It may sound like I’m hemming and hawing with words like “may” and “could”, but that is still the language being used in the literature in 2020. We don’t have a clear grasp on the issue.

So how can this patchwork knowledge about seasonality be applied to COVID-19? SARS and MERS, both caused by coronaviruses, are of no help because the first was contained quickly while the second does not spread very well from human to human. The coronaviruses that cause some of the cases of the common cold behave like the flu: they are out in force in the winter months. The thing about pandemics is that early on, the fact that we’re all so susceptible to them trumps seasonality. It’s possible that COVID-19 will eventually settle into a seasonal cycle, perhaps similar to the flu.

The complex interplay of variables that results in a given virus having such a specific seasonal pattern made me think of the Drake equation. Formulated in 1961, the Drake equation was meant to spark dialogue over the possibility of alien life by taking into account a handful of factors, like the fraction of stars in our galaxy that might have planets and the average number of planets that could support life as we know it. The possibility of extraterrestrial life was not solely determined by one variable but rather by a succession of quantities that needed to be estimated. Likewise, the seasonality of viruses cannot be attributed uniquely to outside temperature, or indoor gatherings, or even shifting humidity levels throughout the year. It’s a result of how all of these factors and more play together with the unique characteristics of individual viruses. If anything, it’s a reminder of the extraordinary complexity of life.

Take-home message:
- Many illnesses in humans caused by viruses peak in a particular season, while others are present year-round
- The reason why many viral illnesses peak in a given season is not simple and involves a web of interactions, some known, others speculated about
- Some of these reasons include humidity, the life cycle of animal vectors for the virus, human behaviour, and how the environment from season to season alters our human biology


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