Nature will not help us against COVID-19 in the American Spring

It’s the beginning of the official spring of 2020, and the United States of America is now in the midst of a massive upsurge in positive test results for COVID-19, the illness caused by SARS-Cov-2. Right now, New York City is the major focus. Seattle, which was an early outbreak hotzone has taken a backseat. The frequency and ubiquity of the positive test results suggest to many that this virus has been in these United States for a while.

One issue that keeps coming up: what are the environmental covariates of COVID-19? These are early days yet, but peculiar patterns such as Italy’s high death rate, and Germany’s low death rate, are not understood yet. One issue brought up rather early by President Donald J. Trump, is that weather warming might mitigate the impact of the virus. And there is a seasonality with many respiratory diseases.

But is there reason to assume this would be so with COVID-19? Well, see this piece in Science, Why do dozens of diseases wax and wane with the seasons—and will COVID-19? Here is the most relevant part for me:

Four human coronaviruses that cause colds and other respiratory diseases are more revealing. Three have “marked winter seasonality,” with few or no detections in the summer, molecular biologist Kate Templeton, also at the University of Edinburgh, concluded in a 2010 analysis of 11,661 respiratory samples collected between 2006 and 2009. These three viruses essentially behave like the flu.

The flu is not a coronavirus, but it’s the most famous seasonal illness.

One of the stranger things about the spread of COVID-19 is the relatively slow spread of the disease in many tropical locations. This is glaring in Southeast Asia, which has extensive contact with China (and some early introductions of COVID-19). In contrast, COVID-19 exploded outside of China first in Iran, and then in Italy.

Some early papers suggested there was no correlation with temperature or perhaps a very modest one. Others made a stronger case. The problem is with data. During the early days of the pandemic, there weren’t many data points, and those came from China. Now we have more data, and more analyses are coming out.

A new preprint, Will Coronavirus Pandemic Diminish by Summer?

…While influenza virus has been shown to be affected by weather, it is unknown if COVID19 is similarly affected. In this work, we analyze the effect of local weather on the transmission of the 2019-nCoV virus. Our results indicate that 90% of the 2019-nCoV transmissions have so far occurred within a certain range of temperature (3 to 17C) and absolute humidity (4 to 9g/m3) and the total number of cases in countries with mean Jan-Feb-March temperature >18C and and absolute humidity >9 g/m3 is less than 6%. Current data indicates that transmission of 2019-nCoV virus might have been less efficient in warmer humid climate. We could not differentiate which of the two environmental factors is more important, however, given the tight range of absolute humidity (4 – 9g/m3) across which the majority of the cases are observed, and previous associations between viral transmission and humidity, we believe that absolute humidity might play a bigger role in determining the spread of 2019-nCoV. Theoretical calculations suggest that absolute humidity is always lower than 9 g/m3 for temperature less than 15C and for temperatures between 15 and 25 C, the relative humidity has to be >60% for absolute humidity to be >9g/m3. Therefore if humidity plays a bigger role than temperature, then the chances of 2019-nCoV transmission slowing down due to environmental factors would be fairly limited for regions above 35 degree N due to environmental factors. On the other hand, Asian countries experiencing monsoon from mid-June can see a slowdown in transmission. On the contrary if temperature is more important, then most of the northern hemisphere should see a slow down in the spread of the 2019-nCoV with the approaching summer temperatures. Our hypothesis is based on currently available data and its validity will automatically be tested in the next few weeks with reporting of new cases across the world. The relation between temperature and humidity and 2019-nCoV cases should be closely monitored and if a strong environmental dependence in the spread of 2019-nCOV exists then it should be used to optimize the 2019-nCoV mitigation strategies. Our results in no way suggest that 2019-nCoV would not spread in warm humid regions and effective public health interventions should be implemented across the world to slow down the transmission of 2019-nCoV.

The idea that absolute humidity (basically the amount of water vapor that is present in the air) matters comes in part from a 2009 paper, Absolute humidity modulates influenza survival, transmission, and seasonality. If flu is spread through droplets that are aerosolized, then more absolute humidity means water accrues to the droplets, and they don’t stay in the air as long. Though there is still some controversy about the details of how COVID-19 spreads, often it’s through droplets from coughing or sneezing (though the possible spread from asymptomatic people is troubling, as they would not be coughing or sneezing).

A critique of their data easily presents itself. Russia, at this moment, seems highly likely to be masking their cases. The pandemic is in early stages, and literally every day the media declares that India has the potential to be the next major epicenter. Pretty soon, within four weeks, we’ll probably see if every region of the world is going through the exponential increase that we’re seeing in the United States of America, making the climate modifier model moot. But we’re not there yet.

Figure 4 from the preprint presents their primary result (recapitulating earlier work), that most of the infections seem to occur at a particular temperature/humidity range:

You see here that the infections are occurring in the range of absolute humidity between 4 and 8 g/m3. There are all sorts of reasons these are artifacts, but this clearly comports with intuition when you look at the map of where infections are. As is clear in the preprint, the authors are not claiming that climate is the only variable that constraints or shapes the spread of the disease. To name some off the top of my head, density, cultural practices (e.g., physical greetings that require contact), age structure, and frequency of comorbidities and other infections probably matter.

Using a temperature and humidity table I computed when cities get “warm enough” to reduce the risk of COVID-19 transmission (I ignored the cold as a mitigator because I don’t think we really have enough reliable data):

Metro AreaThe month when it gets humid enough
New YorkJune
Los AngelesJune
Washington DCMay
Miami(all year within the zone)
San Francisco(all year outside of zone)
Mumbai(all year within the zone)
Karachi(all year within the zone)

The key point to note is that absolute humidity is dependent upon relative humidity and temperature. Very dry cities, such as Cairo and Tehran don’t do so well, because even though they get warm rapidly in spring, they remain dry. There should be a huge difference in Pakistan, between balmy Karachi, and Lahore inland, which is drier and more continental.

Unfortunately, San Francisco is too cool all year, though the whole region has many microclimates, so I wouldn’t overgeneralize. Seattle summers tend to be dry and only moderately warm.

Another major wild-card here is that air-conditioning is now very popular and widespread. This reduces absolute humidity in the environments that many people live in. Rural residents of tropical countries, who have less access to air-conditioning (and live at lower densities), may actually be relatively lightly impacted by COVID-19 compared to their jet-setting urban compatriots, who work in air-conditioned offices.

Temp – CTemp – F10.00%20.00%30.00%40.00%50.00%
Temp – CTemp – F60.00%70.00%80.00%90.00%100.00%

18 thoughts on “Nature will not help us against COVID-19 in the American Spring

  1. Re: asymptomatics, droplets and aerosols are really a continuum, progressing from larger to smaller sizes, and aerosols can be more problematic, because some testing has already shown that in enclosed spaces, particularly artificially heated spaces where you get rising air currents, aerosols can hang in the air for as long as 3 hours, and you can be infected by those.

    Covid-19 invades the upper respiratory tract as well as the lower, and even asymptomatics who are not coughing or sneezing are breathing out warm moist air carrying a lot of coronavirus particles (which are tiny), which is an aerosol. Not only can aerosols hang in the air for 3 hours, a clever bit of detective work by some Chinese scientists showed that an aerosol can infect someone as far away as 4.5 metres, which is a lot further than the 2m distancing rule that people are being taught. They reconstructed what happened on a bus that was sealed and heated during a 4 hour bus trip, and not only were people infected who were sitting as far away as 4.5m from one infected guy who got on, one woman was infected 30 minutes after he got off, i.e. she didn’t get onto the bus until 30 mins after he had got off. Perversely, people who were sitting right next to him, immediately in front and immediately behind him were not infected – it was all determined by the air currents and where the aerosols that he was breathing ended up.

    Higher temperatures and humidity should help with aerosols, maybe even more than they can help with droplets, and they can help with reducing the life spans of coronavirus particles deposited on fomites, if this coronavirus is anything like the SARS coronavirus in terms of temperature and humidity sensitivity. I’m saying IF – I’m gob smacked that no one has done any proper testing of the sensitivity of SARS-CoV-2 to temperature and humidity yet; it’s not that hard. Give my daughter a lab and some samples of the coronavirus and she could do it.

    My theory on why the low death rate in Germany is that they started testing early and often, and they have been steadily cranking out 12,000 tests/day ever since they got their first few cases, so they have picked up a lot of asymptomatic and mild cases in their total data set, even more than the South Koreans have. Maybe they don’t have a completely random population sample, but they might be closer to it than anyone else. Plus they reportedly have a first class health system which has not come close to being over-stretched yet, whereas in northern Italy the health system has clearly been completely overwhelmed and they are having to make awful decisions about who to try to keep alive and who to let go, and evidently a lot of elderly Italians are preferring to die at home surrounded by family rather than go into isolation in a hospital where they are going to die anyway. Supplementary reasons are age and health profile for Germany and northern Italy respectively.

  2. I think, like I said in the last comments made on the issue, that its a combination of sun/UV-rays, humidity and temperature which will make a huge difference and that those three factors work independently as well as together.

    – The best weather is sunny/high UV with higher temperature and humidity.
    – The worst weather is cloudy/low UV with low temperature and humidity.

    That those three factors
    – UV rays
    – temperature
    – humidity

    have all an effect on the virus should be a given. The real question is which effect every factor has on its own and how they effect in combination.

    There is also the problem of human behaviour. Like if people work in darkened open plan office with artificial light and air conditioning, you basically knock a large portion of the weather effect out.

    If people work on the field, in the open, with fresh air, all weather effects will have their full impact.

    So basically, places like hospitals, old people’s homes, shopping malls and offices are now and might be in the future breeding grounds for the virus. The more urban and modernised, the less effective the weather effects might be.

    I wouldn’t wonder if a large portion of the community infections in places like Brazil and India would happen under the same circumstances in an “artificial environment”.

    But right now we have in Central Europe the “ideal weather” for the spread of the virus the next days and still good weather for the infectiousness probably for many weeks to come. Like you listed, for many places its months to go, which is a huge challenge and burden for the economy and society. Probably too much for keeping up isolation, especially if the exact extent of the weather effect is still unknown.

    The more Southern and Western people tend to touch each other more often and live close. In Italy people touch each other more often, come closer, older people are better integrated, live oftentimes in the house of their children and grandchildren.

    If you isolate old people’s homes and hospitals earlier and better, tell old people which have their own home to stay at home, you gain a lot of time and prevent a lot of infections. This was done in Germany, but wasn’t done at all in Italy, where it would be much harder to archive anyway.

    I think that’s one of the major factors for the different death rates so far.

    In Italy the spread of the virus to the South is much slower and less severe, which proves, once more, the weather effect.

  3. As a doc I’m in communication with friends working in the English speaking world. Sydney is a hotspot, Tom Hanks caught it on the Gold Coast south of Brisbane, warm and humid, Brisbane ICU’s filling. London ICU’s in crisis. I’m in Central Massachusetts and this morning 50% ICU beds have CORVID cases.

    You can sit here and look at these papers. Understand they are 3 weeks behind reality. Watch Indonesia and Malaysia as precursors to the tropics.

    This will likely become a seasonal virus – but only when herd immunity reaches 50-60% (per my epi friends). They expect continued waves for spread until that happens.

  4. To be a bit more careful and precise, if you are close enough to an infected person to cop a droplet in the face, the weather won’t save you no matter how good it is, and among chaotic, dense, impoverished populations crowded together in India and other countries that could happen a lot. If you are inside a large, enclosed, air conditioned space full of people in Singapore, likewise, from aerosols. There will not be zero transmission in tropical and hot, sunny places. In 2003 Singapore had a cluster of SARS infections inside a large public hospital – the external tropical weather conditions were totally irrelevant.

    In watching what happens in Africa, the particular living conditions will be very relevant to trying to interpret events.

  5. We have plenty of examples in the US. Miami and FL should be seeing a lot of deaths if the temp/humidity thing is wrong. If you look at demographics and the smart thermometer reads out fever is 3% higher than normal in Florida, but we’re seeing miminal cases and deaths.

    California generally is also very low. Hawaii is as well but given that is a special case.

    HVAC is an issue. However I think the larger factor is epithelia health. Improves with humidity. Mucus dries up and hardens in the winter with low humidity, becomes looser with higher humidity. Loose musus is being renewed sooner with higher humidity levels and people with low viral exposure won’t get infected.

    The problem is low levels of viral transmission exist now, and the panic is requiring lockdowns when there is an almost zero risk of infection — see Florida.

    Infections patters looks like every other respiratory illness — you catch it from people you live with. Children will be the biggest spreading agent. The difference here is health care staff is the larger driver of infections.

  6. Warm, humid air helps with pneumonia too. It makes me wonder if a bunch of mild cases in Africa, Latin America, and India just aren’t getting reported by the people who get them, because they don’t have real access to doctors anyways. How many people die in slums without being reported to any kind of central authority?

  7. The importance of absolute humidity as opposed to relative humidity is difficult to understand if the mechanism is some direct effect on aerosols. Tiny droplets very quickly reach the same temperature as the surrounding air, and then their growth or shrinkage is purely determined by the chemical potential of water inside and outside the droplet. If the droplet were pure water, it would shrink for any relative humidity under 100%. In fact it contains solutes, so the threshold for shrinkage would be a bit under 100% relative humidity.

    Absolute humidity would be more relevant if the key steps occur somewhere at fixed temperature where the atmospheric temperature is not very relevant. That would be inside the body. Therefore, if it really holds up hat absolute not relative humidity is the key parameter, the mechanisms are physiological. That suggests possible strategies.

  8. ” The problem is low levels of viral transmission exist now, and the panic is requiring lockdowns when there is an almost zero risk of infection — see Florida.”

    If, even before this current lockdown, the median Floridian spent more than 5 minutes daily in an un-airconditioned communal airspace (consisting soley and wholly of scurrying through sauna-esque parking lots, to and fro the frigidized car) between April and October, then I’ll respect charlie’s opinionatin’.

  9. I read analyses like these, where deep understanding is beyond me, at least what I have the time and attention for, and I wonder about Australia. Why is no mention made of that country? It is hot there now, or at least warm, and it seems to have many cases of Covid19 (either ~1.7K confirmed cases up by ~7% from yesterday or ~4.1K confirmed cases up by ~12% from yesterday)*. Is it just that throughout much of the continent, the climate is dry — low absolute humidity?

    *NB: The figures on these pages change daily, so the figures that I have included here will not be the same after today.

  10. A Berry:

    You have crows in the ICU?

    More seriously: I’m also in Central MA. I’ve been watching the Mass. DPH reports of numbers of hospitalized COVID-19 patients. As of 12:30 pm yesterday, they report a total of 71 hospitalized (not ICU) cases for the entire state. Does that number seem correct, based on what you’re seeing? (The numbers are reported in the PDF file linked from this page:

    Thanks, and stay safe!

  11. @Stuck In The Muddle With You;

    As I said HVAC is a problem.

    But I was pointing out two separate issue; the physics of droplets moving in humid air but also the effects of living in a humid and warm climate on mucus flows in your nose. So yes you average Floridian has more HVAC exposure but you’ll see better health overall.

    Besides it isn’t oppressively hot right now in Florida. Just in the 80s.

    For fun, check out the

    It is accurate? Definitely shows the NYC region as a hotspot, as well as all of Florida. Demographics in FL also make sense. Does FL have a spike in deaths?

    And yes, temperature and humdity does not mean zero transmission. Just reduced fatality and reduced spread. With the pol pot level panic out there to stamp out the evil virus that won’t be enough to convince people to normalize.

  12. Unfortunately, Iran has severe air pollution and additional environmental contaminations, as a result, many Iranians already have compromised lungs and underlying health issues. Almost all big Iranian cities are covered in smog and the arid climate does not help either!!!

  13. In my county, a smaller city just west of Chicago, we have 6 confirmed cases as of today, at least two in the hospital I work in. Details are unavailable as to their condition, and even which ward they are in. Doctors have asked me (nurse) if I know where they are.

    The lack of information is annoying, as is the lack of sufficient PPE. We are reusing disposable gowns and masks.

  14. Jim

    I’m in a hospital to the West of you. You should be aware the Mass DPH numbers can run 36hrs behind. This week that can make a big difference.

    For whatever reason we admitted a few high intensity patients yesterday. Other hospital in city not bad yet. But this week likely to be bad. To any HCW’s out there keep yourself and family safe.

  15. @ Marcel Proust;

    Temperature/Humidity doesn’t stop a virus from spreading, it just slows it down. You can catch the flu in July. It is just incredibly rare.

    Looking at Australia, again the number of new cases is largely about testing, but a 7% daily growth rate is much slower that what models would suggest.

    Looking at your dashboard, the number I’d look at is the “serious” cases which seem to be either hospitalizations or ICU cases and in Australia it is zero. Likewise my Florida example. I can completely believe that the virus is raging in FL hard but why limited ICU admissions?

    Everyone need to start reading up on viral loads. Pretty clear at this point that low hits of the virus — with healthier immune systems – are resulting in very asymptotic infections. Hitting healthy young people with high loads (medical staff) will result in bad infections, as hitting older people with low levels.


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