Following up on my earlier question, which asked about the implications of coronavirus infecting >10% of the world, I'm now more interested in discussing how likely it is that that will happen. To that end, I'm asking about "saving throws," i.e. reasons why the virus might be stopped before then.

I know of three plausible ones so far:

1. Warm weather is coming and might dramatically slow or even stop the virus.

2. A vaccine might be found and deployed.

3. World governments might follow in China's footsteps and initiate massive quarantines etc.

Are there more?

And how plausible are these three?

My sense right now is that 1 is somewhat probable, 2 is improbable, and 3 is improbable.

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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

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

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 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?

Just wanted to add that quarantine has now been put in effect for mine and other regions of Italy, after 5000 cases, so that has just become an extremely likely measure for other nations as well.

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.

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.

1. Iran isn't especially warm at this time of year. Temperatures were between -2°C and 12°C this february.

3. There's loads of 'liberal' measures that governments can take to change the distribution of cases over time. Many of the estimates epidemiologists give are explicitly for scenarios without countermeasures. (For example, the estimate that 10% of the population will be infected at the peak of the epidemic.)

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, 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!)

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I think the most plausible saving throw is

  1. Current reports on the infectiousness and/or mortality of this coronavirus are exaggerated and total damage will not be as high as expected.

I can't help but see strong similarities to the Swine Flu (H1N1) pandemic/scare of 2009 which ended up not causing any more yearly deaths than seasonal influenza. The biggest similarity I see is extensive media reporting and third-party analysis of very little factual first-hand information. The Health and Human Services whistleblower is reported on 5x more so than what the information they are providing deserves. There is boundless speculation and analysis on mortality from the same or slightly different set of facts and every new % chance of dying is reported on as if it's a whole new story.

The ambiguous info about coronavirus perfectly lends itself to the type of media overhype that has become so prevalent in the years since Swine Flu played its course.

ETA: I should make clear, 2019 coronavirus will likely cause damage on a worse scale than Swine Flu did. This is the most likely saving throw, but it is not more likely than not to be correct.

Might be interesting to look at information that was available at the start of H1N1 and how accurate it turned out to be in retrospect (though there's no guarantee that we'd make errors in the same direction this time around).

I would have suggested broadening your questions to include the possibility that >10% are infected but it's still not a big deal. Then my ideas would be things like: a less virulent strain is the one that becomes widespread; or maybe the same strain but it's less virulent in most people (or less transmissive) compared to the groups that have been studied so far (air quality, smoking or other behavioral quirks, genetic quirks, locally prevalent diseases of previous years, etc.). Antivirals or some other miracle cure make it easy to treat at scale and at home. Lots of people are immune altogether, maybe because they caught a particular cold a decade ago, or just random genetic reasons, etc. (There's also the possibility that COVID-19 is just plain less virulent than the statistics suggest, even in Wuhan, due to mild cases not being detected. But AFAICT there is pretty good evidence that this is not the case.) Plus what you said. There should be data to rule some of these in or out, but I don't have time to figure it out.

Seems I was right! I didn't make quantitative predictions though.

2 is improbable

This one is a matter of time I think. (If things are delayed long enough, then maybe it'll be effective.)

Might also consider a 3a case: people use the information we have until now and start doing their own self-quarantining and other segregating activities. I know some of that will be views as racists or xenophobic or simply prejudiced but it will achieve much the same result. It also seems to be something of the way in which people dealt with such problems in the past -- think of leper colonies and fear or outsider type reaction.

In that regard perhaps we get something of a bifrucated outcome. One set is of those not adjusting their behaviors (probably the same group everyone seems to point to as why western democracies could not impose the same lock-down as China did). This group will have an R0 greater than one, have more deaths and the spread will be the epidemic path. The other group will limit their interactions, control for contagion and generally produce a R0 < 1.

The relative proportions of those two attitudes in the general population will perhaps determine both the need to the government level actions as well as the over all impact observed in each country (and this probably scales globally too, to some extent seems to be playing out).

An odd source to be sure but this story seems to fit with your thoughts that 1 is the most likely of the three paths. Here is a link to the bio on the HK professor making then statements for those who want to decide how credible a source he might be.