Hygiene hypothesis’ tested: Poorer water, sanitation associated with fewer COVID-19 deaths

Reviewed by Emily Henderson, B.Sc.Nov 2 2020

Indian researchers from the formidable Council of Scientific and Industrial Research (CSIR) and two other top-notch institutes have put to test the "hygiene hypothesis" with a study suggesting that areas with high prevalence of infectious diseases are likely to see fewer COVID-19 deaths.

They cite the fact that Bihar state, notorious for the worst human development indices in this country of 1.3 billion people, have so far shown a COVID death rate of 0.5 per cent, which is about third of the national average.

Published 19 October in MedRxiv, the study awaits peer review but states boldly: "Paradoxically, better sanitation leads to poorer 'immune training' and thus could be leading to higher deaths per million."

While several developmental parameters were involved in the study most particularly it found that the poorer the water and sanitation indices, the lower the deaths per million.

That might seem to be a blow for the hygiene hypothesis, which holds that as the incidence of infections decreases in developed and developing countries there is a corresponding rise in allergies and autoimmune diseases.

However, the study hastens to warn against pursuing weak hygiene as a strategy in dealing with COVID-19 or other diseases. "Although we provide a possible explanation based on sanitation practices on the CFR (death rate) differences among economically stronger and weaker countries, this should not be inferred as our advocating a move towards weaker hygiene practices for handling future pandemics."

So, what are the researchers pushing? They are looking at new possibilities for "immune training" and at microbiome therapies that may supplement conventional hygiene and sanitation practices.

Earlier studies have also suggested that exposure to pathogens boost the immune system against infections, but this is still to stand up to close scientific scrutiny. Indeed, research in Sub-Saharan Africa, again awaiting peer-review, associates higher COVID-19 death rates with poor access to WASH, acronym for Water, Sanitation and Hygiene.

So, anyone seeking comfort in India's situation of poor WASH indices is doomed to disappointment. India has undertaken a drive to eliminate open defecation and other practices that are linked to a high burden of water-borne infections that are linked to stunting and avoidable infections.

A rapid 'review' by a peer, Manu Raj, professor and senior consultant at the Amrita Institute of Medical Sciences and Research Centre, Kochi, Kerala, holds that the study has too many assumptions. "I'm sure the second wave will destroy all their conclusions and new assumptions will come into play," Raj tells SciDev.Net.

"The biggest hurdle for such data is that poor countries don't have accuracy for variables that we use in the model. They are just some assumptions while in the rich countries, these are more accurate," says Raj, pointing to "differential reporting of deaths from countries like China, Iran, Russia, Turkey and many Latin American countries."

The corona scene is highly volatile and most early assumptions are gone. So, I would just say, looks interesting but we will have a second look post second wave."

Manu Raj, Professor and Senior Consultant, Amrita Institute of Medical Sciences and Research Centre, Kochi, Kerala