Nearly 40 years ago, David P Strachan published a study in the British Medical Journal which discussed data correlating the onset of childhood allergies and household size and cleanliness in British children (Strachan 1998). This so called “hygiene hypothesis” found an interesting trend: on average, children born to large households did not develop as many allergies as those born to smaller households. Additionally, children born to wealthier families, where cleanliness was likely better, experienced more allergies (Strachan 1998). Many years later, scientists shifted their focus from allergies to diseases in order to expand on the idea. The hypothesis was taken further into studies looking at the large-scale onset of autoimmune diseases such are type 1 diabetes and multiple sclerosis in developed countries (Okada et al. 2010). What was drafted from these studies was the idea that certain infections, which developed countries fight with sanitation and modern medicine, protect the body from immune related disorders (Okada et al. 2010). For the sake of my project, I am looking at the hygiene hypothesis in a new light with a disease that has spread rapidly world-wide in the last few years: COVID-19. Using data collected by the World Health Organization and studies published since the onset of the “Coronavirus”, I will be exploring a possible new application of the hygiene hypothesis through the scope of this new viral disease.
In early 2020, a new disease quickly spread through the world infecting millions of people. Since its’ onset, COVID-19 has been recorded as the cause of death for nearly 7 million people world-wide, having a near 1% death rate (World Health Organization 2023). Of the 763 million cases reported throughout the globe, symptoms range from mild fever and flu-like symptoms to respiratory arrest and other deadly outcomes (Sehrawat and Rouse 2020). What is interesting is that of those 7 million deaths caused by the disease, over 5 million of them were reported in developed countries (the Americas and Europe) while the remaining 2 million occurred in lesser-developed areas of the globe (World Health Organization 2023). There seems to be a correlation in the data which indicates that citizens of less developed countries experienced lower infection rates and less severe symptoms as compared to those of developed countries where sanitation and cleanliness are held at a higher esteem. In India in fact, during the first year of the disease, doctors and scientists reported widespread infection rates but very few occurrences of severe symptoms or death in the Indian people, especially amongst those living in poverty (Chakrabarti et al. 2020). This phenomenon was attributed to cross-immunity due to exposure to other viruses previously in life (Chakrabarti et al. 2020). What the doctors in India were alluding to was essentially the hygiene hypothesis! Scientists in Japan have also identified hygiene as a possible risk factor for severe COVID-19 symptoms. In a 2020 report, Japanese scientists identified a difference in the severity of symptoms from COVID-19 in Central Europe and Eastern Asia and came up with four possible hypotheses to explain why cases in Asia were less severe. Less stringent hygiene practices during childhood was one of these hypotheses (Yamamoto and Bauer 2020). From observations in these countries, the idea of cross-immunity was attributed to why people who were exposed to more viruses and bacteria as children are less likely to experience severe COVID symptoms (Chakrabarti et al. 2020). Cross-immunity is the concept that previous illnesses, such as those experienced in childhood, can help boost the immune system for fighting against future infections such as COVID-19 (Chakrabarti et al. 2020).
The implication that cross-immunity and the hygiene hypothesis can play a role in decreasing the severity of symptoms due to COVID-19 and similar diseases opens the door for exploration into applications for other diseases and disorders, especially in developed countries. We must be careful, however, not to overlook the fact that improved hygiene and modern medicine have played a huge role in eradicating many deadly diseases. While cross-immunity shows promise for protecting people against many ailments, there is also a potential danger in allowing exposure to other diseases by reducing hygiene practices. It is notable that following the development of the COVID-19 vaccination, I was unable to find any studies completed continuing to look at COVID-19 and the hygiene hypothesis. This, I believe, is a grave error. Scientists should continue to explore the implications that the hygiene hypothesis might have on our health and the ability to fight future pandemics before the next one takes over the globe.
For my art project I have drawn comparisons of the immunological defense of a citizen in a developed country without cross-immunity and that of a citizen of an underdeveloped country with cross-immunity. In my images we see a COVID-19 virus and little “immunity soldiers” confronting the virus. In the left image, representing an immune system without cross-immunity, the immunity soldiers do not recognize the virus or how they might fight it. Conversely, in the right image, we see the immunity soldiers recognize that this virus is something similar to what they have fought before, and they know how to fight it. This represents how the hygiene hypothesis is predicted to aid in the reduced symptoms of COVID-19 due to a quicker immune response.
Chakrabarti S.S., Kaur U., Singh A., Chakrabarti S., Krishnatreya M., Agrawal B.K., Mittal A., Singh
A., Khanna R., Gambhir I.S., Jin K., Chakrabarti S. 2020. Of cross-immunity, herd immunity, and country-specific plans: Experiences from COVID-19 in India. Aging and Disease. 11: 1339-1344. doi.org/10.14336/AD.2020.1104.
Okada H., Kuhn C., Feillet H., Bach J.F. 2010. The ‘Hygiene Hypothesis’ for autoimmune and
allergic diseases: an update. Clinical and Experimental Immunology. 160: 1-9. doi:10.1111/j.1365-2249.2010.04139.x
Sehrawat S., Rouse B.T. 2020. Does the hygiene hypothesis apply to COVID-19 susceptibility?.
Microbes and Infection. 22: 400-402. doi.org/10.1016/j.micinf.2020.07.002.
Strachan D.P. 1989. Hay fever, hygiene, and household size. The British Medical Journal. 299:
World Health Organization. 2023. COVID-19 Weekly Epidemiological Update: 13 April 2023. 138.
Yamamoto N., Bauer G. 2020. Apparent difference in fatalities between Central Europe and East
Asia due to SARS-COVID-2 and COVID-19: Four hypotheses for possible explanation. Medical Hypotheses. 144: 110160. doi.org/10.1016/j.mehy.2020.110160.