It’s almost four years since the World Health Organization declared COVID-19 a pandemic. As the specter of lockdowns loomed in the West, scientists had already been scrambling for weeks to learn as much as they could about the emerging threat. One key question centered around whether there could be a seasonal pattern to infections – the first reports of disease had cropped up during the wintertime, and we’re used to seeing other respiratory bugs wax and wane across the year. But so far, at least, COVID-19 is proving to be a different beast.

Why are some viruses seasonal?

The idea that COVID-19 could be a seasonal disease was a logical one when we consider how many other viral infections follow a seasonal pattern. 

Respiratory infections like the common cold, flu, and respiratory syncytial virus (RSV) all peak during the winter months. In countries with temperate climates, polio tends to peak during the summer.  

For many winter illnesses, the cold, dry weather is a big factor in helping them to spread. While it’s a myth that going out in the cold will give you a cold, it is true that influenza viruses, for example, thrive much better when the air is cool and less humid, giving them a much greater chance of finding an unwilling human to latch onto. 

Another factor that can come into play during the dark, gray winter months is vitamin D – or, more specifically, the lack thereof. Writing for The Conversation, immunologist Margherita T. Cantorna explained that vitamin D deficiency during the winter, a particular problem for people with deeper skin tones and those living at northerly latitudes, could impact the immune system’s ability to fight off infections.

Then there’s our behavior. In many regions, winter brings months of freezing, treacherous weather. Combine that with the festivities that also crop up this time of year, like Christmas and Hanukkah in the Northern Hemisphere, and what are you left with? Lots of people cozying up together indoors, sharing food, laughter – and all the germs they’ve picked up along the way.

So, is COVID-19 seasonal?

For a while, at least, it looked like COVID-19 was following these familiar patterns. As a disease that primarily causes respiratory symptoms, it was easy for many of us to start thinking of it in a similar way to the flu.

At various points in the pandemic, the evidence seemed to agree with this assumption. One study in 2021 concluded that its findings “support[ed] the view of COVID-19 as a true seasonal low-temperature infection,” based on data from the initial surge of the virus before mitigation measures like lockdowns were put in place. The authors found that higher transmission rates were linked with lower temperatures and humidity levels, much as we see every year with flu.

However, earlier this year a study published in the journal npj Viruses found that, while the SARS-CoV-2 virus is more stable in wintry conditions, this did not seem to translate to higher levels of airborne transmission when tested in the lab. 

While we have undeniably seen spikes of COVID-19 cases during the winter over the past few years, some experts say it is too soon to define a “COVID season”. For one thing, these peaks did not happen at the same time each year.

Instead, what could be more important – at least at our current stage of learning to live with this virus – is human behavior.

“You might look at that data and think, maybe this is just a biannual virus,” infectious disease physician Cameron Wolfe told Wired, referring to the summer and winter bumps in infection rates that we’ve previously observed with various COVID variants. “But that gets much harder to say when you factor in that as a society we behave very differently, seasonally.”

And it’s not just normal seasonal changes in behavior, as we talked about earlier. During this pandemic, people in different regions have been subject to ever-changing rafts of mitigation measures, from mask mandates to social distancing, to shelter-in-place orders. We know now that the biggest risk of infection comes from prolonged contact – the person who shares your desk in the office, or your neighbor at the theater or cinema. It’s only comparatively recently that these sorts of activities have been restarted in lots of places.

While COVID-19 may not have yet settled into a reliable seasonal pattern, that doesn’t mean it never will, as virologist Vincent Munster explained to Science News. Rather than being solely dependent on the weather though, Munster believes any seasonality is more likely to be determined by behavioral factors, and fluctuating immunity due to immunization and prior infection. 

Ultimately, science has a lot to figure out about seasonality, even for viruses that have been with us for centuries. COVID-19 has been in our lives for less than five years. Unfortunately, that probably means that we still have a lot of “waiting and seeing” ahead of us. 

The content of this article is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of qualified health providers with questions you may have regarding medical conditions.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.