Infectious diseases are common at different times of the year - respiratory viruses, like a cold or the flu, are more common in winter, while Polio is far more common in summer. It’s also something we see in animals too - even though they don’t share the same winter behaviors as humans, like spending more time indoors - which suggests there are other factors involved.
Respiratory infections decrease in summer
One of the factors used to explain this phenomenon is that respiratory viruses such as COVID-19, influenza or cold viruses, survive better in cold and humid environments, as their structure is more unstable in high temperatures and dry air (1). But this answer doesn’t explain why many diseases are also seasonal in tropical regions where temperature and humidity are constant.
Another explanation is in our immune system, as the composition and function of our immune systems varies with seasonal changes throughout the year (2).
“How our immune system responds to seasonal changes in our environment isn’t fully understood. This remains an important area of research given the importance of our immune system to our health and the future challenge of how we stay healthy in a changing climate.”
Says Tom Hayday, Chief Research Officer at Melio and researcher at the Division of Immunology, Infection and Inflammatory Disease of the King’s College.
Cold seems to have an impact on our immune system and its ability to fight infections, both in cellular (T & B Cells) and humoral (antibody) responses (3).
In addition, cold air decreases the mobility of respiratory cilia, which are structures in the form of small hairs present in much of our respiratory tract and which are responsible for continuously dragging mucus along with dust and pathogens to the nose, expelling them out of the body or into our digestive tract.
However, heat waves can also affect the adaptive response of the immune system
Studies in animals have shown that acute heat stress can impact the ability of the immune system to fight off infection and mount an effective response to vaccination (4). Vaccines have become an essential weapon in our society to fight off many diseases, including those with pandemic potential such as COVID-19. If the same effect happens in humans, exposure to high temperatures for long periods of time may have to be avoided after you’ve been vaccinated.
According to a study from the University of Tokyo (5), heat waves can decrease our ability to defend against diseases such as influenza. Researchers placed healthy adult mice in chambers at 4, 22 and 36 °C and, after 8 days, infected them with the influenza virus. The mice in the 36°C chamber had significantly fewer antibodies and fewer lymphocytes against the influenza virus. This, together with other data from the study, indicates that their adaptive immune response was severely impaired.
Warm nights and sleep
In some cities, summer nights can be very warm, making it difficult to get a good night’s sleep and optimal immune function requires adequate sleep. Studies show that people who don't get quality sleep or enough sleep are more likely to get sick after being exposed to a virus, such as a common cold virus (6).
All the biochemical and physiological mechanisms by which sleep deprivation affects the immune function haven’t been identified and explained, but some are related to changes in the production of cytokines (immune signaling molecules) and to changes in hormones of the circadian rhythm (7). Both cytokines and hormones (such as cortisol and the growth hormone) affect the interaction between the antigen presenting cells and T helper cells, a process needed for the formation of immunological memory.
A recent study discovered that sleep deprivation can affect the adhesion of T cells to virus-infected cells or cancer cells, due to an increase in adrenaline and noradrenaline (8). Furthermore, modest sleep loss is associated with an increased secretion of the proinflammatory cytokines IL-6 and TNF-⍺ (9)
Good sleep must be a priority in our lives, because it not only affects immune function but many other aspects of our health, such as mental, cardiovascular and metabolic health. In fact, an analysis of data from three separate studies suggests that sleeping five or fewer hours per night may increase mortality risk by as much as 15 percent (10).
Sunscreens and vitamin D
Vitamin D's main function is to regulate the amount of calcium and phosphate needed to keep bones and teeth healthy and maintain strong muscles. But vitamin D is also essential for our immune function and low vitamin D levels can increase the risk of infection of respiratory diseases (including COVID-19) as well as increase autoimmunity (when the immune system ‘attacks’ the body).
Our bodies make vitamin D when our skin is exposed to sunlight, but due to the modern lifestyle (spending too much time indoors and covering our skin), vitamin D deficiency is very common even in sunny countries such as Spain. According to the Spanish Society of Endocrinology this deficit affects 40% of people under 65%.
With this in mind, many people may wonder if sunscreen can block vitamin D synthesis. The truth is that very high SPF sunscreen can indeed block vitamin D synthesis, however some epidemiological studies have found that the use of sunscreen doesn’t decrease the values of vitamin D in your blood (11, 12). This was attributed in part to the lack of total skin cover at all times.
So in practice, unless you apply sunscreen generously, and religiously wear hats and SPF-protected clothing when exposed to sunlight at all times, you shouldn’t worry about vitamin D deficiency in summer. You shouldn’t endanger your skin to obtain the optimal dose of vitamin D.
The exact time of sun exposure needed to meet daily requirements is unknown, because it depends on many factors such as your type of skin, the hour of the day, the season of the year and the latitude you are in. In general, it is considered enough to expose our unprotected hands, forearms and legs for 5-10 min at midday or 30 min in the morning or evening.
The study of the immune system
"The immune system is an incredibly complex network of specialized cell types that work together to fight off infection. Understanding how these cells operate has provided a huge amount of insight into the immune system as it relates to disease, but this requires highly specialized research techniques. "
Says Dr Adam Laing, Chief Scientific Officer at Melio and researcher in the Department of Immunobiology at the King's College London. He also talks about how we will obtain much personalized and detailed information about our immune system in the future:
"As the technology needed to measure the complexity of the immune system improves, we can begin to think about immune health on an individual basis - how an individual's immune system is shaped by their environment and how this might impact the ability of their immune system to function properly”
Salamanca-Fernández, E., Rodríguez, M., Sánchez, M.J. (2021). Influencia de la temperatura ambiental y la contaminación en la transmisión del SARS-COV-2. Rev Esp Salud Pública. 2021; Vol. 95: 20 de enero e1-8.
Wyse, C., O’Malley, G., Coogan, A. N., McConkey, S. & Smith, D. J. Seasonal and daytime variation in multiple immune parameters in humans: Evidence from 329,261 participants of the UK Biobank cohort. iScience 24, 102255 (2021).
Brenner, I. K. et al. Immune changes in humans during cold exposure: effects of prior heating and exercise. Journal of Applied Physiology 87, 699–710 (1999).
Moriyama M and T Ichinohe. High ambient temperature dampens adaptive immune responses to influenza A virus infection. PNAS, 2019 DOI: 10.1073/pnas.1815029116
Hirakawa, R. et al. Heat Stress Causes Immune Abnormalities via Massive Damage to Effect Proliferation and Differentiation of Lymphocytes in Broiler Chickens. Frontiers in Veterinary Science 7, (2020).
Cohen, S., Doyle, W. J., Alper, C. M., Janicki-Deverts, D. & Turner, R. B. Sleep Habits and Susceptibility to the Common Cold. Archives of Internal Medicine 169, 62 (2009).
Besedovsky, L., Lange, T. & Born, J. Sleep and immune function. Pflügers Archiv - European Journal of Physiology 463, 121–137 (2011).
Dimitrov, S. et al. Gαs-coupled receptor signaling and sleep regulate integrin activation of human antigen-specific T cells. Journal of Experimental Medicine 216, 517–526 (2019).
Vgontzas, A. N. et al. Adverse Effects of Modest Sleep Restriction on Sleepiness, Performance, and Inflammatory Cytokines. The Journal of Clinical Endocrinology & Metabolism 89, 2119–2126 (2004).
Sleep Disorders and Sleep Deprivation. (National Academies Press, 2006). doi:10.17226/11617.
Neale, R. E. et al. The effect of sunscreen on vitamin D: a review. British Journal of Dermatology 181, 907–915 (2019).
Marks R;Foley PA;Jolley D;Knight KR;Harrison J;Thompson SC. The effect of regular sunscreen use on vitamin D levels in an Australian population. Results of a randomized controlled trial. Archives of dermatology 131, (2017).