Some of my assumptions come from Zvi's articles. Zvi is the former co-founder of MetaMed and I trust him. This said, he hasn't read or approved this post, and even if you trust Zvi, you might not trust my reading skills.
The things I think are as follows.
Sources: Zvi — Covid 9/10: Vitamin D and [somebody on Twitter].
Vitamin D improves your chances significantly if you have Covid.
Now we have a full RCT (although not a blind one), and the effect size is gigantic. They treated hospital patients with or without calcifediol (effectively D3), and used ICU admission as the end point. There were 50 patients in the treatment group. None were admitted to the ICU. There were 26 patients in the control group. Half of them, 13 out of 26, were admitted to the ICU. So 13/26 vs. 0/50. Two of the control group died, none of the treatment group.
The treatment in this RCT was soft capsules of 532 mcg 25(OH)D on day 1 of admission to the hospital, followed by 266 mcg on days 3 and 7, and then 266 mcg once a week until discharge, ICU admission, or death.
This is equivalent to 106,400 IU vitamin D on day 1, 53,200 IU on days 3 and 7, and 53,200 IU weekly thereafter. If this were given as daily doses, it would be the equivalent of 30,400 per day for the first week, followed by a maintenance dose of 7,600 IU per day.
If I suspect I have Covid, I will do what they did in the study. 100,000 IU on day 1, 50,000 IU on days 3–7, and then I'll resume my normal regiment.
In the study quoted above, they used calcifediol (a vitamin D metabolite), and it takes a while for your body to produce calcifediol out of vitamin D, so your chances are not as good as the study participants' chances. Might still be useful.
Vitamin D is commonly thought to be dangerous, but Wikipedia says it's much less dangerous than people think. (Except if you have hyperparathyroidism, but only 0.1–0.4% of people have it.)
However, if you are scared, at least try to get rid of the vitamin D deficiency that you probably have. This is useful too, Covid-wise.
If you want to do this really well—measure your blood level of vitamin D several times. You want to get to 30ng/mL. You might also want to supplement with vitamin K2 so that you don't get your calcium levels raised. I am not doing that, though. I am just taking vitamin D.
I was taking 4,000 IU daily, but currently I'm bumping that to 12,000 IU because I have a single anecdotal case from [somebody on Twitter] that higher doses can get you to the good vitamin D level much faster than lower doses. 4,000 IU might turn out to be completely useless.
Sources: Zvi — Covid-19: My Current Model.
Covid mostly transmits via small respiratory droplets. Thus: breathing normally is fine-ish. Talking is bad. Shouting is very bad. Singing is super bad.
Masks do not protect you against Covid, but they protect others against getting Covid from you. If you have Covid and wear a mask while talking to somebody, they will probably not get infected. UPDATE Nov 15: Zvi says masks do protect you against Covid after all, so I was wrong. It's unclear how much they protect, though.
In the absence of singing/yelling, ventilation, wind, etc, everything will probably be fine if you stay 6-ish meters away from people. I admit I pulled the number "6" from my ass, not from Zvi's articles, so—the more, the better. The point being—if you're going on a walk outside and don't talk to anybody, you probably don't need to wear a mask. And if you talk to somebody, don't talk in their direction.
Wiping surfaces or washing hands super-rigorously helps very little compared to everything else.
Sources: Zvi — Taking Initial Viral Load Seriously.
The initial viral load (the quantity of virus you get when infected) is super important. If you get a lot of Covid at the start, your immune system won't have time to develop a Covid-specific response and you might die.
Being in a shop with a Covid-infected person is much better than having a dinner with a Covid-infected person, even if you get infected in both cases. And if you are infected, don't kiss your partner or talk to them without a mask, even tho they will probably get infected anyway.