Featured Image: Covid-19 Global Case Map, Source: John Hopkins, https://coronavirus.jhu.edu/map.html
My interest in viruses has exponentially increased over the past 3 weeks in a similar trajectory to the Covid-19 caseload curve. An article published last week by a professor from Beihang University, Jingyuan Wang and others, indicates that the current Covid-19 pandemic may slow down as the northern hemisphere enters spring and summer. Upon seeing the headline, I, like everyone else in the world right now, capitulated to the Covid-19 click bait, frantically searching for some good news and a realistic possibility that quarantine and the pandemic might end before I dissolve into my bed in a puddle of drool and tears. I found this headline particularly interesting as I had recently heard the president of the United States declare that the pandemic would fizzle as the weather warms over the next couple of months. Then, I listened to the follow up comments about the lack of evidence supporting his statement from a chorus of public health officials. I ended up very confused and assumed this article could shed some light on the issue.
The recently published study looked at the relationship between the effective reproductive number or R value of the Covid-19 virus and absolute humidity (the amount of water in the air) and temperature across 100 Chinese cities. The R value measures the severity of infectiousness of a virus or how easily it is transmitted between people. After accounting for GDP per capita (gross domestic product per person or a metric of economic welfare) and population density (how many people per area) differences between cities, the authors found that there was a negative relationship between the R factor of Covid-19 and the Jan. 21 – 23 average temperature and absolute humidity of the 100 Chinese cities examined. The authors estimated the R value for a given temperature and absolute humidity and produced a map that indicated the Covid-19 transmission rate should decrease in the northern hemisphere as summer approaches.
This paper used a large amount of data to show an important correlation, but I was really interested in the mechanism. Why are respiratory viruses more successful at spreading when it is cold and dry? My subsequent exploration found a hot debate.
Density of people
I have heard that viruses spread more easily in the winter because when it is cold, more people are inside in closed spaces. Being from the south, I never really bought this argument considering I spend more time inside in the summer. However, this could explain increased transmission for more temperate latitudes and Winter also corresponds with many holidays. Regardless of the temperature, holiday parties and gift searching at the mall can be a viral exchange haven. This explanation is really just increased density within confined spaces. Density is an extremely important factor, but it doesn’t explain the correlation in the recently published paper and is far from the whole story as I found out.
Humans don’t perform optimally in cold weather but respiratory viruses do
Another relatively common explanation is that cold weather reduces the strength of our immune systems. There is evidence that low humidity and low temperature conditions impact human defenses against airborne viral infections. For example, cold air has been shown to slow down the defensive action in our noses that combat contaminants (Eccles 2002). In addition, low humidity has been shown to reduce the effectiveness of our mucus in protecting against potential viral threats (Salah et al. 1988). Basically, the nose and throat cilia and mucus that act as the last line of defense against contaminants do not work as effectively under cold and dry conditions. Other evidence points to reduced stability of some of these respiratory viruses as temperature and humidity increase. Generally, colder temperatures allow for the preservation of organic material. Think about entire bodies of woolly mammoths being preserved in Arctic ice. It is similar for viruses in that viral viability is higher in colder temperatures.
Viruses can spread easier in low humidity conditions
The role of humidity on viral stability and transmission interested me the most as a physical scientist. Respiratory viruses are generally enveloped in droplets of water (Fig. 1). Your lungs are at 100% humidity (the air in your lungs is completely saturated with water) so when you cough, those droplets enter ambient air that is less than completely saturated (unless you are in a sauna or south Louisiana in early August). The amount of water that the air can hold is temperature dependent. Warmer air can hold more water, colder air can hold less (Fig. 2). The humidity of ambient air that the water enveloped virus enters controls the amount of evaporation that will occur from the virus surface (Fig. 1).
For example, think about a simplified evaporation above a lake. If the ambient air above the lake surface is already close to saturation, the atmospheric demand for additional water will be low, whereas if the air is dry, the demand for lake water will be higher due to concentration gradients and subsequent diffusion (Fig. 3). Similarly, if you have low humidity conditions, more evaporation will take place from the viral envelope. This can affect the concentration of salts in contact with the virus (Yang et al. 2012, Fig. 1). Studies have shown that at very high relative humidity little evaporation takes place and physiological conditions are maintained, as is viral stability. At low relative humidity, complete evaporation of the envelope may take place. In this situation, the salts crystallize out of solution (because no water is left) and do not affect viral stability. However at intermediate humidity, evaporation leads to a high salt concentration still dissolved in the smaller liquid envelope which can inactivate the virus (Fig. 1).
Humidity can also affect the travel distance of a viral droplet (Fig. 4). Under low humidity conditions, the liquid envelope evaporates, reducing the size of the viral droplet (Tellier 2009). Smaller viral droplets can stay airborne for longer periods of time compared to larger droplets. The time it takes for particles to settle is explained by Stoke’s law which is largely dependent on the size of the particle. Thus, low humidity conditions may allow the virus to stick around longer in the air, increasing the chances that another person may inhale it rather than the viral droplet quickly falling to a surface.
To be clear, respiratory viruses are around all the time, but they are more easily transmitted from person to person and thus statistically more abundant under cold and dry conditions. Summer colds aren’t impossible but there is a reduced chance of getting them. Unfortunately, all of this research has been conducted on the normal cold and flu viruses. Covid-19 presents a novel situation. There is little mechanistic information on the effects of temperature and humidity on the viability of this virus. However, due to the similarities of the Covid-19 transmission to other respiratory viruses, the physical rules governing how long particles of certain sizes can stay airborne and the physical ability of our immune systems to protect against infection under various environmental circumstances should hold. For now, only time will tell if the severity of spread will be reduced in northern latitudes as summer arrives. However one thing is for sure, reducing the amount of people you come into contact with by self-quarantining, increasing the amount of space between you and others when you have to go out (self-distancing), or using a mask (reducing droplet air travel distance) will reduce the risk of spread regardless of the temperature and humidity conditions. Stay safe and stay inside!
Wang, Jingyuan and Tang, Ke and Feng, Kai and Lv, Weifeng, High Temperature and High Humidity Reduce the Transmission of COVID-19 (March 9, 2020). Available at SSRN: https://ssrn.com/abstract=3551767 or http://dx.doi.org/10.2139/ssrn.3551767
R. Eccles, An explanation for the seasonality of acute upper respiratory tract viral infections. Acta Otolaryngol. 122, 183-191 (2002). doi: 10.1080/000164802 52814207
Salah B, Dinh Xuan AT, Fouilladieu JL, Lockhart A, Regnard J. 1988. Nasal mucociliary transport in healthy subjects is slower when breathing dry air. Eur. Respir. J. 1:852–855.
Yang W, Elankumaran S, Marr LC. 2012. Relationship between humidity and influenza A viability in droplets and implications for influenza’s seasonality. PLoS One 7:e46789. doi:10.1371/journal.pone.0046789.
R. Tellier, Aerosol transmission of influenza A virus: a review of new studies. J. R. Soc. Interface 6, S783–S790 (2009). doi: 10.1098/rsif.2009.0302.focus.
A. C. Lowen, J. Steel, Roles of humidity and temperature in shaping influenza seasonality. J. Virol. 88, 7692–7695 (2014). doi: 10.1128/JVI.03544-13.