“Did you get your flu shot?” If your friends are anything like mine, you heard this question at least a dozen times before Thanksgiving. You probably got your fair share of disdainful looks too, if you answered “No.” But why are we worried about getting the flu shot now and not in May? Why is there a flu season at all? After all, what does a virus living in a host who provides a dependable, cozy incubation chamber of 98°F, care whether it is freezing and snowy outside or warm and sunny? This question has bothered people for a long time, but only recently have we begun to understand the answer.
What is the Flu?
In order to discuss why we have a flu season, we must first understand what the flu is. The flu, also called influenza, is a viral respiratory illness. A virus is a microscopic infectious agent that invades the cells of your body and makes you sick. The flu is often confused with another virus, the common cold, because of the similarity in symptoms, which can include a cough, sore throat, and stuffy nose. However, flu symptoms also include fever, cold sweats, aches throughout the body, headache, exhaustion, and even some gastro-intestinal symptoms, such as vomiting and diarrhea (1).
The flu is highly contagious. Adults are able to spread the virus one day prior to the appearance of symptoms and up to seven days after symptoms begin. Influenza is typically spread via the coughs and sneezes of an infected person (1). Around 200,000 people in the United States are hospitalized each year because of the flu, and of these people, about 36,000 die. The flu is most serious for the elderly, the very young, or people who have a weakened immune system (1).
The Flu Season
The flu season in the U.S. can begin as early as October, but usually does not get into full swing until December. The season generally reaches its peak in February and ends in March (2). In the southern hemisphere, however, where winter comes during our summer months, the flu season falls between June and September. In other words, wherever there is winter, there is flu (3). In fact, even its name, “influenza” may be a reference to its original Italian name, influenza di freddo, meaning “influence of the cold” (4).
A common misconception is that the flu is caused by cold temperatures. However, the influenza virus is necessary to have the flu, so cold temperatures can only be a contributing factor. In fact, some people have argued that it is not cold temperatures that make the flu more common in the winter. Rather, they attest that the lack of sunlight or the different lifestyles people lead in winter months are the primary contributing factors. Here are the most popular theories about why the flu strikes in winter:
1) During the winter, people spend more time indoors with the windows sealed, so they are more likely to breathe the same air as someone who has the flu and thus contract the virus (3).
2) Days are shorter during the winter, and lack of sunlight leads to low levels of vitamin D and melatonin, both of which require sunlight for their generation. This compromises our immune systems, which in turn decreases ability to fight the virus (3).
3) The influenza virus may survive better in colder, drier climates, and therefore be able to infect more people (3).
The Flu Likes Cold, Dry Weather
For many years, it was impossible to test these hypotheses, since most lab animals do not catch the flu like humans do, and using humans as test subjects for this sort of thing is generally frowned upon. Around 2007, however, a researcher named Dr. Peter Palese found a peculiar comment in an old paper published after the 1918 flu pandemic: the author of the 1919 paper stated that upon the arrival of the flu virus to Camp Cody in New Mexico, the guinea pigs in the lab began to get sick and die (4). Palese tried infecting a few guinea pigs with influenza, and sure enough, the guinea pigs got sick. Importantly, not only did the guinea pigs exhibit flu symptoms when they were inoculated by Palese, but the virus was transmitted from one guinea pig to another (4).
Now that Palese had a model organism, he was able to begin experiments to get to the bottom of the flu season. He decided to first test whether or not the flu is transmitted better in a cold, dry climate than a warm, humid one. To test this, Palese infected batches of guinea pigs and placed them in cages adjacent to uninfected guinea pigs to allow the virus to spread from one cage to the other. The pairs of guinea pig cages were kept at varying temperatures (41°F, 68°F, and 86°F) and humidity (20%-80%). Palese found that the virus was transmitted better at low temperatures and low humidity than at high temperatures and high humidity (see Figure 1).
Figure 1 ~ Experimental Setup. Guinea pigs were housed in adjacent cages. Guinea pigs in cage 1 were infected by Palese with influenza. Palese observed how many guinea pigs in cage 2 became infected from the guinea pigs in cage 1 at different temperatures and levels of humidity. B, C) Transmission rates were 100% at low humidity, regardless of temperature. At high humidity, transmission occurred only at the lower temperature.
However, Palese’s initial experiment did not explain why the virus was transmitted best at cooler temperatures and low humidity. Palese tested the immune systems of the animals to find out if the immune system functions poorly at low temperatures and low humidity, but he found no difference in innate immunity among the guinea pigs (5). A paper from the 1960s may provide an alternate explanation. The study tested the survival time of different viruses (i.e. the amount of time the virus remains viable and capable of causing disease) at contrasting temperatures and levels of humidity. The results from the study suggest that influenza actually survives longer at low humidity and low temperatures. At 43°F with very low humidity, most of the virus was able to survive more than 23 hours, whereas at high humidity and a temperature of 90°F, survival was diminished at even one hour into incubation (3).
The data from these studies are supported by a third study that reports higher numbers of flu infections the month after a very dry period (6). In case you’re wondering, this is only the case in places that experience winter. In warmer climates, oddly enough, flu infection rates are correlated most closely with high humidity and lots of rain (6). Unfortunately, not much research has been done to explain these contradictory results, so it’s unclear why the flu behaves so differently in disparate environments. This emphasizes the need for continued influenza research. Therefore, we can conclude that, at least in regions that have a winter season, the influenza virus survives longer in cold, dry air, so it has a greater chance of infecting another person.
Although other factors probably contribute as well, the main reason we have a flu season may simply be that the influenza virus is happier in cold, dry weather and thus better able to invade our bodies. So, as the temperature and humidity keep dropping, your best bet for warding off this nasty bug is to get your flu shot ASAP, stay warm, and invest in a humidifier.
Hannah Foster is a PhD candidate in the Molecules, Cells, and Organisms program at Harvard University.
For more information about the flu, check out this video:
References
1) Medical News Today. What is flu? What is influenza? What are the symptoms of flu? < >[2 November, 2014]
2) Centers for Disease Control. The Flu Season. <http://www.cdc.gov/flu/about/season/flu-season.htm> [2 November, 2014]
3) Elert, E. 2013. FYI: Why is There a Winter Flu Season? Popular Science. <http://www.popsci.com/science/article/2013-01/fyi-why-winter-flu-season> [2 November, 2014]
4) Kolata, G. 2007. Study Shows Why the Flu Likes Winter. New York Times. <http://www.nytimes.com/2007/12/05/health/research/05flu.html?_r=1&> [2 November, 2014]
5) Lowen, A.C., S. Mubareka, J. Steel, and P. Palese. 2007. Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature. PLOS Pathogens. 3(10):e151.
6) Roos, R. 2013. Study: Flu likes weather cold and dry or humid and rainy. University of Minnesota Center for Infectious Disease Research and Policy. <http://www.cidrap.umn.edu/news-perspective/2013/03/study-flu-likes-weather-cold-and-dry-or-humid-and-rainy> [14 November, 2014]
Have we got evidence that the infection rate is seasonal or is it the severity of symptoms (and deaths) that’s actually peaking in the winter? We could randomly test the population all year round to find out if there really is a seasonal variation in infections but it doesn’t really matter. What matters, and is undeniable, is that more people die of flu in the Winter. But why should we assume that the weather is affecting the virus when it’s likely affecting the hosts (us) to a greater degree? With Covid-19 we are at last randomly testing the population and it turns out many people have it with virtually no symptoms, so it may be that people in Summer have flu just as much but display far less severe symptoms – or none at all. Now that the stats show that darker skinned people in northern latitudes are more likely to die from Covid-19, I’m convinced that sunlight and Vitamin D are paramount in explaining seasonal variation of deaths from corona viruses.
Maybe there is a correlation between flu season and the NASA research (link at bottom) showing how wind moves dust from the Sahara to the Amazon mainly in Boreal Winter and Spring (i.e. maybe virus particles are carried by increased wind during that time, enabling a more rapid spread across greater distance than when relying purely on host mobility ).
The NASA study also shows that the amount of dust transported has an inverse correlation to higher rainfall in the previous year – which seems to be in line with the third study you mention where flu infection is increased following dryer months in Winter climates.
One possible explanation for increased flu infections after rain or high humidity in warm climates could be that air humidity affects the weight of dust particles (and so I assume virus particles as well) due to increased condensation and causes them to lose altitude, which could bring previously “out of range” particles into contact with humans, increasing infection. So in cold climates dryer air lightens particles, allowing the wind to carry them farther, which increases the spread of infection; and in warmer climates, humidity lowers high altitude particles, bringing them into contact with humans, and increasing the number of infections.
Another factor could be that in Winter the average UV index is 1/6th of what it is in the Summer (using New York as an example), so maybe a lower percentage of virus particles survive inter host travel in Summer?
Anyway, sorry, just some random thoughts. Link to NASA article below. Thanks. Nick.
https://www.nasa.gov/content/goddard/nasa-satellite-reveals-how-much-saharan-dust-feeds-amazon-s-plants
So where do people get infected? Outdoors or indoors? It would need the people to get in touch with each others outdoors, because even in winter the temperatures are kept high indoors. So using the public transport, go shopping etc should not lead to more flu infections, if the temperatures are high enough and given it is the direct influence of the temperature to the virus, no?
Please comment on another (apparently not so popular theory) which I believe may agree with the guinea pig experiments as well: That cold dry weather dries out the mucinous surfaces in the nose leaving the virus with better access to infectable cells either due to cracks or due to lack of a sheet of protective mucus. Thank you.
From the ridiculous to the asinine is but a single step.
I believe the reason that flu spreads more in dry air and colder temperatures (winter months) is because it’s easier for the influenza droplets to spread in dry air. When the air is humid it will way down the droplets and make it fall to the ground sooner. And to the person that never got sick from a virus, even after being exposed, you’re propably the carrier. My daughter of 14 never got a childhood disease but her brother did and he coughed in her face.
I always believed that the humidity keeps some of the air born virus down to the ground. I’ve always noticed on humid days we always get less dust particles float into our paint jobs at our auto body shop. In fact, some of the older painters used to hose down their paint booths to create humidity on dry days.
You guys are dancing so close to the fire that you are burning. Consider this and if you agree, help get the word out.
WHAT CAUSES AND STOPS THE SPREAD OF VIRUSES
In 2007 Dr. Palese, Professor and Chair of Microbiology of the Icahn Medical School of Mt. Sinai, NY, discovered that viruses transmit 100% in low humidity. Low humidity is created in the winter months when our furnace heat dries our inside air. This heat dries our skin as well as our nasal passages, making them a perfect breeding ground for viruses. If you keep your nasal passages moist, you decrease the chance of infections attaching and multiplying in your nose and moving into your lungs. Monitor it closely during the winter months.
How I Have Stopped My Viruses
Due to my weakened immune system, I have studied various illnesses for many years and have found 14 activities that have effectively stopped my viruses. They are most effective if implemented at the first sign of symptoms or upon exposure. I can usually stop viruses by doing just a few of the activities below. Using several of the suggestions in 1-7 and all of the steps in 9-14 might prevent more severe viruses. Start them at the first sign of symptoms.
Keep the Nasal Passages Moist (1-7)
1. Take a brisk 20-min. walk outside, morning, and night.
2. Open your bedroom windows to get fresh air in.
3. Set your thermostat at 55° and dress to go skiing.
4. Sniff water from your clean fingertips.
5. Use a nasal saline spray; it’s not fun, but it works.
6. Use a neti pot or a Navage.
7. Run a humidifier.
Stimulate Your Immune System (8-14)
8. Gargle with a small amt. of Listerine, every hour on the hour of the first 4 days of symptoms.
On the half hour, eat or drink small portions of the following items.
9. hot tea
10. orange, pineapple, grapefruit, or tomato pieces
11. orange, pineapple, grapefruit, or tomato juices
12. chicken noodle soup, canned is fine
13. frozen lemonade concentrate into hot water & honey
14. cinnamon twirls or rolls without the icing.
Keep thinking!