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Comment by rrauenza | Hacker News Reader

What I'd like to understand is why viruses are less prevalent in the summer due to more humid air, but yet they can exist in the moist body and spread from moist spray from coughs / sneezes.

I haven't been able to phrase the question just right to find the answer to what seems like a conflict.

I think the spray is just a method of transmission with the virus "hoping" to dry out quickly on a dry surface before it falls apart.

But inside the body?

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This paper confirms the finding (google search: droplet transmission "winter"). They're uncertain of the mechanistic cause, though. They summarize:

> Cold and/or dry conditions impact the stability of influenza virus particles, the innate defense of host nasal epithelia, and the production of infectious bioaerosols. Each of these effects contributes a plausible explanation for the impact of [relative humidity] and temperature on respiratory droplet transmission, and more than one mechanism most likely contributes to the observed transmission outcomes.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4097773/

Heat, humidity and sunlight all reduce infections during summer.

Humidity seems to reduce infection multiple ways.

1. Testing shows that there is less virus particles in air after it has been humidified. Stuff like droplets, dust and micro-particles fall down faster.

2. In dry air viruses may have more direct way into lungs and other tissues when the protective moist layer is evaporating away.

> Testing shows that there is less virus particles in air after it has been humidified. Stuff like droplets, dust and micro-particles fall down faster.

Seems feasible but can you provide a citation? Thanks!

Short answer: we don't know.

"The exact mechanism behind the seasonal nature of influenza outbreaks is unknown."

https://en.wikipedia.org/wiki/Flu_season#Cause

My personal theory is that in places like England in the winter people close the windows and cough at each other hence spreading colds, flus etc. In the summer they open the windows, go outside, the cough outputs blow away. I was recently in Bali and Singapore and noticed no colds in Bali where everything is outdoor and transport is mostly scooters and a lot in Singapore where everyone sits in aircon boxes - offices, busses etc. Also there's a fair bit of coronavirus in Singapore and not much it seems in Bali in spite of Bali having many Chinese tourists.

I was under the impression it was heat, not humidity, that caused viruses to become less effective.

I don't know the impact factor of these journals, and a bit too lazy to look up, but it's probably a very complex problem, otherwise it'd be settled by now.

A 2018 study claims that it's independent of humidity, but their 2019 one seems to show a trend with absolute humidity.

[2018] https://www.health24.com/Medical/Flu/News/humidity-doesnt-sl...

https://academic.oup.com/jid/article/218/5/739/5025997

[2019] https://royalsocietypublishing.org/doi/full/10.1098/rsif.201...

It's hotter inside your body on most days of the year, even if you live close to the equator.

Also more humid!

Perhaps the humidity only affects when it's not attached to a host cell?

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