The
evidence has been changing, but we know a lot more now than we did a few months
ago.
By Sara
Chodosh and Claire Maldarelli July 10, 2020
Over the past several
months, the idea of wearing a face covering has been hotly debated. In the
beginning of the pandemic, the general recommendation was that masks should be
reserved for medical professionals only. At the time, this made sense. There was
a major mask shortage, and virologists and epidemiologists didn’t have enough
information about how the coronavirus spread to know whether masks would
prevent the coronavirus from spreading outside of a medical setting.
But a mounting body of
evidence makes it clear that masks do prevent the spread of the
virus—especially the more people wear them properly.
One key factor that sealed
the deal for general mask-wearing was the understanding that a significant
percentage of COVID-19 positive folks don’t show signs of the disease, known as
asymptomatic spreaders. This group includes people who test positive but never
show signs of the disease (an estimated 30-45 percent of people who test positive for the
virus) and those who have tested positive and will later show symptoms but
haven’t yet.
So far, studies suggest
that these asymptomatic people play a notable role in spreading the virus. A
high quality and widely
shared paper published back in April in the journal Nature suggests
that people are most infectious two days before actually showing any symptoms.
Additionally, this past week, a conglomerate of 239 scientists from various
fields of study, from epidemiology to virology to physics, signed an open letter to the World Health Organization
with evidence of the airborne nature of the virus.
“If you have spread from
people who don’t have symptoms, you have to do two things,” explains Monica
Gandhi, an infectious disease researcher at the Division of HIV, Infectious
Diseases, and Global Medicine at UCSF/San Francisco General Hospital. “The
first thing is you have to mask them to prevent them from spreading, and the
second thing is what we did with social distancing and lockdown.” There’s still
a lot of research to be done, but the evidence increasingly points to masks as
one of the most crucial parts of our fight against COVID-19.
Scientists now know that SARS-CoV-2
travels and spreads from one person to another via aerosols, which are droplets
of bodily fluid that carry the virus. Big droplets move by coughing and
sneezing, but these researchers warn that a growing body of evidence suggests
much smaller particles can spread the disease, too. This new evidence makes
wearing a mask even more important.
Prior to the coronavirus
pandemic, most studies done on the effectiveness of masks weren’t robust at
all. Most of them were observational in nature—not the gold standard
double-blind ones, where one group wears masks and the other group doesn’t—and
were done in the hospital setting. That means even these low-quality results
wouldn’t necessarily translate to the general population.
However, over the past few
months, there have been a number of well-done studies that investigated what
prevents the spread of the virus. A meta-analysis, published in The Lancet at the beginning of
June, looked at 172 studies that investigated how COVID-19, SARS, and MERS
spread. The study found that mask wearing was strongly correlated with reduced
risk of viral transmission and that “face mask use could result in a large
reduction in risk of infection with stronger associations with N95 or similar
respirators compared with disposable surgical masks or similar [coverings].”
In a more recent
study published last week in the journal Physics of
Fluids, researchers at Florida Atlantic University used a visualization
technique to demonstrate how a variety of masks helped stop the spread of these
aerosol droplets. The team looked at three types of masks: A single-layer
bandana-style covering, a homemade mask that had two layers of cotton, and a
non-sterile cone-style mask. They chose these three on purpose, says study
co-author Siddhartha Verma, a professor in the department of Ocean and
Mechanical Engineering at Florida Atlantic University because they were the
three that seem to be most often used by the general public. The researchers
also looked at how far aerosols spread with no mask on at all.
In a lab, the team put
masks on mannequins and used a mixture of water and glycerin to create a fog
that emulated how droplets would travel by talking, coughing, and sneezing. The
researchers found that without a mask, droplets traveled more than 8 feet. With
a bandana, they reached an average of 3 feet and 7 inches, versus 1 foot and 3
inches with a two-layered cotton homemade mask, and about 8 inches with a
cone-style mask.
What amazed Verma the most
about the study was just how widespread droplets traveled without a mask on.
“Seeing it with my own eyes was a bit surprising.” Equally interesting, he
says, was how much slower the spread was when the masks fit better on the
mannequin’s face. Based on these findings, as well as prior studies, he suggests
this tip when choosing an effective mask: Hold the mask up to a light—the less
you can see through it, the more effective it is.
Other research and
government institutions have made similar recommendations on best practices for
mask-wearing. Johns Hopkins University also recommends holding the mask up to a light and
using “thicker, more densely woven cotton fabrics,” finding a proper balance
between ensuring it fits properly and that you can still breathe with it on.
That’s also why N95 masks worn by medical professionals work
better than homemade masks: They require a fit test and seal check which
significantly reduces the risk of leakage.
Of course, Verma’s work was
done in laboratory settings, and real-world usage won’t be exactly the same.
Further, Verma says that while his study and others like it all make it clear
that masks reduce the spread of the coronavirus, it’s important to note that in
his study all the masks—even the ones with multiple layers of well-fitted
fabric—tended to have some leakage. “Using a mask doesn’t reduce the
transmission to zero.” However, it’s abundantly clear they effectively slow the
spread.
Up to
date as of July 9, 2020Infographic by Sara Chodosh
Across the country, though,
we’ve been slow to adopt mask usage. Pennsylvania
and New Jersey are the latest to join just 10 other states that have
mandatory face covering rules put in place. Others include: North Carolina, California,
Nevada, Rhode Island, New York, Delaware, Connecticut, New Mexico, Illinois and
Washington.
Meanwhile, the association
between countries that wear masks regularly and those with low death rates from
coronavirus is clear. “If you look at countries that SARS had hit, they just
immediately slapped on these masks because they were used to it, The Czech
Republic weren’t used to it but made a decision in early March,” says Gandhi.
“If you look at these countries there’s a near perfect correlation.”
Part of this may be that,
though masks don’t eliminate risk, they do significantly reduce the amount of
virus a person is exposed to. We know from other viruses, like influenza and
norovirus (which causes vomiting and diarrhea), that the
more viral particles you’re exposed to, the more sick you’re likely to get.
That means wearing a mask may not prevent you from getting sick, but it is
likely to give you a milder case should you catch COVID-19.
The apparent shift in
advice has been frustrating for many people—and that’s understandable. Everyone
was originally told that masks were likely to do very little, but now it seems
like the experts have changed their minds. Part of the problem is that some top
officials advised against mask-wearing not because of any scientific evidence,
but because of concern over shortages for health professionals who really
needed it. But another part of this is that we simply didn’t know a lot about
mask efficacy to begin with. It’s crucial for science to adapt as new evidence
comes in, and that’s exactly what happened here. Armed with better knowledge,
we now know the importance of wearing a mask—and if we want to control this
virus, we all have to abide.
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