Photo Credit PHIL (Public Health Image Library)
Although just about everybody knows that winter heralds the arrival of influenza season, one of the enduring mysteries about influenza is why it is predominantly a winter phenomenon – at least in temperate zones of the world.
Numerous theories abound, including;
- During the winter people tend to gather indoors, with less outside ventilation.
- Diminished sunlight exposure may reduced Vitamin D levels (see Study: Vitamin D And Flu-Like Illnesses)
- With schools in session, millions of children co-mingle and more efficiently share viruses
While these may be factors, they don’t satisfactorily explain our yearly winter flu season. Nor are they very helpful in explaining why the flu transmits pretty well in the tropics, where there is little temperature variability.
In recent years, researchers have been looking at relative humidity (RH) and absolute humidity (AB)as factors in aiding influenza virus survival (IVS).
In 2007, we looked at a study (see Cold And Dry Statistics) that appeared in PLoS Pathogens entitled Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature by Anice C. Lowen, Samira Mubareka, John Steel, and Peter Palese.
Using a guinea pig as a model host, they showed that airborne spread of the influenza virus was at least partially dependent upon both ambient relative humidity and temperature.
The link was significant, but not overwhelming.
The following year researchers Jeffrey Shaman and Melvin Kohn and found an even stronger correlation between the AH (Absolute Humidity) and the survival, and transmission of the influenza virus (see It's Not So Much The Heat, It's The Humidity).
Jeffrey Shaman, and Melvin Kohn
In early 2010, Jeffrey Shaman returned – joined by Virginia E. Pitzer, Cécile Viboud, Bryan T. Grenfell and Marc Lipsitch – to pen a study published in PLoS Biology called:
Here we extend these findings to the human population level, showing that the onset of increased wintertime influenza-related mortality in the United States is associated with anomalously low absolute humidity levels during the prior weeks. We then use an epidemiological model, in which observed absolute humidity conditions temper influenza transmission rates, to successfully simulate the seasonal cycle of observed influenza-related mortality.
During the summer, ambient air often contains 4 times as much water as it does on a cool dry winter's day. This, increasingly, is beginning to look as if it is a significant factor in the spread of influenza viruses.
As a side note, the Chinese have long boiled vinegar in their homes to ward off respiratory ailments, such as influenza.
The noxious odor was supposed to `purify' the air inside the home, and newspapers still recommend this practice during their influenza season.
It may well be that the `active ingredient' is really the water vapor being released, raising the absolute humidity in their homes to an unfavorable level for influenza virus survival and transmission.
All of which is neat and tidy until you consider that flu transmits readily in the tropics, where the atmosphere is often nearly saturated with water vapor.
Enter researchers from Virginia Tech who have found that both extremely low and extremely high levels of humidity appear to aid and abet the viability of the flu virus – at least when it resides in mucus and respiratory fluids like those found in your nose, throat, or lungs.
This is an open access article, available from PloS One.
Relationship between Humidity and Influenza A Viability in Droplets and Implications for Influenza’s Seasonality
PLoS ONE 7(10): e46789. doi:10.1371/journal.pone.0046789
Wan Yang, Subbiah Elankumaran, Linsey C. Marr
Abstract (excerpts reparagraphed for readability)
Humidity has been associated with influenza’s seasonality, but the mechanisms underlying the relationship remain unclear. There is no consistent explanation for influenza’s transmission patterns that applies to both temperate and tropical regions.
This study aimed to determine the relationship between ambient humidity and viability of the influenza A virus (IAV) during transmission between hosts and to explain the mechanisms underlying it.
We measured the viability of IAV in droplets consisting of various model media, chosen to isolate effects of salts and proteins found in respiratory fluid, and in human mucus, at relative humidities (RH) ranging from 17% to 100%.
In all media and mucus, viability was highest when RH was either close to 100% or below ~50%. When RH decreased from 84% to 50%, the relationship between viability and RH depended on droplet composition: viability decreased in saline solutions, did not change significantly in solutions supplemented with proteins, and increased dramatically in mucus.
Essentially, these researchers inoculated droplets of simulated respiratory fluids (containing salts & proteins) with influenza viruses, and tested their survivability at different humidity levels.
- At low humidity (< 50%) the droplets evaporated quickly, and the virus survived well in a dry environment.
- At high humidity (near 100%), the droplets were stable, and the virus survived as well.
But at humidity levels in-between, the droplets slowly dried out, increasing the concentration of salts and proteins to which the viruses were exposed, decreasing their survival rate.
As these experiments were conducted on relatively large droplets, how these findings would relate to viruses carried by smaller, aerosolized particles (that would desiccate much faster, even at higher humidities) remains untested.
Still, it’s fascinating research, and it adds incrementally to our understanding of how the influenza virus survives outside of a host.
You may recall I featured another study by these same researchers in early 2011 (see Why Size Matters) where they analyzed the amount of influenza virus suspended in the air in several environments, including a daycare center, a healthcare waiting room, and aboard commercial aircraft.
They found airborne virus particles in half of the air samples tested, and in quantities they believe sufficient to enable transmission of the virus.
Many of these virus particles were very small, less than 2.5 micrometers, which can remain aloft on a room’s air currents for hours. Larger droplets would settle far sooner, and theoretically present less of a threat.
"As a whole," the three authors concluded in the Journal of the Royal Society Interface, "our results provide quantitative support for the possibility of airborne transmission of influenza in indoor environments."
All of which makes the influenza virus a formidable foe, and highlights the importance of maintaining good flu hygiene (hand washing, covering coughs & sneezes & staying home when sick), and the wisdom of getting that flu shot every year.