Exactly How the Flu Spreads, According to Infectious Disease Experts (msn.com)
Richard Watkins, M.D., an infectious disease physician and professor of internal medicine at the Northeast Ohio Medical University.
Amesh A. Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security.
William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine
Please note: The gents quoted in the article above are not simple laypersons, hanging out on fish forums.
“The data would indicate this only happens under certain circumstances,” he says. “The flu is spread overwhelmingly through close personal contact.” However, the flu may be more likely to hover in the air in the winter, when the air is drier, Dr. Schaffner points out. “That little bit of fluid that covers the particles evaporates and they’re not as heavy,” he says. “That’s one of the reasons it’s thought that influenza is more common in the winter—the air is drier then.”
Will COVID-19 prevention measures protect people from the flu, too?
Definitely. “Mask wearing, social distancing, and hand washing are effective prevention methods for flu avoidance,” Dr. Watkins says. “Which is why last year we saw a record low number of flu cases.”
“Last season, we had almost no flu and all of us were staying home, washing our hands, and wearing masks,” Dr. Schaffner says. “It all had a profound effect on reducing the influenza virus.”
That was the simple version. Below is a more detailed version of the actual science behind how temperature and humidity can potentially affect the transmission of airborne influenza aerosols/droplets. Where I live, RH is generally 20-30% for 5-6 months of the year (winter) , with outside ambient temperatures as low as -50F, with the vast majority of people, spending the vast majority of their time, indoors.
Mechanistic insights into the effect of humidity on airborne influenza virus survival, transmission and incidence | Journal of The Royal Society Interface (royalsocietypublishing.org)
3.6. Mechanisms
When influenza viruses are emitted into air from an infected host by coughing, sneezing, talking or breathing [
53–
55], they are just one component of respiratory droplets that are a complex mixture of inorganic and organic ions, protein and surfactant [
56]. The respiratory droplets can range in size from submicrometre to thousands of micrometres [
57–
64], although those larger than 100 µm remain suspended for less than 5 s before settling to the ground from a height of 1.5 m. We expect temperature to affect the stability of viruses in the environment, including those that are airborne.
In a review of the effect of environmental parameters on the survival of airborne pathogens, Tang [26] concluded that temperature and RH always interact to affect the survival of aerosolized viruses. One mystery is how RH could affect viruses in a respiratory droplet because unless they are present at the surface of the droplet, they are not exposed to ambient air and so would not directly interact with water vapour in the air.
Upon contact with ambient air, respiratory droplets are subject to evaporation until they reach equilibrium. Evaporation may be partial, in which case some water remains in liquid or semi-solid form, or complete, in which case only solutes and the virus remain, and likely some trapped water molecules. Air expired from the respiratory tract is saturated [
65], whereas ambient air usually is not, so there is a driving force in the form of a vapour pressure gradient between the surface of a droplet and ambient air. The extent of evaporation and final, equilibrium size of the droplet is determined by ambient RH, not AH. This equilibrium size, which is attained within a few seconds at most [
3,
36,
66], has important effects on the droplet's physics and chemistry, although inactivation of any virus contained in the droplet appears to proceed more slowly [
13].
Combining the effects of curvature at the air–liquid interface (i.e. the Kelvin effect) and of solutes on the saturation vapour pressure of a droplet produces an equation that can be used to predict the equilibrium diameter of a droplet as a function of RH [
67].
Figure 3 shows how the equilibrium size of a droplet of model respiratory fluid depends on RH in terms of the ratio of the equilibrium diameter to the initial diameter. Initially, the model fluid contains 9 mg ml−1 salt as NaCl, 3 or 76 mg ml−1 protein (range of values found in nasal surface airway fluid [
10] and aerosols in exhaled breath condensate [
11], respectively) and 0.5 mg ml−1 surfactant as DPPC [
12]. For reference, two other studies report total protein contents of respiratory fluid that are closer to the lower end of the range: 4 mg ml−1 in unstimulated saliva [
68] and 7 mg ml−1 in nasal mucus [
69].
The ratio shown in
figure 3 applies for droplets of initial diameter as small as approximately 0.5 µm, at which point the Kelvin effect becomes important.
Figure 3 shows that at 100% RH, there is no evaporation, and the ratio is 1. At 90% RH, the droplets are dramatically smaller at equilibrium, 0.28 or 0.51 of their initial size for low and high-protein contents (3 mg ml−1 and 76 mg ml−1), respectively. At 50% RH, these values are 0.19 and 0.41, respectively. For comparison, Liu
et al. [
70] predicted that dried droplet nuclei would be 0.32 of their original diameter. In fact, at RH below 64% for the low-protein droplets and below 42% for the high-protein droplets, the equilibrium diameter is unchanged because all bulk liquid water has been lost and only the solutes, or what some have called the droplet nucleus, remain.
The equilibrium droplet size is critical in determining its fate (e.g. [71–74]). Size determines how long the droplet remains suspended in air before it is removed by gravitational settling and where it deposits in the respiratory system if inhaled. The transformation of a droplet subject to evaporation in ambient air has a dramatic impact on its lifetime. We calculated the settling time for a droplet initially 10 µm in diameter containing 9 mg ml−1 salt as NaCl and 3 mg ml−1 protein at RHs of 100%, 90% and 64% and lower.
Table 2 shows that the droplet shrinks to 2.8 µm at 90% RH and to 1.9 µm at less than 64% RH.
These diameters correspond to vastly different settling times; the 10 µm droplet at 100% RH remains suspended for only 8 min, whereas the 1.9 µm droplet at less than 64% RH remains suspended for more than 3 h, posing a much longer opportunity for airborne transmission. Xie
et al. [
74] and Parienta
et al. [
72] present a much more complete analysis of droplet transformation and transport distance as a function of initial size and RH.
Furthermore, we can calculate the deposition efficiency in different regions of the respiratory system to contrast where the droplets might deposit if inhaled [75]. Table 2 shows that the 10 µm droplet has a high chance of being deposited in the head airways (81%) and only a small chance of being deposited in the alveolar region (2%), whereas a 1.9 µm droplet has a lower chance of being deposited in the head airways (57%) and a relatively higher chance of being deposited in the alveolar region (12%) than does the 10 µm droplet. This calculation does not account for hygroscopic growth of droplets upon entering the saturated airway, although we have shown that model respiratory droplets containing surfactant do not reabsorb water even after extended exposure to saturated conditions [
56].
A review of aerosol transmission of influenza suggests that infection initiated in the lower respiratory tract requires a lower dose and produces more severe symptoms compared to intranasal inoculation [76]. In summary, at lower RH, droplets stay suspended longer and are more likely to deposit in the lower respiratory tract. As mentioned earlier, the dynamics and deposition efficiencies of aerosols in animal models are likely to be quite different, for example in ferrets due to their more horizontal posture and much smaller airway diameters compared to those of humans [
37,
38].
Besides its effects on physics of a droplet, evaporation also affects its chemistry, which could affect the stability of a virus contained within the droplet. The mass of solutes in a respiratory fluid droplet containing one virus is expected to be at least five orders of magnitude larger than the mass of the virus [
56], so these components should not be ignored when considering virus viability in droplets and aerosols.
Table 2 shows the concentration factor, calculated as the ratio of the initial mass of water in the droplet to that remaining at equilibrium. This is the factor by which solutes that remain in the aqueous phase become concentrated due to loss of water if they do not precipitate out of solution. The solubility of NaCl is 360 mg ml−1, or 40 times the initial concentration of 9 mg ml−1. Thus, we expect it to crystallize at an ambient RH of approximately 90% (concentration factor of 65) and lower, and indeed, we have observed crystallization of salt in model respiratory fluid droplets exposed to varying RH [
25,
56]. Under these concentrated conditions in droplets, protein may form aggregates [
56], and there may be spatial gradients in pH [
77]. In studies of atmospheric aerosols, researchers have observed phase separation, crystallization and changes in pH under conditions of changing RH [
78–
81]. While such changes in droplet chemistry could very well cause virus decay, the exact mechanism of inactivation is not known. Various possibilities have been proposed, such as osmotic bursting [
82] or pH-sensitive changes in protein structure [
24], but more studies are needed.
4. Conclusion
The debate over the importance of AH versus RH for influenza virus survival and transmission dates to the mid-twentieth century. Writing in Nature in 1960, Hemmes et al. [83] claimed, ‘it became evident that the relative humidity and not the absolute humidity is the determining factor’, for virus survival in aerosols. Analyses of the early twenty-first century have pointed to AH as the modulating factor. While it is appealing to try to isolate a single controlling environmental factor that modulates influenza virus survival, transmission and incidence, our analysis suggests that the combination of temperature and RH provides a consistent, mechanistically sound explanation of the observations. Temperature can be considered an intrinsic factor in virus stability because rates of inactivation of proteins and nucleic acid are expected to increase with temperature [
16]. RH can be considered an extrinsic factor in virus stability because it controls evaporation, which affects a droplet's size and physical fate and its chemical microenvironment. By contrast, there is no mechanism by which AH is expected to affect droplet diameter except through its relationship with RH.
The story is not yet complete, but we know that RH determines the extent of evaporation of an airborne droplet and thus the resulting chemical microenvironment to which the virus is exposed. If virus survival in the environment is a dominant factor in influenza transmission, then RH is expected to influence incidence and seasonality, too.
The short version
- if you want to help reduce the spread of covid (or the flu) while indoors with others, wear a mask. If you don't care about others, then I guess don't bother.
