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Tuesday 1 June 2021

Three Things I Don’t Know (Part II): ‘Flu

So, I asked myself, what unanswered scientific questions do I have, and are there answers out there for me? I had a think. And I came up with a list of three questions – and did my research. So here is the first of my three “Things I don’t/didn’t know” – let’s find out whether there’s an answer!

Why do we get colds and ‘flu more in winter?

In places with temperate climates, like the UK, colds and ‘flu are more common in winter. There’s a predictable seasonal pattern. Vaccines come out each year to help tackle new strains. But what drives this? Since we know about the germ theory, it’s clear that we don’t catch colds from chill winds, but instead from microbes that are passed from person to person. Since microbes thrive in the 37 degree environments of our bodies, why, I wonder, do they transmit more when it’s cold?

I turned to the internet…

The first really interesting thing I found was that the cold weather pattern is not always true! It’s true in temperate climates, yes, but where it’s tropical, we instead see more infections when it’s rainy and humid[1]. And this is definitely something scientists don’t fully understand – but do have theories about.

One leading theory is the immune effect. This theory states that when winter comes and it’s darker for longer, we get less sunlight and have lower vitamin D levels – and D is a key ingredient for making antimicrobials in the body. Indeed, one study found that giving children daily vitamin D boosted their immune system and meant they were less likely to get influenza A[2].

An extension of this theory states that nasal vasoconstriction (as we try to keep warm) limits white blood cells, stopping them getting to and killing off microbes entering our nostrils!

via Wikipedia Commons

Another key theory states that we cluster during the cold. We close our windows, restricting air flow and fencing ourselves in to a room filled with the breaths of others. We meet other people inside, and we stand closer. And thus we transmit more. Whilst this isn’t sufficient to explain the entire trend, it’s likely to be factor.

Thirdly, scientists claim that colds and ‘flus transmit faster in the cold. Cold air carries less water vapour, meaning more will fall out as rain, leaving the air itself drier, even when the weather seems wetter. And dry air is better at carrying viruses[3]. But what about tropical areas? Conclusions conflict. The best existing explanation is that outbreaks in the tropics are dominated not by dry air transmission, but by surface contact. Because when it’s hot and wet, viruses are less likely to be airborne, and more likely to stick to the stuff we touch[4].

Scientists are still exploring these theories, and measuring the extent of their effects. Hopefully, they’ll lead to better understanding of “‘flu season”, and help us reduce its prevalence year long.

If you can find papers or know anyone working on the topic, please leave a comment or Tweet us @TWeDK to share the information!

[1] Paynter, S. Humidity and respiratory virus transmission in tropical and temperate settings. Epidemiology & Infection 143.6 (2015): 1110-1118.
[2] Urashima, Mitsuyoshi, et al. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. The American journal of clinical nutrition 91.5 (2010): 1255-1260.
[3] Shaman, Jeffrey, et al. Absolute humidity and the seasonal onset of influenza in the continental United States. PLoS Biol 8.2 (2010): e1000316.
[4] Hirve, Siddhivinayak, et al. Influenza seasonality in the tropics and subtropics–when to vaccinate?. PloS one 11.4 (2016): e0153003.

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