What Will Winter Bring for Covid-19?
How our understanding of influenza can give us clues for what to expect this winter with the pandemic.
Covid-19 cases are on the rise as the dark night of winter looms large over our country, as well as many other countries in the northern hemisphere. A question that is on many people’s minds is, “What will winter mean for the coronavirus?” It seems second nature that winter brings with it cold and flu season, and by extension, yet another wave of coronavirus cases. But why is this the case? Why are respiratory viruses seasonal?
This has been one of my favorite things to learn about since joining a flu research lab because the simple answer to this question is, well, we don’t really know. Fascinating, right? Though we have known about the seasonality of respiratory diseases for quite some time, we actually don’t have a clear understanding of why they are seasonal. It is even possible that this coronavirus pandemic could give us crucial data to help us answer this question.
But most normal people (aka, not nerdy scientists) would like an answer to this question now so we can be prepared. Without data on a new virus, the best we can do is compare it to the behavior patterns of similar viruses to get an idea of what might happen with the coronavirus in winter. So, what do we know about influenza seasonality?
The problem
We are so accustomed to the seasonality of cold and flu that it is intuitive that it is inherently tied to winter. That’s just the way it works, right? Not so fast! Influenza does go through strong seasonal patterns in temperate regions; however, seasonal patterns have also been documented in tropical regions. And just to complicate it further, in some regions, the flu doesn’t show evidence of seasonality at all!
What’s going on here? This is a harder question to answer than it may seem. There does seem to be an association between environmental factors and flu seasonality, as even in non-temperate regions, flu seasonality can correlate with weather patterns. However, this also suggests that seasonality isn’t inherently tied to the winter.
What causes seasonality? Although we don’t have a conclusive answer to this question, there are several theories for the mechanism of seasonality. The most discussed theories can be broadly divided into three categories.
Three theories
- Environmental factors
This category might be the one that is most intuitive to us. Theories that focus on environmental factors to explain flu seasonality usually highlight weather measurements such as temperature, humidity, or solar radiation. This category of theories has been around for many years and is probably one of the first sets of explanations for flu seasons (at least in modern medical journals). In fact, it seems that the very term ‘influenza’ was coined on the basis that cold temperatures were implicated for influenza seasonality when it was derived from the Italian phrase ‘un influenza de freddo’, or ‘the influence of the cold’ in 1743.
Humidity levels have also been implicated as a possible mechanism to drive seasonality since 1962, and virus survival at different levels of humidity was studied as far back as 1943 and 1950. More recent studies have also looked at the effects of humidity, temperature, and solar radiation and found some evidence that each of these might influence seasonality.
2. Human behavioral factors
Perhaps more relevant to the control measures put in place to combat the coronavirus, a second class of theories to explain flu seasonality focuses on the seasonal behavior patterns of humans, usually highlighting our tendency to spend more time indoors during the winter, or the fact that much of our school year (and thus large aggregates of students) is concentrated in the winter months. Being in close proximity, especially in direct contact, to others increases the exposure and risk of transmission of the flu from an infected person.
Several studies have shown that school closures (and openings) have a significant impact on influenza rates among children, suggesting that behavioral factors (such as increased contact and enclosed spaces during the school year) could play a major role in seasonality. However, most of these studies did not find any significant reduction in influenza rates among adults associated with school closures, and at least one study found that school closures for less than 8-weeks did not significantly affect flu epidemics. Other studies have tied influenza spread with work travel behaviors. Interestingly, the current pandemic has the potential to give us more data that might support this group of theories (see below).
3. Externally driven body cycles
This third class of theories is somewhat of a hybrid of the first two, focusing on different cycles that our bodies go through that are driven by seasonal fluctuations. For example, it has been shown that temperature and humidity can have an effect on nasal mucus clearance, which in turn could have an effect on how well viruses can infect us. Further, there have been many studies showing strong evidence that vitamin D regulates innate immune responses, and that vitamin D levels may provide a protective effect against influenza. Since vitamin D production is influenced by sunlight, and during the winter we typically spend less time in the sun, this could be a mechanism of seasonality. Similarly, melatonin levels in our body, regulated by daily light/dark cycles, have also been suggested as a mechanism of seasonality, as days are shorter in the winter in temperate regions.
What will the winter mean for COVID?
The actual mechanism that drives flu seasonality is likely to be a combination of all of these influences (and possibly more). It is still up for debate which mechanisms are the most important. However, what we can certainly take away is that respiratory viruses typically have strong seasonality in temperate zones. I think we can take the knowledge we have and tentatively answer some questions about what the winter holds for COVID-19.
Will Covid-19 be seasonal? The answer to this question is likely yes, but probably not for a couple of years or so. When a novel virus emerges, there is little to no immunity in the human population and the virus has a plethora of new hosts it can infect. In this case, it is likely that the sheer number of people that the virus can infect will overpower any seasonal influences, and it will simply look like a continuous outbreak. But this leads to another question.
Then why have there been at least two (if not three for the US) ‘waves’, or peaks, of coronavirus infection already? I think this has to do with initial outbreaks and our responses to the outbreaks. The first peak happened early on when the virus hit large coastal cities such as New York. This initial shock spurred most states into action, encouraging social distancing and masking. I think there is evidence that these policies had an effect on the transmission of the virus, as shown by the peak and then decline in daily cases. The second ‘wave’ then hit the south, with states such as Texas, Florida, and California (I know that Cali wasn’t always included in this grouping, but their cases were soaring at the same time) experienced large increases in cases. Further responses (and the natural cycle of viral infection and transmission) lead to lowered daily cases after a large peak. Now we are in the third ‘wave’, as many of the midwestern and western states are experiencing a similar explosion of cases. My theory is that this pattern has a lot to do with the population densities and delays coming from our responses to the pandemic.
However, it should be noted that if this is true, then there really haven’t been three distinct waves of the virus. It has been one continuous wave with several peaks. Note that even after the peak was passed for each state, their cases did not drop to zero, but rather continued on at a steady pace. This likely has something to do with why each of our peaks has been higher than the last.
Fortunately, it seems that we have bought a little bit of time, and with that time we have optimized treatment protocols such that even though more people are getting the virus, a smaller percentage are dying. But this in no way means we are out of the woods, or that many, many people are not still dying. And if the 1918 Spanish flu is any indication, then a true second wave could claim many more lives than the first. To be clear, I hope this isn’t the case, and I think that the differences between influenza viruses and coronaviruses indicate that it likely won’t be. However, we can’t know for sure. We are still very much in the middle of the pandemic.
So, what will winter mean for Covid? Well, if it behaves anything like the flu, it is likely that we will continue to see large increases and peaks in the number of cases as winter comes, whatever the driving mechanism behind the increase in winter is. And if it is true that our social distancing responses did have an effect on viral transmission, flattening the curve so to speak, then it is likely that the holiday season will spark even more cases as large family get-togethers take place.[1] Interestingly, this would give indirect evidence that group 2 theories (human behavior) might play a dominant role in the seasonality of viruses.
Conclusion
I’m not here to tell you what you should or shouldn’t do. I know that social isolation also has detrimental effects on us. I know that virtual learning is not the same as person-to-person instruction. I know that stay-at-home orders have a disproportionate effect on small business owners and service-industry workers, which in turn can have detrimental health effects. In my opinion, most of the tension surrounding this topic stems from pretending like one side of the fence has all the right answers and the other side just doesn’t care. This is far from the truth.
I’m just here to think about the interesting science questions. However, these answers certainly have real-world applications. And using what we know, we can make informed decisions (however fallible they may be) as to what we do this holiday season. Please be careful, especially around those who are in vulnerable populations, such as the immunocompromised and the elderly. It is likely that COVID is not going away any time soon. The recent news revealing the efficacy of several trial vaccines is very encouraging. But even then, the rollout of the vaccine to the general public probably won’t happen until April at the earliest. There’s a lot of time between now and April. A lot of time and dark winter.
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[1] Interestingly, at least one group has surveyed several countries in Europe and used mathematical modeling to suggest that during a normal year, contact is reduced during holiday periods. Similarly, another group found that holidays lead to a 20–29% decrease in influenza transmission among school children, though there was no reduction in rates among adults. However, it is hard to say if these findings would hold true for this year, as work and school contacts have already been reduced via working from home and virtual learning options. It seems more likely that the holidays will increase contact this year since the current threshold of contact is low. Further, in the case of the pandemic virus, traveling can spread the virus to new places where it hasn’t yet gained a foothold, whereas seasonal influenza is typically already endemic across the nation.
