Introduction

Acute viral respiratory infections affect people of all ages and represent the most common illnesses globally.¹ They can range in severity, from that of the common cold to COVID-19. Generally, these infections are self-limiting, meaning they go away on their own. Symptoms typically include a cough, fever, sore throat, and runny nose; however, these can vary. For example, a loss of sense of taste or smell can occur in COVID-19 infection and the impacts of this virus can be long-lasting.

Viruses are pathogens (disease-causing microorganisms). They are far smaller than bacteria. Respiratory viruses are typically spread via tiny, infected water droplets in the air (aerosols) that are released by an infected person when they sneeze, cough or talk. These infections can also be spread by direct or indirect physical contact, for example, by touching a contaminated surface or object.²

The arrival of autumn and winter sparks the fear that we are more susceptible to catching a cold or the flu – or something worse. Unfortunately, whilst we enjoy the seasonal festivities associated with the colder months, we are also more at danger of viral infections. Why do we always seem to catch colds at Christmas? This article aims to answer this question.

The four seasons

In tropical zones, located close to the equator, the temperature is generally hot and does not change very much throughout the year. The main seasons are usually characterised by wet and dry periods. 

Temperate climates, however, are located in the areas between the equator and the polar regions. It is in these climates that seasons exist. Temperatures here are typically cooler and can vary a lot more over the course of a year. Seasons are therefore more distinct in these areas. The environmental factors that fluctuate with each season include temperature, humidity, and levels of sunlight. 

How do seasonal behavioural changes increase our risk of viruses?

The first factor that will be addressed is a fairly obvious one that has less to do with viruses and more to do with our own behaviour.

With the temperature plummeting outside in autumn and winter, we find ourselves preferring to stay warm and cosy indoors, rather than socialise outside. Due to this crowding indoors, people are in much closer contact, and thus, the risk of viruses spreading increases.

Again, with the start of school terms in late summer, children congregating in classrooms creates a hub for viral transmission. This is why the incidence of viruses like measles and chickenpox is lower over the summer months, when students are on school holidays with less contact with other children.³

The effect of temperature and humidity on viral respiratory infections

At some point, everyone has been told that they should wrap up warm on wintry days to ward off colds. But does this claim have any scientific basis?

It has in fact been reported that emergency department visits for respiratory diseases are more common when the environmental temperature is colder.⁴

When we breathe in cold air, this affects our airways, making them more sensitive, inflamed, and narrowed. Furthermore, cold temperatures may lead to impaired mucociliary clearance. Mucus is a substance that is produced by the cells that line our respiratory airways, including our noses. The role of mucus is to trap pathogens that enter the body via these respiratory airways and prevent them travelling further. The mucus is then cleared by specialised hair cells (cilia) that beat and waft the mucus away, which is referred to as mucociliary clearance.

Mucociliary clearance is at its most effective at core body temperature (around 36-37˚C) and in high humidity.⁵ When the temperature is cooler, or when the air is dry, this dries up the mucus in our respiratory airways. This means that pathogens can bypass this mucus barrier and enter our bodies more easily.

Some viruses might be able to survive better in colder temperatures and low-humidity conditions. Certain types of coronaviruses have been found to persist in the environment for up to roughly a month at 4˚C. These viruses were more rapidly inactivated at higher temperatures.⁶ This suggests that during autumn and winter, when the environment is colder, viruses could survive outside of the body for longer, ultimately meaning they survive longer on contaminated surfaces, objects, or in the air.

Conditions of low-humidity also encourage viral transmission because they allow viruses, such as influenza, to survive for longer.⁷ Indoor heating systems that reduce humidity in the air might contribute to this effect.⁸ One theory as to why low-humidity favours viral survival proposes that with less water in the air, there are fewer antiviral compounds that kill these viruses. Reactive oxygen species (ROS) are molecules that contain oxygen and that react easily with other molecules. One such ROS is hydrogen peroxide, which is commonly used in household cleaning products and is able to destroy viruses. With increased humidity comes increased ROS concentration, meaning viruses in the air or on surfaces are more likely to be killed.⁹

On the contrary, one study has found that out of five airborne viruses—measles, influenza, SARS-CoV-2 (COVID-19), human rhinovirus, and adenovirus—greater humidity only decreased the risk of infection with influenza.¹⁰ Therefore, the scientific evidence supporting the role of low-humidity and temperature in viral transmission is conflicting.

How does our immune system change over the year?

The functioning of our immune systems exhibits seasonal variations. There is evidence to suggest that this has an important link to the different levels of sunlight that we are exposed to throughout the year.

Tuberculosis used to be treated by exposing infected patients to sunlight.¹¹ We now know that vitamin D, which is important for not only bone strength but also immune system function, is essentially activated by exposure to ultraviolet rays present in sunlight.¹² This may be why the sun’s rays could help our defence against infections and was traditionally used as a treatment.

As the days become shorter in winter, we are exposed to less and less sunlight. Naturally, the levels of vitamin D in the body tend to dwindle as a result, leading to various health risks including rickets (soft and weak bones, causing pain and poor growth in children). This is why the NHS recommends that everyone takes vitamin D supplements in autumn and winter.¹³ This vitamin contributes to normal immune function by promoting the production and maturation of immune cells and the destruction of pathogens, along with a multitude of other effects.

Melatonin is another substance that is key for immune function. This is a hormone that is likewise influenced by exposure to light; its levels are in fact dictated by the body’s circadian rhythm (the natural variations in biological processes over the course of a day). Therefore, much like vitamin D, its levels in the body vary with the changes in light in different seasons. Melatonin impacts immune function by promoting the production and inhibition of certain immune cells. For example, it helps increase the number of fast-acting natural killer cells, but decreases the number of slower-acting cells involved in the long-term immune response, such as CD8+ cells.¹⁴

Examples of seasonal respiratory viruses

There are some specific viruses that are known to display seasonal variations. For example, seasonal influenza, which causes flu, shows patterns of outbreaks in different seasons and climates. Influenza epidemics (an increase in cases of an illness in one geographic area) tend to occur in winter, specifically between November and March in the northern hemisphere.¹⁵

Respiratory syncytial virus (RSV) is another virus commonly seen over the winter months. It is more serious in infants and the elderly as it can make breathing especially difficult in those with small airways and weak immune systems. It is responsible for around 30,000 hospitalisations of children under 5 years in the UK each year.¹⁶ RSV epidemics last for roughly 2 to 5 months around winter time.¹⁷

Coronaviruses, including SARS-CoV-2 (the virus that causes COVID-19), also exhibit seasonality. From November to April, COVID-19 cases spike. However, since SARS-CoV-2 is more transmissible than influenza, the number of COVID-19 cases may be higher than those of other viruses year-round.¹⁸

Summary

• Acute respiratory viruses are incredibly widespread, causing the most common types of illnesses globally. We are more likely to catch these viruses in autumn and winter
• Increased indoor crowding during the colder months and being in closer proximity to others means we are more likely to catch viruses from infected people
• Low temperature and humidity can lead to the mucus lining of our airways drying out and not effectively protecting us from infection. Viruses also survive longer in colder and drier environments, where fewer natural antiviral molecules are present
• Our immune systems may be worse at fighting off viruses in autumn and winter due to decreased levels of vitamin D and melatonin in the body, ultimately as a result of reduced exposure to sunlight