Face-masks can give the immune system a helpful hand

FACE-MASKS HELP reduce the spread of SARS-CoV-2, the virus that causes covid-19. Several studies have reported the more surprising finding that, even if wearers do become infected, their disease is usually milder. Now Joseph Courtney and Ad Bax, a pair of researchers at the National Institutes of Health in Bethesda, Maryland, think they may have worked out why. As they report in the Biophysical Journal, it comes down to humidity, the immune system, and the protective powers of mucus.

At first glance, there may not seem much of a mystery to unravel. Masks cut the number of infectious particles entering the nose and mouth. One might expect, therefore, that severe disease would be less likely. But it is not so. One vital factor which predicts disease severity is how far viral particles make it into a person's lungs. Cheap cotton face-masks struggle to block the smallest aerosols, which are the most likely to penetrate deeply. Dr Courtney and Dr Bax wondered if something else might explain their protective effect.

One of the body’s first lines of defence against airborne pathogens is known as the “mucociliary clearance mechanism”. Sticky mucus in the nose and respiratory tract snares viruses and bacteria. Little hairs known as cilia push the mucus into the throat. From there it is swallowed and...