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Epidemiology

Droplets, Fomites, and COVID-19

The disease COVID-19 is caused by the virus SARS-CoV-2. This is a member of the coronavirus family. These are viruses that can infect humans, other mammals, and birds. Most coronaviruses are rather benign respiratory viruses that account for about 5-10% of annual colds and flu-like illnesses. But occasionally, a coronavirus will successfully jump from one species of mammal to humans. When this happens, humans have never encountered that particular strain of coronavirus and so we have no immunity against it. Recent examples of coronaviruses making this zoonotic jump include SARS (from bats) and MERS (from camels). Although we do not know for sure, it is believed that SARS-CoV-2 originated in bats.

Coronavirus are transmitted the same way as other respiratory viruses are transmitted, by droplets of respiratory secretions. There are two ways that droplets can spread the virus. Either an infected person can cough or sneeze in another person’s face with the result that those respiratory droplets land on the second person’s nose, lips, or face. Or, the the infected person can get those respiratory droplets on their hands by coughing into their hand or touching their nose or mouth with their hand; when the infected person’s hand then touches a surface such as a door knob, light switch, keyboard, or television remote control, then those droplets get passed to those surfaces.

Those surfaces now become fomites. A fomite is any surface that that can serve as a intermediary conduit to get a virus or bacteria from an infected person into another person so that the infection spreads. Fomites are coronaviruses best friends. Certain types of surfaces make better fomites than others. For example, copper surfaces kill viruses whereas stainless steel surfaces do not kill viruses. Smooth surfaces, such as metal of glass, make better fomites than porous surfaces, such as upholstery or fabric, because viruses and bacteria tend to get wedged in between the fibers of fabrics making them harder to be picked up by another person’s hands.

Face masks can help prevent spread of viral-laden droplets from an infected person to an non-infected person, both in the hospital and in public. In the hospital, doctors, nurses, and respiratory therapists wear masks when caring for patients with respiratory viruses such as coronaviruses because they often have to get close to infected patients who can cough or sneeze in their faces. So in the hospital, it is the uninfected people who should wear masks.

In public, it is just the opposite; the infected person should wear the mask and not the average uninfected person. As long as you are maintaining social distancing by standing 6 feet from someone who is infected, even if that person coughs, those droplets are going to fall to the ground by gravity before they reach you. By having the infected person wear the mask, when they cough, they cough into the mask, thus substantially reducing the ability of those respiratory droplets to reach someone else. In fact, when healthy people wear masks when they are out in public, those masks get moist from the humidity of breath and most masks tend to get colonized with bacteria and fungus that can then make that previously healthy person sick.

So, it turns out that most respiratory viruses don’t get transmitted through the air, they get transmitted by fomites. And that means that the most important way to prevent spread of those viruses is by washing one’s hands after touching fomites.

If I could only have one thing to reduce the spread of COVID-19, it would not be a surgical mask or a face shield, or an N-95 mask. What I would want instead is soap and water.

March 30, 2020

By James Allen, MD

I am a Professor of Internal Medicine at the Ohio State University and the Medical Director of Ohio State University East Hospital