I managed to find a literature review on post-covid studies, which only targeted people with "mild" cases.

Much of the data is self-evaluation, and everything here is obviously inferior to the kind of data that could be procured by a specialist firm on Wall Street (let alone the military). It also goes along with the usual suggestion that brain damage isn't permanent and that everyone will be fine if they just wait long enough and adjust. 

It is also probably the best data you've ever seen on Long Covid, by a massive margin. Because that's the kind of world we live in right now.

There is a data point that suggests that cognitive dysfunction persists after 3 months in mild cases, but most of the data points indicate fatigue that persists 3 months after a mild infection. This is notable because fatigue reduces intelligence, if nothing else by dramatically increasing how taxing your workday is. There are also some data points indicating that fatigue persists from around half of all mild cases.

People on Lesswrong and elsewhere seem to think that P100 masks are the best bet for protection, and I really hope they're correct because the CDC is once again saying nothing on the topic (and I wouldn't trust them if they did, since these are the same bozos who knowingly pumped out broken COVID tests and told everyone to wear cloth masks for two whole years). Amazon is obsessed with labeling P100 masks as "dust masks" and cloth/surgical masks as "COVID masks". I have a lot riding on my P100 mask during my flight tomorrow, so I really hope they got it right, but it looks like they did.

If you like the idea of being smart, either right now or in the future, then you should probably stop worrying about Nootropics or Nutrition, and start worrying about respiratory aerosols, room ventilation, and P100 masks. Because at this point, it's clear that reality is allowed to give you debilitating brain damage, and that COVID is a brain virus.

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I'm confused by the title. I'm not seeing "Hard evidence that mild COVID cases frequently reduce intelligence" in your post?

The strongest possible protection is a positive air pressure respirator; when in high-risk situations I use a Versaflo TR-300N+ and I think it's basically invincibility. P100 is a lot cheaper, quieter and doesn't have batteries to manage, though.

Technically, the best protection is a self-contained breathing apparatus (SCBA) which is a fancy way of referring to a respirator connected to an oxygen tank, but that thing too impractical and overkill for most people.

The amount of protection offered by a positive air pressure respirator (PAPR) depends on what kind of hood is being used and may offer about the same (or more) protection than a reusable elastomeric respirator. Assigned Protection Factor (APF) is a measure of the level of protection offered by types of respirators; PAPRs range from 25 to 1,000 APF, whereas reusable elastomeric respirators offer an APF of 10 (probably around 25 in reality) for half-facepiece respirators and 50 for full-facepiece respirators. Besides a potentially higher APF of 1,000, PAPRs also don't require a fit test (which most people aren't going to bother doing), so leaks are less likely, even though the APF might be similar or even less than a full-facepiece respirator. They're also more comfortable than any other respirator. However, one hard-to-avoid disadvantage of a PAPR is that you'll have to carry around an elastomeric respirator as a backup.

To mitigate the cost and bulk issues, it's possible to DIY a PAPR (a plastic bag connected to filters, fans, and a battery). A DIY PAPR might be easier to repair and could also filter exhaust air, unlike commercial PAPRs. I haven't bothered to DIY it yet, but others have definitely used these things successfully.

DIY PAPR intro info

https://viralhelmets.medium.com/15-viral-helmet-version-of-a-2000-hospital-mask-papr-4950905ae2cc
https://www.youtube.com/watch?v=zj_C4GrxfNM&t=102s
https://www.youtube.com/watch?v=xfaswOLIHoQ

DIY PAPR in-depth info

https://www.designnews.com/industry/beyond-n95-hackathon-produces-air-purifying-respirator
https://devpost.com/software/bunnypapr-for-jcrmrg-hackathon
https://bunnyscience.dozuki.com/Guide/Bunny+Science%E2%84%A2+PAPR/4
https://web.archive.org/web/20210202224805/https://bunnypapr.org
https://web.archive.org/web/20210825175428/https://www.viralhelmets.com
https://www.instructables.com/Viral-Helmets-9999-Hospital-Grade-Viral-Protection/
https://www.reddit.com/r/viralHelmets/

I think it would be a lot cheaper and less conspicuous to put a really cheap scuba kit under a cloth mask/bandana and a normal backpack, so that only the tube going between them is visible. Easily 1/3 the price too. Maybe including swim goggles or airtight lab goggles, I haven't found any info on how likely the virus is to enter the eyes via respiratory droplets.

One way or another, the best thing to do is to be careful in apartment common areas, and to meet people outside, especially if eating takes place.

Amazon is obsessed with labeling P100 masks as "dust masks" and cloth/surgical masks as "COVID masks".

Masks designed as dust masks usually have vents to allow exhaling of unfiltered air. For the use-case of preventing inhalation of dust it's not necessary to filter outgoing air. From a public health perspective it makes some sense to encourage people to use masks that also filter outgoing air for COVID protection.

Now I want to know whether the average score on IQ tests (or similar tests like the SAT) has gone down since the start of the pandemic -- and if so, how frequent drops of that magnitude or greater have been.

SAT decreases due to school disruption seem pretty likely

I'm really sorry that I can't go into details here, but that's exactly the kind of information that you'll have a hard time finding in the modern era (without paying a handsome sum of money to a well-connected, exclusive firm you've actively established a relationship of trust with).

If that kind of problem ends up routinely causing you significant trouble in the future, we can eventually talk about it via a call. SAT trends in particular are harder than they look, especially if they start using/already have been using different question combinations/ordering for different people, which I predict will happen by 2027 if it hasn't already. I also predict that publicly available SAT overviews will either become less available, less representative, or otherwise significantly less accurate, in the same timeframe.

Correlation is not causation. If the "best" data is garbage, it's still garbage. We should not update our priors based on garbage data.

It would be helpful to hear more specific thoughts on whether / why this data is "garbage".

An initial thought on that (since I don't have time to dive deeper on this today) is that the first source linked in this post says most of the studies did not use a control group:

For the assessment of the quality of the included studies, we used the Newcastle-Ottawa Scale for observational studies.(22) Not every item from this scale was relevant for the included studies as most included studies did not use a control group. Therefore, we used two items from Selection (representativeness and ascertainment of outcome) and the three items from Outcome (assessment of outcome, follow-up long enough, adequacy of follow-up; see Supplementary Material).

Regarding the table in the OP, there seem to be strong selection effects that are involved. For example, the "recruitment setting" for the "Goërtz 2020" study is described as:

Recruitment from Facebook groups for COVID-19 patients with persistent symptoms and registries on the website of the Lung Foundation on COVID-19 information

Thanks for this. Prompted by this and other recent posts, I'm trying to mobilize more of a systematic effort to maintain an updated assessment of Long COVID risk -- if you're interested, please chime in here! https://www.lesswrong.com/posts/4z3FBfmEHmqnz3NEY/long-covid-risk-how-to-maintain-an-up-to-date-risk