What "Saving throws" does the world have against coronavirus? (And how plausible are they?)

Virus mutates to a less severe form, quarantine measures select for the less severe form, fighting off less severe form provides immunity against more severe form, severe form dies out.

According to https://en.wikipedia.org/wiki/Spanish_flu

Another theory holds that the 1918 virus mutated extremely rapidly to a less lethal strain. This is a common occurrence with influenza viruses: There is a tendency for pathogenic viruses to become less lethal with time, as the hosts of more dangerous strains tend to die out[15] (see also "Deadly Second Wave", above).

Article today suggested that COV19 has already split into two strains and hypothesized that selection pressure from quarantine changed the relative frequencies of the strains, don't think there's evidence about whether one strain is more severe https://academic.oup.com/nsr/advance-article/doi/10.1093/nsr/nwaa036/5775463?searchresult=1

I'm not an expert and this isn't great evidence, so it's maybe in the "improbable" category

There's a parameter "How aggressive does an anti-transmission intervention (social distancing, contact-tracing, etc.) have to be to get Re < 1?" (i.e., low enough transmission to stop exponential growth). (Glossary on "R0" vs "Re".) This parameter is obviously important and (to my knowledge) still largely unknown.

We do know that doing nothing at all is not sufficient to get Re<1 (obviously). And we also know that what China did in Wuhan is sufficient to get Re<1 (i.e., sometimes welding people into their apartments, deploying 1800 5-person teams of contact-tracers, etc. etc.). So, the answer is somewhere between those two extremes. But I don't think we have much information about where it is on that interval. At least I don't know.

So, maybe we can entertain the hopeful theory that if you just post signs encouraging people to wash their hands, that's all you need! That's good enough to get Re < 1!! Or the theory that you need to do that and also cancel live sporting events? If it's something like that, then presumably every community in the world will just do those trivial things (at least, they'll do it after they get their first rash of cases), and the infection won't become widespread. I don't have any good basis for guessing how likely this is.

(Update: See my other answer [LW(p) · GW(p)] for a better mental image of what this means.)

I feel that 3 would not be improbable as we get to a certain rate of infections (my guess would be around 10000-20000 cases in a nation, based on how much mental pressure that number would put on the population and the government) that milder measures didn't manage to slow down enough. They'd of course try to be as less threatening as possible about its enforcement, but in Italy some small areas where the virus showed up first are already in quarantine (though they are talking about opening them up again now, since the virus is clearly out of them as well). If the economic damage keeps piling up at this rate they might consider end up copying what China did to stop it more quickly, and after that many cases I guess that people would accept it less begrudgingly.

I'd also say a fourth one is possible, the milder measures to contain it growing in efficacy as governments get more examples on the consequences of a determined reaction. In Italy my perception is that politicians acted in the first period in a very disorganised way, everyone tried to show they were doing something to gain consensus, nobody wanted to do anything that would cause too much economic damage. As people's reaction made clear that economic damage would ensue anyway, they started coordinating more. As more nations try stuff politicians might learn from their successes and failures, and would be more justified in doing the stuff that worked somewhere else even if it would be unpopular otherwise. And the advice they receive would grow more accurate as well. From what the government is saying here, we should understand during this week how much mild countermeasures work.

The effectiveness of it would still likely be influenced by how people perceive the situation, and that also seems to be getting more accurate. In Italy we had medias making everyone panic for the first cases, sponsors made them calm down, so half people are panicking and half are thinking it wasn't so serious after all and mocking those who did (I'm kinda guilty as charged with this as my earliest reaction). I'd have reasonable hopes that other nations would get the hang on what to say to the population a bit quicker, seeing how we... didn't.

This might still be not enough than the loss of efficacy caused by how much the infection has spread for early mistakes, but it still might slow it down.

5, virus being selected to become less severe is looking kinda good so far, given the news on the two strains and its interactions with quarantine and countermeasures. From the news I've read it seems that the more aggressive form was responsible for 70% of cases in the first stages, but dropped quickly due to the human intervention that selected the less aggressive one. Given that this is very recent news I wouldn't put complete faith in it, but it's the best one I saw so far.

Even if 2. usually takes a year, there aren't really precedents of a virus doing so much economic damage to so many nations all at the same time. Wouldn't this concentrate an unprecedented amount of resources on finding cures and vaccines, greatly decreasing the time needed? I get that ten people can't finish a project in a tenth of the time, but I'd expect it would at least be quicker than usual?

1. We can slow down the spread through hand-washing, social distancing etc for long enough to develop a vaccine (or other measures) on time.

2. A vaccine is brought to market without the usual safety testing. Apparently we already have one that works in mice (from personal communication).

3. >10% get infected but the death rate has been greatly overestimated due to sampling bias. That one seems probable to me.

4. Antivirals

Are there more?

Speaking as a layperson, it seems to me plausible that we'll see a "successful saving throw" in the form of a new coronavirus testing method (perhaps powered by machine learning) that will be cheap, quick and accurate. It will then be used in a massive scale all over the world and will allow governments to quarantine people much more effectively.

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1. Maybe, but the virus seems to be prevalent in both Iran (warm and dry) and Singapore (warm and humid). I think hoping for the warm weather is more to reduce the burden on the health system from the normal seasonal flu.

2. Unlikely from what I've read. These things usually take at least a year, and by that time it may have already spread so far through the population that the damage is done. It might be useful going forward if Covid-19 becomes endemic and seasonal.

3. Unlikely again. Would the governments of liberal democracies even have the resources, let alone the willpower to enforce mass quarantines.

The best discussion I've found of warm weather is this blog post. The way I'm thinking of it now is: Different cities / regions / countries will institute different levels of anti-transmission interventions (social distancing, contact tracing, etc.). As in my other answer [LW(p) · GW(p)], there's some threshold of intervention which is good enough to stop exponential growth. Some regions will cross that threshold very early on the exponential growth curve, such that only a small fraction of their populations will catch it. Other regions will cross it very late, or not at all, and get it very bad. Warm weather will (more likely than not) help move the threshold in a favorable direction. Based on that blog post above, I think it's unlikely (but not impossible) that warm weather will move the threshold all the way to "no exponential growth even without any anti-transmission interventions whatsoever". But that's not necessarily relevant anyway; there will be interventions! Thus, warm weather will increase the number of regions that avoid problems. (Unless, of course, governments get cocky and relax the interventions in warmer weather, undoing the effects!)