[ExI] Cloth masks do protect the wearer

Dave Sill sparge at gmail.com
Mon Aug 31 13:34:33 UTC 2020


Masks slow the spread of SARS-CoV-2 by reducing how much infected people
spray the virus into the environment around them when they cough or talk.
Evidence from laboratory experiments, hospitals and whole countries show
that masks work, and the Centers for Disease Control and Prevention
recommends face coverings for the U.S. public. With all this evidence, mask
wearing has become the norm in many places.

I am an infectious disease doctor and a professor of medicine at the
University of California, San Francisco. As governments and workplaces
began to recommend or mandate mask wearing, my colleagues and I noticed an
interesting trend. In places where most people wore masks, those who did
get infected seemed dramatically less likely to get severely ill compared
to places with less mask-wearing.

It seems people get less sick if they wear a mask.

When you wear a mask – even a cloth mask – you typically are exposed to a
lower dose of the coronavirus than if you didn’t. Both recent experiments
in animal models using coronavirus and nearly a hundred years of viral
research show that lower viral doses usually means less severe disease.

No mask is perfect, and wearing one might not prevent you from getting
infected. But it might be the difference between a case of COVID-19 that
sends you to the hospital and a case so mild you don’t even realize you’re

Exposure dose determines severity of disease

When you breathe in a respiratory virus, it immediately begins hijacking
any cells it lands near to turn them into virus production machines. The
immune system tries to stop this process to halt the spread of the virus.

The amount of virus that you’re exposed to – called the viral inoculum, or
dose – has a lot to do with how sick you get. If the exposure dose is very
high, the immune response can become overwhelmed. Between the virus taking
over huge numbers of cells and the immune system’s drastic efforts to
contain the infection, a lot of damage is done to the body and a person can
become very sick.

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. If this
happens, the person experiences fewer symptoms, if any.

This concept of viral dose being related to disease severity has been
around for almost a century. Many animal studies have shown that the higher
the dose of a virus you give an animal, the more sick it becomes. In 2015,
researchers tested this concept in human volunteers using a nonlethal flu
virus and found the same result. The higher the flu virus dose given to the
volunteers, the sicker they became.

In July, researchers published a paper showing that viral dose was related
to disease severity in hamsters exposed to the coronavirus. Hamsters who
were given a higher viral dose got more sick than hamsters given a lower

Based on this body of research, it seems very likely that if you are
exposed to SARS-CoV-2, the lower the dose, the less sick you will get.

So what can a person do to lower the exposure dose?

Masks reduce viral dose

Most infectious disease researchers and epidemiologists believe that the
coronavirus is mostly spread by airborne droplets and, to a lesser extent,
tiny aerosols. Research shows that both cloth and surgical masks can block
the majority of particles that could contain SARS-CoV-2. While no mask is
perfect, the goal is not to block all of the virus, but simply reduce the
amount that you might inhale. Almost any mask will successfully block some

Laboratory experiments have shown that good cloth masks and surgical masks
could block at least 80% of viral particles from entering your nose and
mouth. Those particles and other contaminants will get trapped in the
fibers of the mask, so the CDC recommends washing your cloth mask after
each use if possible.

The final piece of experimental evidence showing that masks reduce 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 cages next to the masked and
unmasked hamsters, and air was pumped from the infected cages into the
cages with uninfected hamsters.

As expected, the masked hamsters were less likely to get infected with
COVID-19. But when some of the masked hamsters did get infected, they had
more mild disease than the unmasked hamsters.

Masks increase rate of asymptomatic cases

In July, the CDC estimated that around 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 to be much higher. In an outbreak on an Australian cruise
ship called the Greg Mortimer in late March, the passengers were all given
surgical masks and the staff were given N95 masks after the first case of
COVID-19 was identified. Mask usage was apparently very high, and even
though 128 of the 217 passengers and staff eventually tested positive for
the coronavirus, 81% of the infected people remained asymptomatic.

Further evidence has come from two more recent outbreaks, the first at a
seafood processing plant in Oregon and the second at a chicken processing
plant in Arkansas. In both places, the workers were provided masks and
required to wear them at all times. In the outbreaks from both plants,
nearly 95% of infected people were asymptomatic.

There is no doubt that universal mask wearing slows the spread of the
coronavirus. My colleagues and I believe that evidence from laboratory
experiments, case studies like the cruise ship and food processing plant
outbreaks and long-known biological principles make a strong case that
masks protect the wearer too.

The goal of any tool to fight this pandemic is to slow the spread of the
virus and save lives. Universal masking will do both.

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