Masks slow down the spread of SARS-CoV-2 by reducing the number of infected people spewing the virus in the environment around them when they cough or talk. Evidence from laboratory experiments, hospitals and entire countries shows that masks work, and the Centers for Disease Control and Prevention recommends face covering for the American public. With all this evidence, mask wear has become the norm in many places.
I’m a doctor of infection and a professor of medicine at the University of California, San Francisco. When governments and workplaces began to recommend wearing masks, my colleagues and I noticed an interesting trend. In places where most people wore masks, those who became infected were dramatically less likely to become seriously ill compared to places with less mask wear.
It seems that people get less sick when they wear a mask.
If you wear a mask – even a cloth mask – you are typically exposed to a lower dose of the coronavirus than if you did not have one. Both recent experiments in animal models using coronavirus and nearly one hundred years of viral research show that lower viral doses usually mean less serious disease.
No mask is perfect, and wearing one can not prevent you from becoming infected. But it can be the difference between a case of COVID-19 sending you to the hospital and a case so mild that you do not even realize you are infected.
AP Photo / Kathy Willens
Exposure dose determines the severity of the disease
When you inhale a respiratory virus, it immediately begins to hijack all the cells that come to the land to make them into virus-producing machines. The immune system tries to stop this process in order to stop the spread of the virus.
The amount of virus you are exposed to – called the viral inoculum, as a dose – has a lot to do with how sick you become. If the exposure dose is very high, the immune response may be overcome. Between the virus that takes over large numbers of cells and the drastic attempt of the immune system to contain the infection, a lot of damage is done to the body and a person can become very ill.
On the other hand, if the initial dose of the virus is small, the immune system is able to contain the virus with less drastic measures. When this happens, the person experiences fewer symptoms, than one.
This concept of viral dose related to pregnancy of diseases exists for almost a century. Many animal studies have shown that the higher the dose of a virus you give an animal, the safer it becomes. In 2015, researchers tested this concept in human volunteers with a nonlet half-flu virus and found the same result. The higher the dose of flu virus given to the volunteers, the safer they became.
In July, researchers published a paper showing that viral dose was related to disease progression in hamsters exposed to the coronavirus. Hamsters given a higher viral dose were sicker than hamsters given a lower dose.
Based on this body of research, it seems very plausible that if you are exposed to SARS-CoV-2, the lower the dose, the less ill you will get.
So what can a person do to reduce the exposure dose?
Masks reduce viral dose
AP Photo / Marcio Jose Sanchez, File
Most researchers and epidemiologists of infectious disease believe that the coronavirus is most commonly spread by droplets in the air and, to a lesser extent, small aerosols. Research shows that both dust and surgical masks can block the majority of particles that can contain SARS-CoV-2. While no mask is perfect, the goal is not to block all the virus, but simply reduce the amount you can inhale. Almost every mask will successfully block some amount.
Laboratory experiments have shown that good cloth masks and surgical masks can block at least 80% of viral particles in your nose and mouth. Those particles and other contaminants will be trapped in the fibers of the mask, so the CDC recommends washing your dust mask after each use, if possible.
The latest piece of experimental evidence shows that masks reduce the viral dose comes from another hamster experiment. Hamsters were divided into an unmasked group and a masked group by placing surgical mask material over the pipes that brought air into the cages of the masked group. Hamsters infected with the coronavirus were placed in boxes next to the masked and unmasked hamsters, and air was pumped from the infected cages into the cage with uninfected hamsters.
As expected, the masked hamsters were less likely to be infected with COVID-19. But when some of the masked hamsters got infected, they had more mild disease than the unmasked hamsters.
AP Photo / Matilde Campodonico
Masks increase rate of asymptomatic cases
In July, the CDC estimates that about 40% of people infected with SARS-CoV-2 are asymptomatic, and a number of other studies have confirmed this number.
However, in places where everyone wears masks, the rate of asymptomatic infection seems much higher. In an outbreak on an Australian cruise ship named Greg Mortimer in late March, the passengers were all given surgical masks and staff were given N95 masks after the first case of COVID-19 was identified. Mask use was apparently very high, and although 128 of the 217 passengers and staff ended up testing positive for the coronavirus, 81% of infected people remained asymptomatic.
Further evidence has emerged from two more recent outbreaks, the first at a seabird processing plant in Oregon and the second at a chicken processing plant in Arkansas. In both places the workers were provided with masks and obliged to wear them at all times. In outbreaks of both plants, almost 95% of infected people were asymptomatic.
There is no doubt that values of universal mask slow down the spread of coronavirus. My colleagues and I believe that evidence from laboratory experiments, case studies such as the outbreak of the cruise ship and food processing plants and long known biological principles make a strong case that masks also protect the wearer.
The goal of every tool to combat this pandemic is to slow down the spread of the virus and save lives. Universal masking will do both.
