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Notes -
On your comment on the differences in innate immunity in different human lineages, you're absolutely right and I was being imprecise in my wording. Mea culpa.
On the broader point, I think that you are potentially neglecting the common phenomenon of traveller's flu for the South to North traveller. People do get ill due to e.g. diffetences in circulating influenza strains. Agree that illness in the North to South traveller is more likely and severe due to the broader range and exposure to excitingly virulent pathogens.
You are of course right to say that the innate does a lot of the work, but for potential pathogens that can evade the initial response, then as you know a major factor in the severity of the illness (I.e length and how debiliating it is) is the existence of relevant B- or T- memory cells. My argument is that fewer relevant memory cells (etc) exist if you have experienced fewer relevant infections due to reduced exposure. Vaccines can close some but not all of the gap simply due to sheer range of potential pathogens.
Perhaps a relevant example would be a person from 1910 time travelling to 1925, at which point the Spanish Flu is still circulating. They would have a higher risk of dying from Spanish Flu than the average 1925 person because they do not have previous exposure to the strain itself, or to the various similar strains circulating post-1917 and generating relevant cross-reactions.
I think the only way masks work out for reducing severe illness long-term is if there is indeed a dose effect that results in exposure to the same pathogens but milder illness. If masks are so effective that one only gets ill vanishingly irregularly, then the risk of that occasional illness being severe are increased due to lack of relevant previous exposure. If masks are not effective at all such that one has a similar immune profile to the general population, then there is little point in wearing them.
I have never heard of Traveler's flu before, and as far as Google tells me, there's no clinically recognized condition by that name.
The closest I can find is this publication:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5505480/
I don't think this has much to do with traveling to a new place as much as it does with being crammed in tight spaces with poor ventilation with hundreds of others.
Similarly, if you travel to a new country during the flu season, you're at increased risk of catching it, but so are the locals. I am not aware of any literature claiming that new immigrants are at additional risk of catching flu and similar diseases, relative to the native population.
Masks vary greatly in terms of effectiveness, both from the specific type in question and proper use and adherence. But for a typical adult who begins masking in public, I do not expect that the the decreased exposure to pathogens will significantly impact their susceptibility to novel strains. In the case of the common cold or the flu, prior infection to a previous strain isn't protective for the next one that does the rounds, which is why flu vaccines are refreshed annual (and designed pre-emptively, based on models of what the next strain might be).
If someone masked and avoided the flu, for say, 2 years, I strongly doubt they are at any additional risk if they stop masking or catch the next one despite masking.
This is a different consideration from children, since I know that atopy is associated with insufficient exposure to a diverse array of microbes in childhood. Everything from peanut allergies to asthma can be mitigated by exposing them early and often. But for an adult, I do not expect it to make a difference.
Mind you, I don't disagree that wearing masks is largely pointless. Unless you're immunocompromised or are working with people who are, they are unlikely to help, unless maybe if you wear an N-95 on the plane and so on. What I disagree with is that they are meaningfully harmful on net to an average adult, which is a different question entirely from whether they provide benefits.
There are places, like China and Japan, where masking in public was common before the pandemic, and I have seen no claims that makes them more susceptible overall to respiratory illness when they do get it.
I think I disagree with the statement that various types of flu/colds do not generate any immunity: they simply fail to generate sufficient immunity to avoid symptomatic infection, which is why the new strains are able to circulate in the fairst place.
Now you can reasonably disagree that this cross-reactivity actually matters in terms of disease length/prognosis if you're still not immune enough to avoid getting ill, and I'm not sure whether it has been tested empirically. But it makes intuitive sense at least, and the principle seems to be accepted e.g. in this 2009 paper looking at swine flu immunity in the general population https://www.pnas.org/doi/abs/10.1073/pnas.0911580106
Overall, seems like a hard thing to study. Would be interesting if someone did an East/West study on whether flu is actually worse in the East due to reduced exposure (noting that it might still be less prelevant on the net due to effective masking). I would fall back on the statement that either masks are effective and decrease your exposure to disease and therefore your immunity relative to the unmasked population, or ineffective and therefore pointless.
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