Does the Hygiene Hypothesis Explain COVID-19 Cases and Death? A Global Analysis

PURPOSE: At the start of the COVID-19 pandemic there was widespread speculation on the role of the hygiene hypothesis in the incidence and mortality of COVID-19. This study sought to determine if such a correlation exists via a global analysis using many surrogate factors representing the hygiene hypothesis. METHODS & MATERIALS: Publicly available data from 190 countries were gathered. These data included COVID-19 total case numbers and deaths through December 28, 2020; water, sanitation, and hygiene (WaSH) metrics; data on mortality due to various types of air pollution; and other factors such as control of solid waste, emission growth rate of methane and carbon dioxide, and daily adjusted life years lost to unsafe drinking water and sanitation. Using IBM SPSS 27.0, these factors were analyzed using multiple regression analyses to determine the combination of factors most predictive of COVID-19 total cases and deaths. Separate regressions were conducted for the two criterion variables. RESULTS: The analysis revealed positive correlations between two predictor variables: a nation's mortality due to air pollution (MDAP) and control of solid waste (CSW), with COVID-19 total number of cases. The combination of predictors accounted for approximately 28% of the variance in total number of cases. Concerning the number of COVID-19 deaths, 9.6% of the variance was accounted for by MDAP. Findings support previous studies pointing to the likelihood of air pollution as a potential catalyst for COVID-19 spread. CONCLUSION: Decreasing air pollution is an important mitigating strategy for dealing with respiratory viruses, as it correlates with decreased damage to the respiratory system and decreased time for the virus to circulate in denser particles of polluted air. Thus, MDAP is an effective predictor of COVID-19 cases, and to a lesser extent, deaths. The positive correlation with cases and CSW indicates a likelihood that lockdowns throughout the world wreaked havoc on solid waste disposal systems, particularly in nations with prior effective CSW mechanisms. Although the hygiene hypothesis is not supported, findings should encourage societies to implement policies which focus on minimizing air pollution and strengthen systems to CSW and attenuate a descent into another global pandemic.