I have argued that there is a strong case for universal masking to contain the spread of Covid-19. Here I dig a little deeper into the science of why.
The first thing to understand is that there are three primary routes of exposure to SARS-CoV-2, the coronavirus that causes Covid-19: (1) Respiratory droplets greater than 10 microns that are expelled while breathing, talking, sneezing, or coughing; (2) Aerosols that are like droplets, but less than 10 microns in size; and (3) Fomites, i.e. surfaces like doorknobs and light switches that are contaminated through direct touch or through droplets that have landed.
How much each of these contributes to SARS-CoV-2 transmission is not known. Aerosol transmission is likely involved in a relatively small number of exposure events. However, it may have a disproportionate role in the number of individual exposures because aerosols can expose many more people per event.
In public spaces, the major interventions against these routes of exposure are face masks (primarily for droplets, but also for aerosols), ventilation (for aerosols), and hand hygiene (for fomites).
Being in a class, work environment, or living situation with an infected person does not itself cause infection. To become infected, the virus needs to reach epithelial cells in a person’s nose, eyes, airway, or lungs. Physical barriers such as respirators and face masks can prevent this from happening.
A big part of the controversy over face masks is due to the fact that face masks are not a perfect barrier to infection. How effective are they?
Several studies have attempted to answer this question.
One thing generally agreed upon is that N95 masks, or respirator masks, are the most effective of all mask types. They can block up to 99% or more of virus particles, whether aerosolized or carried in large droplets. However, especially early in the pandemic, nationwide shortages of face masks in China, the United States, and elsewhere have led those outside healthcare settings to seek alternative types of masks.
One possible alternative is the surgical mask. The efficacy (virus-blocking capacity) of the surgical mask is estimated to be 70% or better, although various studies have produced a wide range of results. Indeed, although surgical masks vary in thickness and permeability and are only designed to filter large respiratory droplets, they may actually even prevent inhalation of aerosolized virus particles.
One downside of surgical masks is that they are generally a type of single-use mask, and experts confirm that these should, indeed, be discarded after each use. So, what about the reusable cloth mask? Although the estimated efficacy of a cloth mask is less than 30%, wearing one certainly provides more protection than wearing no mask at all. In addition to reducing emission of virus-laden respiratory droplets, even a cloth mask helps one keep hands away from the nose and mouth.
Regardless of type, there is agreement that face masks are an important piece in prevention of community spread. What is critical is that all mask wearers ensure a proper fit, as gaps between the mask and face reduce effectiveness.
Additionally, although masks help to prevent susceptible people from inhaling infectious virus particles, they are even more effective at preventing exhaled particles from reaching other people. Since a large amount of SARS-CoV-2 transmission is by people that don’t know they’re sick, the maximal protective effect of mask wearing is achieved when everyone wears a mask.
Even with a perfect fit, though, masks are only one piece of the puzzle. First, the use of masks will only be significantly effective in preventing transmission if most members of a community wear them. It’s also important that members of a community practice good hand hygiene and proper social distancing.
It is well documented that we are still a long way from herd immunity. But, every bit helps. How much can face masks help? The theory of epidemics can help to figure that out. A well known mathematical result holds that for an idealized population the immunization threshold is given by the formula 1-1/R0 where R0 is the basic reproduction number (the average number of people an infected person will infect in a population that consists entirely of susceptible people). A lot of scientists and the CDC think that R0 for SARS-CoV-2 is around 3. Plugging this value into the formula suggests that the herd immunity threshold for Covid-19 should be around 67%, which is one way various experts have arrived at the level 60% or 70% that is commonly quoted. But, if we are all wearing masks, the reproduction number drops to about 70% of its natural value, or about 2.1. Plugging this value into the formula gives a herd immunity threshold of just 52%. The difference between 52% and 67% is about 50 million infections averted. For this reason alone, it seems to me, we should all wear face masks.