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.
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 Area||The month when it gets humid enough|
|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 – C||Temp – F||10.00%||20.00%||30.00%||40.00%||50.00%|
|Temp – C||Temp – F||60.00%||70.00%||80.00%||90.00%||100.00%|