Schoolgirl wearing a face mask while washing her hands.
To stop the spread of Coronavirus, the public needs to carry out several physical interventions at the same time. And while the media focuses on the culture war over wearing face masks, we must not forget another intervention that science suggests maybe even more important than a mask: clean hands.
Hand hygiene is central to stopping Covid-19 from spreading by contact transmission, which occurs via routes such as touching a contaminated surface and then your face. Since around 1850, when microbiologists began developing the modern germ theory of disease and doctors started washing their hands, we've know that practicing proper hygiene helps prevent microbes from transmitting infectious diseases from one person to another.
But while there's plenty of research on how good hygiene blocks the spread of respiratory viruses generally, there's relatively little knowledge of how well it works against the SARS-CoV-2 coronavirus specifically.
As a consequence, recommendations from authorities like the World Health Organization and Centres for Disease Control are mainly based on extrapolating from other viruses with a similar structure, especially a fatty envelope that surrounds certain viruses. That envelope is studded with the proteins used to break into cells, and the logic goes that if an intervention is effective against another 'enveloped virus' — influenza, say — then the same should apply to novel coronaviruses.
There are hundreds of studies on interventions that might interrupt or reduce the spread of respiratory viruses, but their results sometimes contradict each other. And when there's no agreement, scientists will perform a systematic review and collect all the available research in order to analyze the quality of work then reach a consensus. That's what was done in the 2010 Cochrane review, led by the Centre for Evidence-Based Medicine at Oxford University. Based on 67 studies, the reviewers found that hand hygiene helps stop the spread of viruses, particularly around young children — probably because kids are less hygienic.
The 2010 review wasn't conclusive, however, as it didn't identify enough studies that compared the intervention with a control. Such experiments allow reviewers to perform a 'meta-analysis' that combines data from multiple trials then offer a conclusion. An as-yet unpublished update to the Cochrane review achieved just that, combining 15 trials involving both adults and children. Those trials weren't carried out in a lab, but took place in homes, offices and classrooms — real-world settings where infections are commonly transmitted.
According to the new review, hand hygiene led to a 16% drop in the number of participants with an acute respiratory illness (ARI) and 36% relative reduction in an associated outcome: people being absent from work or school. The reviewers concluded that "the modest evidence for reducing the burden of ARIs, and related absenteeism, justifies reinforcing the standard recommendation for hand hygiene measures to reduce the spread of respiratory viruses."
Although the 2020 review confirms the intervention's efficacy in limiting viral transmission, it's not specific to Coronavirus. A direct link to SARS-CoV-2 is supported by one study from a Covid-19 hospital in Wuhan, China, however: from a statistical analysis of several risk factors associated with transmitting the virus, researchers found that poor hand hygiene was a major factor, raising the relative risk of infection by around 3%.
The Chinese study also revealed that the higher Covid-19 risk remained even when healthcare workers wore full personal protective equipment (PPE), which suggests that hand hygiene is more important than wearing a face mask. The 2020 review also didn't find much added benefit to wearing a mask along with good hygiene.
Anti-maskers might interpret such findings to mean that masks are worthless, but that would be wrong because the variation in results among studies was too large to draw any strong conclusions. Masks probably do help block viral transmission, but we won't know exactly how effective they are until we have more data.
Employing only a single intervention — such as masks or handwashing — allows an infectious disease to spread because not everyone will follow the recommended guidelines and so infected people slip through the 'holes' in that intervention. When multiple interventions are used simultaneously, however, it's like stacking several slices of Swiss cheese: the more slices you add, the less likely it is that two holes will overlap and let the disease pass every intervention.
While this 'Swiss cheese model' has traditionally been used in medical error reduction, it's relevant to reducing Covid-19 transmission. Regardless of the relative importance of various interventions, we should employ several strategies to stop the spread of Coronavirus.