Freshly Baked Science
Why Do We Catch Colds in Winter?
14th March 2019
Winter time is notorious for sniffles, coughs, and sneezes, which causes most of us to spend at least a week of the winter curled up next to a tissue box, feeling sorry for ourselves. The viruses that cause the common cold and flu can hang around no matter the season, so what is it about winter that sees the number of infections creep up?
Could it be the colder weather? That’s certainly the tale that mums love to use – “Don’t go out without a jacket or you’ll catch a cold!”. Factors such as colder temperatures, lower humidity, and fewer sunshine hours do correlate with flu epidemics, but you may be surprised to learn that seasonal variation in flu infections is also seen in tropical regions, where winter as we know it doesn’t really exist. Scientists believe that there are multiple contributors to the seasonal nature of flu infections, which can be broadly classified into three groups: human contact rate, virus survival, and our own immunity in the winter months.
Contact with other infected humans is a surefire way to pick up a cold or flu – just ask anyone who lives in a house share or works in an office. Once one person gets it, everyone else is left counting down the days until their imminent infection. According to an American database of human activity, people spend on average two hours more time indoors per day when it’s cold outside. This is a relatively small increase, but there are also other factors to consider that see an increase in human contact during winter. For example, more people are likely to be using public transport than cycling or walking to work, increasing the exposure to potentially infected people. Flu epidemics tend to go hand-in-hand with the academic school year as well, another instance where many people are put into close proximity with one another. However, there are some inconsistencies with this theory. For example, in tropical regions there can be multiple epidemic seasons within a year that don’t correlate at all with the school year. Furthermore, summer time is associated with crowded events such as music festivals, which don’t set off epidemics either. Although the risk of transmission may be higher with increased human contact during winter, this is likely a small contributor to seasonal epidemics.
This brings us to the second contributor to winter’s peak infections, the weather. However, it’s more likely to be affecting the virus than you. Virus particles can be spread through aerosolized droplets (i.e. when you cough or sneeze) but can also hang about on surfaces and then be passed on by direct contact (i.e. you touch the door handle that someone used after sneezing into their hand). Studies have found that the molecules that make up the outer protective shell of a virus are more stable at low temperatures. This may improve the virus’ chance of survival and be important for airborne transmission, which was also shown (with guinea pigs) to be enhanced at colder temperatures. Along similar lines, virus particles can be inactivated by UV light, which is of course diminished in the winter months. Again, however, how much this contributes to seasonal epidemics is debatable when you consider that we’re spending more time indoors, where sun exposure is redundant, and the temperature is relatively controlled. Nevertheless, viral fitness in winter conditions is a plausible contributor to the seasonal rates of infection.
Finally, and likely a big influence on seasonal infection rates, is our own immunity. A simple but amazing experiment of manually infecting people with influenza virus showed that people are more likely to get flu symptoms from the infection during common epidemic months than at other times of the year. Our immune function can be influenced by the winter weather conditions – for example, cold air causes blood vessels in the nose and lungs to contract, resulting in less blood flow and therefore less supply of immune cells to these tissues. The dry air can also affect the nasal passages and influence the amount of mucus there, which limits the ability to clear out any nasty viruses that may land there. The fewer sunshine hours available during winter and how this may regulate our immune function is an area of great research interest. For example, molecules such as vitamin D and melatonin, whose synthesis is influenced by UV exposure, can influence immune cell function. Another theory suggests that there is an energy trade-off between our immunity and the need for thermoregulation (i.e. keeping our bodies warm!). Research is still needed in this area, but if you are feeling particularly rundown in winter, at least there may be a scientific explanation for it!
So, given the multiple factors at play, what’s the best way to avoid the winter sniffles? Hygiene is super important to prevent the transmission of the virus, so don’t be gross! Wash your hands, especially after coughing or sneezing! If you are sick, stay at home so you don’t pass it on to your friends and colleagues. And finally, wrap up warm, get enough rest, and eat well during these months where your body’s immune system may be struggling a bit – in other words, don’t make life harder for yourself than it already is!