“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]
Bruh so many conspiracy theories. The vaccine does not give you the flu as @Jim said, and pharmaceutical companies do not make much money off flu jabs.
Bill duquaine hit it on the head with his statement that the cold allows the virus to enter our bodies when we are colder.
This is a proven fact. The old saying that cold doesn’t cause a cold is not true. The rhinovirus or another virus may be laying dormant in your nose. Once the cold air lowers your internal nasal temperature you get signs and symptoms that you did not have before your body got cold. Your immune system could not defend the virus as well as when you were warm as the cold dry air led to vasoconstriction of the internal nose. Now, your immune system is impaired and the virus enters your body and replicates.
The United States Army did a study on this to prove this theory.
So, add this to the rationale in this article to understand why warm humid weather should give our government’s leaders credit for the control of the corona virus.
Now, we have several factual reasons for viruses like influenza to be more contagious in the winter. Most likely the corona virus will follow this seasonal pattern. As the northern part of Italy and Iraq were much more affected than the southern parts of these countries.
Has anyone projected what effect COVID-19 shutdowns would have on this year’s and next year’s flu seasons? Is it reasonable to think what will stop COVID will also stop flu, and prevent it from finding enough hosts to infect and mutate in between our flu seasons? I’m hoping there’s a silver lining here.
This article from Australia gives some hope.
https://www.abc.net.au/news/2020-07-23/coronavirus-restrictions-cause-flu-cases-to-drop-australia/12480190
Has anyone tried to model what effect social isolation imposed for COVID-19 would have on this year and next year’s flu seasons?
I’m assuming it requires a certain number of individuals infected to be passing it on from hemisphere to hemisphere in order to mutate and attack us again next year. That would surely be interrupted by widespread shutdowns and social isolation going on, one would think….
Funny the truth is always obscured. Everybody hates the truth. why
If a person’s comment has no logic to it, just don’t respond. Then maybe, the trolls will thin out & perhaps disappear.
This makes no common sense that a virus needs a warm, moist atmosphere to survive. But winter is when people get the flu virus. Spring and Summer bring warm temperatures and high humidity in the Southern States. So, the flu virus would be more prevalent during these times if the original premise was true.
The fact that this article gives three “theories” tells me that something is fishy when the science doesn’t agree with the facts of what we experience. I firmly believe it is the medical science researchers ommunity that invents these new viruses every year and turns them loose on the nations for enormous profits for doctors, pharmacies, pharma mfgs., retailers and of course the million dollar babies, the hospitals!
Where does Influenza go during the summer? Does it hide in cold places during the summer? Is it possible for us to kill this disease for good? Why do we have a cure for Ebola but not for Influenza?
I didn’t see any comments that mention depression which may weaken your immune system and is more common in the winter.
After the flu season where does this shit go and how does it live until the next flu season? Supposedly it is only to live on a serves for few days!