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COVID-19 and its weather dependency

We’ve been talking about Coronavirus in our house pretty constantly since early February. I’ve come out into the open and admitted my family is doing self-quarantine to reduce spread (we don’t think we’re sick, but we don’t want to spread it by getting sick). I haven’t been very hopeful in a month due to what I see as complacency. The median/modal case scenarios in my head have been getting worse and worse over the weeks.

Over the last hour, I’ve changed and become a bit more hopeful because I’m optimistic that COVID-19 does have a relationship to weather. I’m not confidently optimistic, but there’s now a glimmer of hope. The reason is due to this preprint, Temperature and Latitude Analysis to Predict Potential Spread and Seasonality for COVID-19.

So several things we had talked about in our family:

  • First, why no massive outbreaks in Africa and Southeast Asia? These are areas with lots of Chinese from China. Singapore did a good job, but Indonesia, Cambodia, and Laos are not societies that are nearly as developed. Vietnam mobilized, but to be frank, I’m skeptical the Phillippines could if they wanted to. Africa has massive public health problems, and no capacity for the sort of totalitarian mobilization of China, with the exception of Rwanda and Eritrea.
  • Why the huge outbreaks in Iran, Italy, and now the Pacific Northwest?
  • Why no major outbreak in Russia?

An immediate explanation is bad reporting. But in all these cases? Also, hospital systems get overwhelmed. This seems like it would get out.

The major result from the paper is here:

Further analysis using 2-meter (2m) temperatures from 2020 rather than hPa temperatures yields similar results (Figure 2). In the months of January 2020 in Wuhan and February 2020 in the other affected, there is a striking similarity in the measures of average temperature (5-11 degrees C) and relative humidity (RH, 47-79%) (Table 1). In addition to having similar average temperature, humidity, and latitude profiles, these locations also exhibit a commonality in that the timing of the outbreak coincides with a nadir in the yearly temperature cycle, and thus with relatively stable temperatures over a more than a one month period of time (Supplementary Figure 1). In addition, none of the affected cities have minimum temperatures going below 0 degree C (Supplementary Figure 1).

Here’s the figure:

And here’s a table:

City Nov 2019 Dec 2019 Jan 2020 Feb 2020
Cities with community spreading of COVID-19
Wuhan18C/44%12 C/56%7 C/74%13 C/66%
Tokyo17 C/53%11 C/52%9 C/54%10 C/47%
Qom12 C/52%10 C/58%7 C/59%10 C/47%
Milan11 C/77%8 C/74%7 C/69%11 C/58%
Daegu11 C/64%5 C/62%4 C/68%5 C/62%
Seattle9 C/76%6 C/84%6 C/84%7 C/79%
Mulhouse7 C/84%6 C/82%6 C/80%8 C/74%
Glasgow5C/87%5 C/89%6C/86%4 C/84%
Large cities tentatively predicted to be at risk in the coming weeks
London8 C/78%8 C/80%8 C/80%8 C/70%
Manchester7 C/82%6 C/83%7 C/83%6 C/73%
Berlin8 C/81%5 C/80%5 C/81%6 C/75%
Prague7 C/81%4 C/78%3 C/79%6 C/71%
Hamburg6 C/89%5 C/86%6 C/88%6 C/83%
Vancouver8 C/75%6 C/84%5 C/84%5 C/78%
New York8 C/55%4 C/72%4 C/61%5 C/62%
Warsaw8 C/76%4 C/78%3 C/78%5 C/72%
Glasgow5C/87%5 C/89%6C/86%4 C/84%
Kiev6 C/74%4 C/83%1 C/85%3 C/76%
St. Louis6 C/71%5 C/78%3 C/77%3 C/73%
Beijing9C/33%2 C/43%2 C/41%5 C/45%
Previously predicted city where COVID-19 failed to take hold
Bangkok31 C/52%30 C/45%32 C/50%32 C/51%

Read the whole thing and draw your conclusions. Please put your critiques in the comments if you want.

Update: Some warranted pessimism in the comments.

40 thoughts on “COVID-19 and its weather dependency

  1. I think keeping an eye on Australia might be the most important thing to determine this. Australia does have community spread of the virus, but numbers seem to be growing slower than in other developed countries like the US, which suggests warmer weather at least slows spread of the virus.

  2. Walter Sobchak March 10, 2020 at 5:46 pm

    “I think this thing will blow over. Forecast upper 50s lower 60s here for the rest of the week. I don’t expect spring until mid April hereabouts. But, the warm weather will kill the epidemic. it always does.

    “Winter weather forces people to stay inside and be in contact with infected people. In warmer weather, people can go outside. Are your chances of being infected higher in a gym or running in the park?”

  3. @Erika winter is coming to Australia (literally)

    Australia could be the reservoir that brings back the corona virus in our (northern hemisphere)’s winter

  4. Sydney resident here. Currently Australia is a lot warmer than these temperatures, all capitals at least over 20 degrees C / 78 degrees F. Even in the depths of winter we are not getting down to Wuhan temperatures. Maybe Tasmania and New Zealand’s South Island are close to these temperatures, but they have a population of 500,000 each, so Australasia isn’t going to be much of a reservoir.
    Tropical north of Australia (Cairns, Darwin) are Singapore, Brisbane is Orlando FL, Sydney is LA CA and Melbourne is San Francisco CA. Christchurch NZ might be Seattle WA?

  5. The story goes that in Russia, it’s strict orders to diagnose the outbreaks as whooping cough

  6. Boston averaging 0 and Sydney 23-25 both experiencing community spread. There has been some discussion on Reddit that 78F/26c is some magic ceiling. Maybe 0 is the floor.
    Warm weather spread typically dependent on R0.
    Influenza still around in summer just lower transmission. Corona has a slightly higher R0 (how much still uncertain) so would expect more warm weather spread.
    Time will tell. As a physician I’m not hopeful, although repurposed antimalarials do appear to have legs. Vaccine likely to be trickier just based on prior Influenza designs (80%ish efficacy). Conservative estimates of 20% infection and 1% mortality seem reasonable (80k dead or roughly 2x average flu season).
    Flattening the curve will help, if only so we have some resource to cope with run of the mill heart attacks etc.

  7. Ahh Russiya, the one country where it’s still open season for any honest, “blue checker’ed” American to still take the piss at anytime, for any reason, and it’s never racist or xenophobic! Nastarovia!

    Paid for by PeopleForPutin2036

  8. @Marees – correct. I think we will see this. The Australian government says the country is prepared, and they have secret stockpiles of medical supplies and equipment, but the hospitals do not have the capacity to deal with large numbers of severe cases, and I don’t see any proactive efforts on control/containment in advance of outbreaks of community spread; on the contrary, the government is still harping on about avoidance of disruption to the economy. (Tangentially, Tom Hanks and his wife who are currently in Australia have announced that they have both just tested positive, and I doubt they were infected before they went there because they have been there for quite a long time while he is filming there, so they have picked it up there from somewhere. That means there is community transmission already occurring there that the health surveillance system, such as it is, is not picking up on). To harp on a point myself, reactivity is too late – the disease is already proliferating out of control in the community. Proactivity, good public health surveillance, and *lots of testing* are necessary.

    NBA season has been cancelled, which is no doubt pretty shocking to some people. OK, it was reactive to a Utah Jazz player testing positive, but the thing is that they moved very fast, and very decisively. I give them high marks for that. No NBA games -> no big, dense crowds of spectators in enclosed spaces: good move.

    SARS-CoV-2 does have some temperature (and presumably humidity) sensitivity – if it doesn’t, it would be the first known coronavirus that doesn’t. One Chinese paper states that it survives on surfaces for 2 to 3 days at 37 degrees Celsius. That might not sound great, but it’s actually not bad. The SARS coronavirus was found to survive on a stainless steel surface for 5 days at 40C, and for several weeks at 0C. So it seems that SARS-CoV-2 is somewhat ‘better behaved’ in that respect than the SARS coronavirus. And the SARS epidemic in Hong Kong in 2003 died when we got to June and the ambient air temperature rose above 28C and stayed there.

    But one other thing to be aware of, which is blindingly obvious when stated – Covid-19 cases are detected where testing is done. Most African countries have no testing capacity. Indonesia has had next to no testing, and likewise the Philippines. India? Bangladesh? Pakistan? Iran was behind the curve, but seems to be catching up. Italy failed on public health surveillance and is now paying a huge price for that.

    Having said that, Thailand has a good public health system, and after an early outbreak of cases, because Thailand was the most preferred travel destination for people coming from Wuhan (which is a central travel hub, so a lot of people were going there from Wuhan, not just residents of Wuhan), and they didn’t block entry to those tourists for quite a while because they were worried about the impact on the tourist industry, it has proliferated in Thailand very little.

    Hong Kong has just had a jump in new cases, but they were all HK people who had been on a tour to Egypt, and every single one of them came back infected, and they then infected some of their close contacts (family members). If not for those cases, the epidemic in HK (such as it has been – small sample) had clearly been dying, with next to no new community transmission. We have had a mild winter (warmest January on record since 1884), and have fewer total cases than tropical Singapore (but also a small sample), despite a larger population, a common border with Mainland China and one of the world’s busiest international airports, a very busy port and a fast rail link into the Mainland, only 4.5 hours from Wuhan.

    So I do think there are some grounds for hope with the coming summer in the USA, but watch out for travelers from the southern hemisphere. The President should close the country to travelers from southern hemisphere countries with temperate winters, and he should do it proactively, not wait for imported cases to react to. My country Australia won’t like that, but hard luck. It’s a pandemic.

    Sorry, I seem to have a verbosity problem. I’ll shut up again for a while.

  9. Agree! Great comment.

    My wife and I decided to suspend all our kids’ activities that bring them into contact with others in groups. My oldest is crushed, because he was looking forward to competing in several upcoming Judo and Jiu-Jitsu tournaments, including the youth nationals for Judo. He’s been training extremely hard to compete and win at the highest level, so this is a big letdown for him.

    For the foreseeable future, it’s home training only. We already homeschool, so this is not a hard transition (though all coop and community project activities are suspended). I am seriously considering moving the family to our second home in the hills and staying there, but the problem with that place is distance to a hospital with ICU/trauma capability.

    I hope the pandemic will be a mild one, not the dire predictions of 500,000 dead that is floating around. I worry about my older family and friends.

  10. not the dire predictions of 500,000 dead that is floating around.

    last week i was doing some back-of-the-envelope. 500K was my median/modal estimate based on % infected and mortality rate.

  11. Yes, this is the study I was thinking of when I said yesterday than you are underestimating climate. There are reasons why every other coronavirus is seasonal.

    In terms of the study, obviously testing will make or break this. Another factor is how much time you spend in air conditioned places. The US alone will provide enough test cases. Look at the number of cases in LA versus NYC, or the relative lack of cases in Miami, Dallas and Houston.

    The real danger of epidemics is the over-reaction and we’re seeing this here. Even in a seasonal pattern, you’re going to see reports every week of a “COVID-19” sighting and the panic can continue. I have also no doubt that this will return in the fall.

    I’m waiting for the next shoe to drop, which is when everyone realizes that if you’re under 50 the best thing that can happen is to come down this virus. Two weeks of sick time and isolation and you’re done.

  12. The biggest data point against this right now would seem to be Egypt. Although the official numbers suggest local transmission is limited (67 official cases) there have been numerous reported cases of tourists returning from Egypt with COVID-19 (most recently in the U.S. two people from New Mexico). This seems to suggest to me there’s a large population already infected, although they might be hanging out largely in climate-controlled environs like hotels and airports.

  13. Guangdong had a larger outbreak than anyvother part of china outside hubei. Larger than the outbreaks in tokyo or korea. Doesnt fit the thesis.

  14. Mark Lipsitch wrote a short paper on this a week or two ago –
    His conclusion is that while other endemic coronaviruses are seasonal, it is unlikely that a new coronavirus with no pre-existing immunity will behave in the same way.

    Adding to what he has said:

    I am unconvinced that temperature variation has much to do with the existing pattern of spread, and not optimistic that it will have much effect on the overall trajectory.

    The only equatorial countries doing testing on any significant level are Singapore and Malaysia, which both have plenty of cases despite major social distancing efforts.
    No country in Africa is testing on a significant level. Neither is India, Indonesia, Pakistan or Brazil.
    So to ascribe their apparent lack of cases to warmer temperature is … premature.

  15. @charlie

    we assume that people should have some immunity after recovering from the virus, we don’t know the degree of immunity. It’s likely dependent on a variety of factors.

    Also, this thing can be nasty to ppl who aren’t elderly. The very first confirmed case in Italy just got off a ventilator 2 or 3 days ago, the guy was in his 30s. I don’t know about his comorbidities (or lack thereof), there was no mention of that in the piece, but we know it’s not just ppl over 70 or 80 who can suffer serous consequences.

  16. “Why no massive outbreaks in Africa and Southeast Asia?”

    Massive under-reporting. A friend, male, about 50 yrs, living in Jakarta, Indonesia, where there still was ZERO official cases just ten days ago, told me that it has been rampant there ever since the Chinese New Year (well, direct flights from Wuhan, after all, and a place favored by Chinese tourists), and he himself has suffered it on and off since then, now perhaps alleviating at last. Indeed, it can take five weeks for it to finally go:

    Why did not he seek for a doctor, instead of just buying antivirals from the local pharmacy? Well, nobody wants to be locked in Indonesian quarantine!, was his answer.

    I guess nobody wants to get locked in Vietnamese quarantine either:
    “They’re in a bare room, with six beds in a row with plastic mattresses, with no pillows, no sheets, no towels, and bars on the windows,” she said. From:

    So, same kind of dynamics probably applies all those countries like Egypt, India, Cambodia, Burma (0 cases, hah!), Ethiopia (with a lots of flights still coming a few days ago from all the hotspots: Seoul, Milan, etc), Kenya, etc.

    See also:

  17. “The US alone will provide enough test cases.”

    We’ve had a fair amount of community transmission here in the San Francisco Bay Area, and spring happens now (t-shirt weather). If R0 changes, we should see it soon.

    Sacramento/Central Valley would be an even better, early test case of going from cold-ish to inferno. Not sure if there’s enough community transmission to establish a baseline though. Maybe in Sacramento.

  18. @brian_schmidt I was in san francisco 2 decades ago & missed the warm air of Chennai/Madras a lot 🙂

    (Unlike LA) It was mostly like Bangalore rather; once you get into shade, it would be cool

    About the only time it felt more like chennai/tamil nadu was around Aug when forest fires nearby sent the warm air to the bay area

  19. Finally maybe one small positive from Global Warming 🙂

    Weather Models indicate many parts of TamilNadu & Kerala to see day time TEMPERATURES THAT MAY STAY 3 TO 5°C ABOVE NORMAL FOR THIS TIME OF THE YEAR.

    While there is no consensus yet on whether Covid_19 tends to see lesser impact as temperatures increase, parts of Kerala could see day time max temp around 39°c.

    Interior TamilNadu to see day time high around 37°c.

  20. @T. Greer – Where are you getting your numbers from? They are way off, e.g. S. Korea has had 7979 cases, c.f. Guangdong Province with 1356 cases in double the population.

    Happily, S. Korea is clearly really getting on top of their epidemic and driving down the daily numbers of new infections – they are beating it, so it can be done, and not just in a totalitarian top-down state.

  21. Regarding South Korea, what’s really positive isn’t just the number of new infections. It’s mortality rate is quite low, at 0.8% (Italy’s is 6.3%, China’s 3.8% and even Japan’s is 1.5%).

    Outside Daegu, which is where the high incidence of infections are located (likely due to the wacky cult), the numbers elsewhere (e.g. Seoul) are quite low.

  22. Twinkie – SK’s low rate is because they are doing tons of testing, including random drive-throughs, so they are testing a lot of mild and asymptomatic cases. And their hospitals are not overrun. So their rate is likely to be close to a true rate for mortality among all infected, provided hospitals can provide the necessary care to those who become severely ill.

    In China, outside of Hubei Province, the mortality rate is also around 0.7% – it varies a lot between different provinces but averages out at that. Same thing as SK – lots of testing, including now big random samples of populations, and hospitals not overrun. Inside Hubei is a whole different story, and their numbers dominate the Chinese stats unless they are separated out.

    Italy is a combination of 3 things: old population, tending to test only severely ill people, and their hospitals are overrun.

    I don’t feel like I understand the Japanese numbers – I can’t help feeling we are not getting the whole story, but it could just be old population. They said during the Diamond Princess saga that they didn’t have enough test kits to test the 3,700 passengers + crew. In an advanced hi-tech country? Korea tested 200,000 in the first couple of weeks.

  23. John Massey,

    Since you seem to be a longtime resident in Hong Kong and knowledgeable of affairs there, I was wondering if you would agree with this analysis by Trey Menefee, an American expat analyst there:

    He’s essentially calling the Carrie Lam administration a completely failed government, and Hong Kong was only able to avoid the COVID19 plague by private citizens/civic groups courageously pulling themselves up by their own boot straps. Granted Trey is known for his fervent support of last year’s protest movement, but I do wonder if his views are grounded in truth.


  24. @Riordan – That is not what I would call an analysis; nothing like it.

    Menefee gives scant credit to HK’s very good public health system, no mention of high rates of testing (critical information necessary to managing any epidemic), no mention of our very good Centre for Health Protection (CHP, basically the HK version of the CDC in the USA, but obviously on a much smaller scale), no mention of the role our very good Police Force have been playing in contact tracing in full cooperation with the health authorities, or of some very good specialists from the leading universities here who have been serving as expert advisers to the government, and no mention of government arrangements for quarantining large numbers of people. No mention that government offices were shut down, with people working from home, which gave the lead to private companies to do the same. No mention of quarantine requirements imposed on arrivals from the Mainland and from other countries. No mention of shutting down large public gatherings or closure of government community facilities to enhance social distancing, with professional and public bodies following that lead. No mention that the government instructed the closure of all schools and universities (actually not reopened after the Chinese New Year holiday which began on January 25), not to be reopened until at least after the Easter long weekend and possibly not even then, with teaching being conducted via Internet.

    He also gives no mention of the government’s efforts to air lift HK people who were on the Diamond Princess back to HK, or of their ongoing efforts to air lift back 2,000 HK people who have been stuck in Hubei Province (of whom about 500 were stuck in Wuhan, of whom amazingly only one has tested positive on arrival in HK) since January 23. The HK government representative in Wuhan has been working overtime to help to arrange this, and has stated on his Facebook page that he will be the last HK person to leave Hubei Province, while maintaining a sense of humour, humorously mocking stupid false stories circulating on social media about how the USA planted bioweapons in Wuhan – the way things are going there, there might not be any need for him to leave.

    The CHP has been giving very good public advice and guidance on its excellent website, including things people can do to avoid being infected, and excellent detailed information to keep the public very well informed. In fairness, I should say that most HK people have very painful memories from the 2003 SARS epidemic and learned a lot from that, and most people are pretty good about social distancing. A lot of people are now canceling their plans for overseas travel – better late than never. The government has imposed a 14 day home quarantine requirement on HK people returning from the Mainland (with checks of compliance carried out by the Police); they need to extend that to people returning from Europe and the Middle East, which they are now considering, and which will act as a further deterrent to overseas travel to destinations which are now hot spots. I regret to have to inform that the government is also currently considering extending that to the USA.

    I hope I have illustrated that the management of the epidemic in HK is not down to a few top officials in the government administration (who are mostly little more than figureheads in this very complex situation, which needs to be steered and managed by people who know what they are doing). Of those top officials, my opinion is that some of them (including the Chief Executive and Chief Secretary) are clearly incompetent and should have had the decency to resign a long time ago, but the Financial Secretary Paul Chan (who was out of his depth when he was Secretary for Development) has done an absolutely outstanding job; he has frankly surprised me with how well he has done. Full credit. To me, he is shaping up as a possible candidate for Chief Executive when we finally manage to rid ourselves of Carrie Lam, who I concluded was not up to the job when she badly mishandled the aftermath of a big typhoon that hit HK back in September 2018 (and I was certainly not alone in that assessment of her).

    Menefee lies (or doesn’t know much) about the strike by medical workers (many of whom were low level functionaries like hospital orderlies and such); they were a small minority who formed a new union (as opposed to the existing much larger staff unions). The much larger body of medical workers responded by saying that they considered it to be unethical to withdraw their services, and that they would work overtime to cover any gaps left by the strikers. The proposed strikes fell apart in disarray very quickly after that when the majority of strikers voted to go back to work, against the will of the leadership of the new union (a disgruntled nurse), but she had no choice but to accept the decision of her members.

    Menefee also makes no mention of those of HK’s courageous (aka dumb) citizens who have been kicking the epidemic can down the road by continuing to travel overseas and coming back infected, against advice by the government, or those of our courageous citizens panic buying masks which are in short supply (and needed by front line medical staff), rice and toilet paper which are not in short supply (but which panic buying of toilet paper has humorously been carried over to Australia and elsewhere via scare stories circulating in social media), also against advice by the government. Brave Hong Kongers Trigger Global Shortage of Toilet Paper would be a suitably derisive headline. (The extra funny part about that is that now some HK people are sitting at home surrounded by mountains of toilet paper, which is now in plentiful supply in the supermarkets again; in HK’s very humid climate a lot of that toilet paper will go moldy before it gets used, so the health authorities have been issuing public health warnings about the health risks of using moldy toilet paper – I don’t know what they are, and don’t want to know.)

    I could go on (and on and on), but will that do for now?

    My advice if you want it: find someone else to read.

  25. Twinkie – I think I am beginning to understand the Japanese data. It turns out that they have tested only 10,000 people in total in the whole country, starting right from the beginning. The South Koreans have been testing more than 15,000 people *per day*.

    I read that so far it is estimated that fewer than 5,000 people have been tested in the USA. It avails nothing to keep harping about that, but I am shocked and dismayed, and concerned for my good American friends and cyber-friends.

  26. A difference between the West (and probably also many other places) and India is that routine grandparent-grandchild interaction is a lot higher in India and involves grandparents dropping off grandchildren in schools, looking after them after school till their parents are back from work, preparing and feeding 1-2 of their 3/4 daily meals, etc. etc. I am very concerned about this.

  27. “ I read that so far it is estimated that fewer than 5,000 people have been tested in the USA.”

    We don’t have a surge capacity. Our medical care infrastructure is highly consolidated, our regulatory system is bureaucratic and extremely cautious (not to mention litigious), and we don’t manufacture anything. Our supply chain is complex, just-in-time (yes, hyper efficient), and is brittle to sudden shock.

    Are you surprised? We’ll see how quickly there is a testing center at the parking lot of the local Target or Walmart store.

  28. Follow up comments from Australia: Queensland premier said Friday 13 Mar 2020 there were no COVID-19 cases outside south east queensland (Brisbane – Gold Coast – Sunshine Coast. Brisbane is located at the least tropical end of the state.
    Diamond Princess Cruise ship quarantine in Darwin – no transmission while passengers were in Northern Territory.
    Northern Territory – only imported cases reported. Virtually all the NT population is in the tropical zone.
    Look at the latitudes involved on a map of Australia – this is possibly a great learning experience for transmissibility by latitude as it’s a consistent health system covering from 10 degrees South tropics to 42 degrees South (Seattle like) weather.
    I’d like to get information from that cruise ship that disembarked passengers in Cambodia, that would appear to be an excellent real life infection dose experiment that happened early, if contact tracing reveals no transmission.

  29. @John Massey

    Thank you so very much for that detailed rundown. I am not familiar with the inner workings of the HK bureaucracy, so really appreciated that explanation. I do have one additional question: I have heard from Twitter, anecdotally, that HK’s weather in the past 2 months (Jan. and Feb.) has been significantly warmer and more humid than usual. Any truth to that?

  30. @Antipodean – Aussies from the Diamond Princess quarantined in Darwin were confined to quarters for the duration to avoid cross-infections, hence no transmission. Same for the 370 Hong Kong people got off the DP, and same for the Poms. Septics I don’t know about.

    BTW, the old HK guy fingered as the index patient on the DP says he has been unfairly accused and that it wasn’t him. (He and his close family members who were onboard with him, who did not become infected, have been subjected to a ton of abuse on social media, like he did it deliberately. Brainless people in ravening packs are unattractive.) The Japanese health authorities have backed him up on that, saying that 2 other passengers on the DP showed symptoms 3 days before he did. That’s not a slam dunk, considering the variability of the incubation period, during which people can be infectious, but at the very least it introduces a sizeable measure of doubt and no reason to single him out – and note it is now known that people are at their most infectious in the first few days of being symptomatic – they are shedding the most virus during that period. They have not revealed who the other 2 infected people were. But people need to lay off the 80 year old HK guy – he has been quite unfairly accused.

    While I am at it, I might as well comment on the piece by Marc Lipsitch, which was evidently written to push back against people saying that the coronavirus would disappear in warmer weather, but I think he goes too far. No one with a functioning brain thinks that it is just going to go POOF! and disappear in April.

    In relation to Hong Kong during the 2003 SARS epidemic, he is just plain wrong; I was in HK for the full duration of that, and I know very well what happened. It is certainly not accurate to say that there were “extremely intense public health interventions”; not according to my understanding of what measures would be considered to be extremely intense.

    Schools were closed for 6 weeks in Feb (after it started and spread rapidly) and part of March, but then reopened. Universities did not close. Government offices and the offices of private companies were not closed at any stage. Infected patients were isolated, but at that time hospitals in HK had no negative pressure isolation wards. There was contact tracing and close contacts were asked to quarantine at home. Two staff working in my office became infected (probably by cross infection when they both attended public health clinics) but none of us was asked to quarantine – in fact, we were not even formally notified, and had to find out through the office grapevine. This was in a government department. I was appointed as one of the “skeleton staff” to continue to staff the department if it became necessary to keep all of the other staff at home, but that decision was never made.

    People intuitively engaged in social distancing and avoidance behaviours because at least initially the disease was a complete unknown and terrifying, but there were no formal institutionalised arrangements for this. Patients were treated in open wards with nebulisers, which just created aerosols which cross-infected other patients and medical staff. 8 doctors and nurses died. The early CFR was very high. (The final CFR for HK was 17%, still scarily high.)

    Some adults wore surgical masks when outside the home. Most didn’t. Children were required to wear them when they returned to school, and to have daily temperature checks before setting off for school, usually carried out by Mum feeling Junior’s forehead with her hand and saying “Nah, you’re fine” and signing the requisite form.

    No limitations were imposed on air travel, in or out, at any time. No border controls with the Mainland were imposed, although in 2003 far fewer Mainlanders had freedom to travel, and fewer HK people were crossing the border on a regular basis, but it was a matter of degree – by that time, HK had already been reunited with the Mainland for 6 years, and HK people could freely enter and leave.

    The rate of community transmissions/daily new cases began to gradually diminish as the weather became progressively warmer and more humid in May and early-mid June, and they finally stopped completely in late June when the *minimum* temperatures reached and stayed above 28C (82F), and when humidity had become high.

    Did increasing temperature and humidity play some part in reducing transmissions in HK? Anecdotally, I was carefully noting the daily new transmission, and my observation was that they did. My tennis doubles partner at the time was a medical practitioner who was receiving and treating SARS patients, and that was his opinion also.

    So, how come cases in the wet tropics: Singapore, Malaysia? That puzzled me somewhat in 2003, but I recall clearly that Singapore had one big outbreak cluster inside a large public hospital, which was obviously air conditioned. Singapore and Malaysia are so hot and humid that everyone spends as much of their time as they can in enclosed air conditioned spaces. (I think Karl Zimmerman had a point about that in relation to Egypt.) Plus less transmission does not = zero transmission. As Dr A Bery noted above, influenza still gets transmitted in summer, just less.

    I would not even characterise the measures that HK is taking now against Covid-19 as “extremely intense”, although we are doing a lot more now than we did in 2003. We still have the residual problem of HK people coming back from overseas infected, and I think we are going to have to start imposing mandatory quarantines and testing on those people, the way we are already doing for HK people who return across the border from the Mainland (which is now clearly safer than Europe).

  31. Hi Riordan. January was Hong Kong’s warmest January on record (since records began in 1884). Humidity was above the mean, but January is normally a dry month anyway, so not humid by HK standards.

    February was warmer than the climatic mean, but not as far above the mean as January was, and mean humidity, but February is always moderately humid.

    March so far has been somewhat warmer than the climatic mean, and about mean humidity, so moderate.

    But the extended forecast is that by the end of March we should be getting daily maximum temperatures around 29C (84F), which is early summer weather, and Relative Humidity 70 to 100%, so that will be at least two months early, and minima in the low 20s. The difference is that in summer the daily temperature variation is normally very small, only 2 – 3 degrees C; weirdly it can seem to get warmer after the sun goes down, because the breeze drops, if there is any, and the humidity rises.

    So if SARS-CoV-2 is as temperature sensitive as some say, we might notice a difference starting around end of March and into April, although it will be hard to tell, because we are now getting only a small handful of cases, almost all of which are HK people who have returned infected from overseas, and the very few local transmissions are all between close contacts, which could keep happening anyway, like influenza will in summer.

    HK is now effectively closed to everyone, including the USA, UK and European countries in the Schengen Area, as of yesterday – all arrivals including HK returnees are required to do a mandatory 14 day quarantine on arrival, so arrivals have dropped by 96%, and cross border movements to and from Mainland China were similarly stopped quite a while ago now, except for freight of food and other essential supplies.

    I will be disappointed if SARS-CoV-2 has not burned out completely here by mid April, barring something really unexpected. We’ll see. HK people have already started relaxing somewhat, which I think is a bit premature; it’s not quite over yet.

  32. As a Brazilian I am not convinced it is not just that the transmission rate slows down in hot temperatures. We are right in the summer here, mean temperatures are higher than 25C in almost all the country, and yet that has not prevented the number of confirmed cases to go from 1 to 200 in just two weeks (76 new confirmed cases just in the last 24h). And there is surely some undercounting there, because this is also the rainy season in most of the country, so lots of people with mild symptoms will have thought they just had a common seasonal cold or some harmless virus infection. Besides most people were not really worried about the pandemics until the last few days when the spread of the disease accelerated both in numbers and in geographical extent, including cases in the Amazon and in the Northeastern states close to the Equator.

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