Since most transmission happens within buildings and vehicles, and since the total amount of time spent in these hardly varies throughout the year, the common belief about "spending more time indoors" in winter driving influenza and COVID-19 makes very little sense. Variations in outdoor temperature and humidity hardly matters since most transmission is not outdoors.

What does change, on average, in a profound way, is 25-hydroxyvitamin D levels. Please see my arguments about this, which cite pertinent research. https://nutritionmatters.substack.com/p/covid-19-seasonality-is-primarily . In winter and before real warmth arrives in late spring, in-building and in-vehicle air is hotter and dryer that the outside and than in summer - and it is more likely to be recirculated. I regard this as the second most important factor in this seasonality. It is impossible to quantify it reliably, but I guess 70% or more vitamin D, 20% interior air arrangements, 10% other factors.

If the strong seasonality of influenza and COVID-19 (though COVID-19 is perturbed by new variants, lockdowns and so some extent quasi-vaccines) is primarily due to 25-hydroxyvitamin D levels, then it follows that population wide, proper, vitamin D3 supplementation would reduce the transmission of these diseases, probably below pandemic levels (especially with early treatments for COVID-19) all year round. This is not patented and no-one will make a significant profit from it. Quite the reverse - sales of drugs and vaccines would be very greatly reduced.

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