States with better sanitation more at risk to Covid? Theory needs testing

Studies and data indicate that the hygiene hypothesis may be at play, but experts say that there are more factors influencing deaths

David P Strachan, in his 1989 seminal paper “Hay fever, hygiene and household size”, proposed that lower incidence of infection in early childhood could be an explanation for the rise in allergic diseases such as hay fever. Popularly called the hygiene hypothesis, Strachan’s formulations have since been tested for various diseases and infections.

Hygiene hypothesis is popular again as developing countries and low-income economies have shown more resilience to SARS-Cov-2—the virus that causes Covid-19--than the developed world. Data emanating from the economies suggests this much. The developed world has witnessed more deaths per million than the developing economies.

Researchers, at the start of the Covid-19 pandemic, feared that countries like India would witness millions of deaths, given the size of the population and the density, but the developed economies have had a higher death rate.

Studies published in various journals suggest as much. A paper by Shravan Sehrawat and Barry Rouse argues “that the hygiene hypothesis may apply to Covid-19 susceptibility and also that residence in low hygienic conditions acts to train innate immune defences to minimise the severity of infection.”

Data from Our World in Data shows that while the global average deaths per million are 365, Europe and North America have a much higher incidence. In North America, there have been 1,370 deaths per million until now, and in the EU, this ratio has crossed 1,400. In contrast, Africa has registered just 85 deaths per million, and Asia has registered 93 deaths per million.

 

The UK and the US have witnessed 1,872 and 1,676 deaths per million, while India has just recorded 120 deaths per million and Bangladesh has recorded 56 deaths.

 

A study for Indian states isn’t available but juxtaposing data from the National Family Health Survey-5 on sanitation with deaths per million across states suggests that states that rank higher on sanitation are also suffering from a higher incidence of deaths. So, among states with a sizeable population, those which had over 80 per cent of the people living with better sanitation averaged 267 deaths per million. In contrast, states with 50-70 per cent population residing with improved sanitation recorded just 49 deaths per million.



 

There is also a clear rich-poor and urban, rural divide. Urban centres have had a higher rate of infection and deaths despite having better facilities, with 80 per cent of cases concentrated in these areas, whereas rural areas account for less than 20 per cent of the country’s total death and infection burden.

Increased incidence of testing and better reporting can be one reason for this phenomenon. “States that are lower on the sanitation status also have poorer data reporting systems, so it is hard to say whether the hygiene hypothesis holds up. We need deeper immunobiological insights,” said Gagandeep Kang, a professor at The Wellcome Trust Research Laboratory, Division of Gastrointestinal Sciences, Christian Medical College, Vellore.

However, Dr Srinath Reddy, president, Public Health Foundation of India (PHFI), believes that there can be other factors at play. “Observed death rates have to be adjusted for several factors before comparing different population groups from diverse geographies. States with poorer sanitation have several features which differentiate them from states with better sanitation: lower economic development and urbanisation; lower overall life expectancy and smaller proportion of persons over 60 years of age; lower levels of co-morbidities like hypertension, diabetes and heart disease; lower industrial and automobile emissions causing air pollution. These are all among the factors which influence mortality in Covid-19,” he says.

Thus, drawing a direct causality may be erroneous.

“To attribute observed differences only or mostly to different hygienic conditions, without adjusting for other determinants of death risk would be scientifically incorrect. While it may be that exposure to other microbes in unhygienic conditions may help people to develop non-specific immunity, it is speculative rather than proven in case of Covid-19,” Dr Reddy highlights.

Age is a deterministic factor, as the countries witnessing more deaths also have an older demographic than India or Bangladesh.

With case count increasing again as India braces for a second wave, more evidence may emerge. India recorded over 1 lakh infections on Sunday, for the first time since the start of the pandemic.