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# Discussion

The severity of COVID-19 disease caused by SARS-CoV-2 is highly variable with some individuals and populations being more vulnerable to complications than others. Using information on known factors for probability of severe disease, and mapping it to available data on such factors from the DHS, here we estimate and map this variation in probability of severe disease across Zimbabwe. Results show that the probability of severe disease varies substantially with some districts having a probability of severe disease nearly 30% higher than others. Furthermore, we show that there is a relationship between handwashing and respiratory illness, a potential risk factor for severe disease, and that improving access to handwashing across the country could reduce the probability of severe disease.

From a policy perspective, this study highlights a potential added benefit of improvements to WASH facilities in addition to the already known numerous other benefits improved WASH brings. Hand washing is also an important intervention for gastro-intestinal and worm infections <mark>(Ejemot et al., 2015; Bieri et al., 2013; Aiello et al., 2008; Ejemot et al., 2008; Rabie & Curtis, 2006), in addition to the relationships highlighted here for reducing transmission of respiratory infections. Taken together, this study adds another perspective on why handwashing access, particularly in developing countries, can continue to be considered as an important tool in the fight against COVID-19. Other studies have also shown it to be highly cost-effective in prevention of the spread of the virus (</mark> Bagepally _et al._ , 2021 <mark>), an important consideration in light of the considerable economic and societal costs of the pandemic, and associated restrictions (</mark> World Bank, 2020; Vagliasindi, 2021 <mark>). Indeed, investments in WASH are likely to lead to substantial economic returns in the absence of COVID-19. The pandemic should make this investment all the more urgent.</mark> While the link between handwashing and probability of severe COVID-19 is plausible, there are a number of important limitations and assumptions made as part of this study that deserve discussion. First, we assumed that the prevalence of respiratory illness symptoms in children is representative of the prevalence of illness in the wider population. In reality, rates of symptoms and disease may vary between age-groups. Furthermore, many types of respiratory illness may not be severe enough to constitute being a risk factor for severe COVID-19.

Second, our results rely on the strong relationship between handwashing and respiratory illness risk based on previous studies, the causality of which has been indicated in the literature reviewed in the introduction (Hendley, Wenzel and Gwaltney, 1973; Ansari _et al._ , 1991; Luby _et al._ , 2005). However, this model does not take into account additional factors affecting respiratory illness risk e.g. indoor pollution (Smith _et al._ , 2000; Mocumbi _et al._ , 2019). Further to this point, out of necessity, these analyses are conducted at district level using odds ratios obtained from analysis at the individual level which is not representative of the full population. While we do not expect this to impact the main conclusions of the study, it raises the possibility of the atomistic fallacy.

Third, the comorbidities and risk factors for severe disease included in this study are not exhaustive and are limited to those for which data are available. Other factors that may pose a risk for severe disease, include TB, malaria or hypertension (or yet to be discovered others). Further to this point, we only consider prevalence of _any_ comorbidity as a risk factor. In reality, having more than one