3 days ago there was published a research that swayed me significantly towards the opinion that COVID-19 is spreading better in cold (but not too cold) climates. I believe that the odds for this are probably over 50%, and if it's true there is a practical and scalable (although not extremely cheap) way to slow down significantly the spread of the coronavirus.

Research Claims

It seems that all current pandemic epicenter share a very similar temperature and humidity (5-11OC and 47-79% humidity). Consider the following map:

As you can see all the central outbreak locations are lying only along a narrow east west distribution roughly along the 30-50 N” corridor. This shows a correlation between hyper virus spread and specific climate conditions.

Why do I believe it's probably true?

1. The odds that randomly by pure luck all 6 different epicenters will be with similar climates and latitude seem like a too large of coincidence, even if we take into account that the 30-50N'' corridor does seem more populous than the average. There are many very populated places outside this corridor and none of them got hit as hard.

2. Iran, Italy, China, South Korea are very different places in terms of the political system and government competence, Intuitively it's hard to think about a better explaining cause root that caused the virus to be widely spread specifically in these locations.

3. It could also explain why some places that you would expect to be hit hard, like Thailand (which is the top global destination for Chinese tourism) or Taiwan, for now, don't seem to have it that bad. Both of these countries have a tropical climate. Now granted there could be a higher spread in these countries that is underreported, but if they had it bad as in Italy or Iran it wouldn't go unnoticed.


What are the implications if we believe the research?

on the downside: as the weather warms up on the northern hemisphere more places will warm up enough and could provide optimal conditions to COVID-19, the researches list the following cities as potentially dangerous areas for coronavirus spread

On the upside, if this is true there is something we might be able to do to slow down the virus spread, and the solution is simple: Increase temperatures in closed public spaces, or more simply - turn up the heat. If the coronavirus truly doesn't like heat increasing temperature will kill it faster on surfaces. If we could raise temperatures to hot but still bearable (27C/80F is a good place) in places like supermarkets, public transportation or clinics - and by this might slow down the spreading by reducing connectivity via killing the virus faster while it's on surfaces or in the air.

The main downside is that heating up costs money and we don't know for sure if it will help, A way to know for sure will be to conduct research that tests this assumption directly or even create A/B tests around the country to see if on average areas with high-temperature public areas get less pandemic growth.

Another option is to heat the places only at night to consume electricity outside the peak working-hours when it's pretty cheap anyway. The recent drops in Oil and Gas prices could mean it could be a sustainable and worthwhile act of public policy if it truly works.

EDIT: Found another research from 2011 that finds similar findings regarding the first SARS coronavirus (from 2003). It is a different virus strand but they share similarities.

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Now granted there could be a higher spread in these countries that is underreported, but if they had it bad as in Italy or Iran it wouldn't go unnoticed.

I don't buy it, with the current level of undertesting. The rhetorical work is being done by the red and dark red areas not reporting many cases, but it seems premature to be sure of that, since many of those are places that don't have the medical infrastructure to do widespread testing. It looks like Africa has done 400 tests; some people suspect that's enough for the counts to be accurate, but I wouldn't bank on it yet.

It's not obvious we would know about Seattle if some heroes hadn't tested people for coronavirus despite being told not to, and so it's not clear the higher number of cases reported in Washington state than New York or California is a true difference or testing artifact. Also, Spain (yellow to orange on the map) has more confirmed cases than the entirety of the US.

I can't find data for Shiraz (a hot big city in Iran), and amusingly looking for that data took me to this paper (since one of the authors is affiliated with a university there), but that seems like one of the big tests of this theory.

I also think we have more reason to believe humidity matters than temperature, and wonder if adding that to this map makes this look more or less accurate.

Various places got a lot of traffic from Wuhan before it was shut down: Singapore, Thailand, the US, Europe, Korea, Australia, etc. It's clear that Europe's outbreak is worse than the US/Australia/Singapore. It seems likely that things are worse in the colder parts of the US (vs. Texas or Florida).

Iran was not testing/reporting. There are many tropical / Southern Hemisphere places that could have had an Iran style outbreak and which had a lot more traffic from Wuhan than Iran does. Why Iran?

(I'm mostly replying to this entire thread rather than Owain's comment.)

I'm failry confident in the following two claims:

  • Warm climate slows down transmissions substantially.
  • But not enough to prevent large outbreaks.

My reasoning being:

  • As commenters have pointed out, we see striking correlations between countries that report large numbers of cases and cold-ish temperatures.
  • The alternative hypothesis (poorer testing and detection) seems likely true to me, but not strong enough to overcome this point:
Now granted there could be a higher spread in these countries that is underreported, but if they had it bad as in Italy or Iran it wouldn't go unnoticed.

That said, I think there is circumstantial evidence for a somewhat large underreported outbreak in Indonesia (avg temperature of 31+ Celsius for the last two months), and maybe also the Philippines (but I haven't been following the latter at all). For Indonesia I think it's likely they have upward of 2,500 infections. It seems implausible to me that they'd already be anywhere near Italy or Iran's level of infections, but if one is really cynical, I admit that it can't be ruled out completely.

Some relevant data points:

  • When Indonesia had <20 confirmed diagnoses, 3 Indonesians were tested positive abroad: a married couple in Singapore and an Indonesian woman who travelled to Australia. The Singapore case seems to have been motivated by seeking quality hospital care, which admittedly makes this evidence consistent with a smaller outbreak than if Indonesians who travelled to Singapore for non-health-related reasons had tested positively. Still, I think 2 instances of testing positive abroad at a time when the official count is <20 is quite telling.
  • Indonesia also reported 4 deaths by now. For the first death, a British woman age 53 with several serious previous health conditions, Indonesian authorities labelled the case "imported" because the woman had only been to Indonesia for a couple of days by the point she died. However, I'm not sure whether I should believe the conclusion because there were reports (US nursing home and one from Italy) of people going from symptomatic to dead in <30h. In any case, 3 further deaths were announced yesterday, and no indication was given that they might be imported cases. Even just 3 deaths usually correspond to true rates of infection well above 1,000. The ratio of confirmed diagnoses to deaths is now exceptionally high in Indonesia. (The same applies even more strongly to the Philippines, but I could imagine that some deaths there were definitely imported.)
  • There was this entire discussion about expert modelling suggesting that indonesia's long count of zero diagnoses is evidence for them missing cases throughout February.
  • (Indonesia was one of the last countries to ban incoming travel from Italy or Iran, I believe.)
  • According to this tweet, testing in Indonesia was 970 people. Indonesia's population is >>200 million.

In addition, the incentives for reporting cases diligently seem to be suboptimal, as evidenced by some data points:

  • Indonesian ministers install even less confidence than Trump. Just one example of communication: About the Australia exported case, the Indonesian health official claimed that the woman most likely got infected in Australia because she had travelled for quite a while and because she also visited a Vietnamese restaurant there (implication: Vietnamese restaurants in Australia are risky). However, the Australian source (link above) says clearly that the woman in question started showing symptoms 2 days after leaving Jakarta (so much for "she travelled for quite a while"), and the only reason a Vietnamese restaurant was mentioned anywhere is because the Australian authorities obviously traced the Indonesian woman's contacts in order to interrupt a potentially newly started chain.
  • Maybe the time lag confuses me, but I'm pretty sure there was one time when Indonesia skipped a day of reporting case numbers, and then numbers double and deaths went from 1 to 4. (Could be totally benign if there's some disorganization during a time of crisis, of course.)
  • Indonesian officials seem to be under enormous pressure because the economy is doing very poorly already. Some unusual measures have already been taken a while back, such as disallowing shorting and pledges for government-owned companies to buy back shares.

And here's a concerning but speculative hypothesis:

  • Singapore doctors gave out a warning that patients who tested positive for Dengue fever with a quick-testing kit actually turned out to have Covid-19. They say the diseases are clinically similar and that the widely used quick-test kits may not have failed isolatedly due to extraordinary circumstances. Indonesia currently has a somewhat large Dengue outbreak (and the same goes for the Philippines).

UPDATE: 18 days after I made this comment, Reuters reports that Jakarta's funeral count for March was 1,300 funerals higher than any preceding month since 2018. This looks highly statistically significant because the previous record was only 3,100 (compared to 4,400). (And there was no natural disaster or other unusual disease outbreak.)

Malaysia is up to 428 cases now and rising rapidly: https://www.bloomberg.com/news/articles/2020-03-15/malaysia-virus-cases-spike-after-outbreak-at-16-000-strong-event They've been averaging 24C with peaks of 36C. Not looking good for the heat hypothesis.

I wrote the above before checking today's news: Only 27 further cases, but a government official infected and 1 more death. The article also says that cases were in many different parts of Jakarta, and some in other parts of the country. At the same time, the Indonesian authorities say that the reason why they detected all those cases is because they did "massive contact tracing," suggesting between the lines that this isn't yet evidence of even more widespread community transmission than already acknowledged.

Edit: On the same day, Singapore gets 2 positive tests with a direct connection to Indonesia.

Edit2: Australia may have caught another infected person with Indonesia as travel history. (I say may have because there's a small chance that this refers to the initial case from 5 days ago; however, some of the wording somewhat strongly suggests it's a second, new case.)

A competing hypothesis (this time I am specifically replying to Owain's comment) is that it's easier to catch infected travellers coming from the location where one expects them to come from. Asian countries certainly had it harder in phase 1 when the goal was to detect all the infections with Chinese travel history. However, as soon as even just one country in Europe failed at detection, the traveling dynamics changed and in phase 2 it subsequently became easier for the Asian countries, because they had basically zero incoming travel from Europe by that point. So while everyone was still focused on catching infections with Chinese travel history, Europeans were infecting other places in Europe, but less so places in Asia.

It's maybe some supporting evidence that the Italy outbreak was connected to the early Germany outbreak, and that it only became clear that many countries were going to lose control of the situation once infected travellers had come from many different places. (At the same time, the Seattle outbreak is evidence against.)

Another hypothesis:

Very hot countries have it easier to do contact-tracing. It's plausible that the virus deactivates at 30 degrees Celsius (I think I may have read that in the often-cited Lipsitch article). In the hottest countries, public transport can be 30+ degrees and even grocery stores can be that warm. If infections happen primarily in indoor settings (e.g., business contacts or partying or household), contact tracing is easier, giving hot countries an initial advantage at preventing outbreaks early on. However, once contact tracing fails, that advantage shrinks. What remains is only that the r0 is lowered somewhat by heat interrupting certain types of of transmissions, but not all types. It wouldn't surprise me therefore if hotter countries also need extreme measures to contain the case count from growing exponentionally, even if the doubling time might be generally lower for hot countries.

I agree with the objection regarding Seattle. But I don't think that a situation as bad as in Iran or Italy would go under the radar, the hospitals there are flooded with COVID patients that show the same symptoms over and over and even in the extremely censored China this kind of information eventually leaked.
Spain is only x2 compared to the US right now and this could easily be explained by under testing and proximity to Italy.

Why Iran and Italy and not Thailand and the Philipines?

Why Iran and Italy and not Thailand and the Philipines?

I think it's too soon to say "not Thailand and the Philippines"; three days ago there were 10 reported cases in the Philippines, then 33, then 49. Italy's trend at a comparable level was steeper--3 then 9 then 76--and then here we are, two and a half weeks later.

As for "why Italy and Iran first", there's both the possibility of "lower spread rate" and the possibility of "weird founder effects." If a careless Italian healthcare worker made a mistake three weeks ago, and a careless Philippine healthcare worker made a mistake one week ago, we'd see two additional weeks of growth in the Italian numbers and not the Philippine numbers.

The rate of change being exponential is an extremely important point. The original rate of spread in Wuhan was a doubling every 2 days. If we set another area follows the same rate, then the difference between 32 cases and 4096 cases is just waiting 2 weeks.

Brazil looks like it's also rising pretty quickly, but is especially interesting because both the president and his communications director seem to have it. [Edit: Probably not?]

Note that Bolsonaro does not have the virus.

Looks like he tested positive and then negative? Hopefully that means he doesn't have it, but I seem to recall these tests can be inaccurate in both ways.

My understanding is that he never tested positive; rather, it was reported that he tested positive, and then that he tested negative. (The link you provide says otherwise, but Telesur is not a reliable source.)

Update: it now appears that Bolsonaro may have tested positive, though the situation is still unclear, at least to me. The main evidence in favor of the hypothesis that the Brazilian president has tested positive, according to this London Review of Books article, is that (1) Fox News claims that this is what his son Eduardo initially told them, that (2) Bolsonaro has refused to make the results of his tests public, and that (3) 25 members of his entourage are confirmed to have the virus.

Note that the article shows some signs of bias, such as calling the impeachment of former president Dilma Rousseff a "coup" and describing Bolsonaro's economic minister as having studied "at the University of Chile under Pinochet" (Pinochet was the president of Chile, not the president of the University of Chile). So I'm updating only slightly and would like to see this confirmed by more neutral sources.

Ah, good to know!

Consider possible confounders:

Warmer weather coincides with more sun, which coincides with higher vitamin D levels, related to immune system function. Sunshine does lots of other good things too.

In colder weather people stay indoors, with more opportunities for sharing infections, and get even less sun.

In colder weather people close windows leading to more retention of infectious agents.

I am sure there are many more. Temperature may be a proxy for something else.

Or it might be "the thing" as Joe Biden might put it. Maybe aerosol droplets evaporate fasted in warm air. In this case turning up the aircon might help.

Also a confounder that was only mentioned briefly in the original post: it also seems like human population in general is concentrated on this climate zone. Can we statistically isolate population density from this analysis?

The potential confounders you listed are also relevant to very cold places (northern Europe/Canada) and it seems they are currently less affected.

What do you think about the possibility that there is an undetected outbreak of ~1000 cases in Indonesia? There's indirect evidence based on people traveling from Indonesia to countries with better testing (e.g. Singapore). This post and the Metaculus post I linked both seem convincing, but it's unlikely that they're both true.

I didn't know about it. Large spread in Indonesia definitely weakens the climate theory.

Here's a linear regression using Chinese cities, which also looks some at national data for other countries. They find a significant negative effect on transmission from both temperature and humidity, which look to be of roughly the same strength; however, basically everywhere is still in the "R>1 exponential growth" phase for months. 

Unfortunately for this, it looks to be spreading in South America, and it's probably being under-tested (notably, Bolsonaro himself or at least members of his entourage have been infected).

Just wondering about the display of centers. Is that waviness due to translation of a spherical map onto a flat one or could that be related to wind currents? 20 degree of latitude is pretty wide (1200 miles?) so wondering if something more could be used to narrow any geographical based explanation.

One problem with this theory is Iran. While your graph depicts the Iran cases in the yellow area, much of Iran lies outside of it. Many of the diagnosed cases are in that yellow area, but there are many outside of there, and it is suspected that thousands more lie in the outer regions of the country and are currently undiagnosed.

In addition, Germany and New York are in an entirely different climate zone and are facing outbreaks, so that's a bad sign as well.

Do we have an infection map of Iran? I've only been able to find national numbers, and they're consistent with the disease primarily spreading in Tehran. [That said, I also buy that the numbers in Iran might be a 100-1000x underestimate at this point, which might mean this is just telling us about testing instead of infections.]

Both some German cities and New York are listed as potential cities to be under increased risk soon (see the last image at the post) - they actually have pretty similar temperature/humidity profile.

Now that some more days have passed and more data is in from some countries in Africa and South America I think that the seasonality/temperature claim becomes less plausible: The development of cases in Algeria, Brasil, and Egypt for example show doubling times even worse than the average doubling times in other affected countries (Germany for example which I'm tracking most closely). Source is the Wikipedia COVID19 pages on these countries

I think the remaining explanation is much less international traffic to these locations from China.

This is bad news for the hope of summer fixing things.

I have turned up the thermostat to 27°C anyway.