“Only two of the studies are about Covid and masking in particular ... neither of those studies looked directly at whether people wear masks, but instead at whether people were encouraged or told to wear masks by researchers”
A recent supposed meta-study has claimed that masks have had no impact on the spread of Covid-19. I overheard a guy in line today ranting to the cashier that it was "proven" that "masks did nothing, not even N-95s."
I can tell you this: EK, masked in an N-95, shared a long car ride with someone who turned out to have Covid. EK didn't catch it. Despite all the flying and train riding and concert-going we've done, fully-masked Anne and I have not caught Covid so far.
The NY Times published the human POS Bret Stephens lying about this study. But the Times seems dedicated to getting people killed. (Even The Onion has noticed.)
As you might expect, the study is not all it claims to be. (Kevin Drum's take.) From Kelsey Piper via Vox's Future Perfect:
“There is just no
evidence that they make any difference. Full stop,” Tom Jefferson, the study’s lead
author, said in an interview. Even fitted N95 masks in
health care settings, the interviewer asked? “It makes no difference — none of
it.”
I think Jefferson — an
Oxford University epidemiologist who has a number of eccentric and flatly
nonsensical opinions about Covid-19, including that it didn’t originate in China and may have been circulating
in Europe for years before its global emergence — is overstating his
case. There is something we can learn from the Cochrane paper, but it’s as much
about the process of science as it is about the effectiveness of masks.
First, the reasons I
don’t totally buy the Cochrane review’s conclusions:
The review includes 78
studies. Only six were actually conducted during
the Covid-19 pandemic, so the bulk of the evidence the Cochrane team took into
account wasn’t able to tell us much about what was specifically happening
during the worst pandemic in a century.
Instead, most of them
looked at flu transmission in normal conditions, and many of them were about
other interventions like hand-washing. Only
two of the studies are about Covid and masking in particular.
Furthermore, neither
of those studies looked directly at whether people wear masks, but instead at whether people were encouraged or told to wear
masks by researchers.
If telling people to wear masks doesn’t lead to reduced infections,
it may be because masks just don’t work, or
it could be because people don’t wear masks when they’re told, or
aren’t wearing them correctly. [And yet, as Kevin Drum notes, both of those studies DID show masks helped!]
There’s no clear way
to distinguish between those possibilities without more original research —
which is not what a meta-analysis of existing work can do.
Digging
into the research on masks and Covid
Those studies that did
take on Covid and masks directly often painted a different picture than the
broader conclusions from the meta-analysis.
One of the largest studies of mask-wearing during the Covid pandemic
was conducted in Bangladesh, with more than 170,000 people in the intervention
group and similar numbers in the control group. The authors studied a series of
public announcements and mask distributions which raised the frequency of
mask-wearing. In the end around 40 percent of the experimental group wore
masks, compared to around 10 percent in the control group.
The result, the study
found, was a substantial reduction in the share of people with Covid-19-like
symptoms, and in antibodies that would suggest a Covid-19 infection: “In
surgical mask villages, we observe a 35.3% reduction in symptomatic
seroprevalence among individuals ≥60 years old ... We see larger reductions in
symptoms and symptomatic seropositivity in villages that experienced larger
increases in mask use.”
That looks like pretty
substantial evidence that mask-wearing reduces Covid-19! And this article is
one of only two studies of mask-wearing included in the Cochrane review which
happened during the Covid-19 pandemic. The other, a study in Denmark, assigned
people to wear masks (though, of course, not all of the people told to wear
masks did so consistently or correctly) and had a control group that generally
did not wear masks. The group that was told to wear masks had slightly lower
infection rates than the group that didn’t wear masks, but the sample was too
small for the effect to be significant.
Given that — one study
finding very solid evidence for the benefits of masks, and one finding limited
but encouraging evidence — how did Cochrane arrive at its conclusion that mask
wearing “probably makes little or no difference?” Because their meta-analysis
mixes these studies, with many more pieces of research that were conducted
before Covid-19 and found little effect of masks on the transmission of other
illnesses like influenza.
Science is full of judgment
calls
The new Cochrane
review paper strikes me, and may strike you, as something of a scientifically
irresponsible way to represent these findings. It gets at one of the core
challenges of science: there is no methodology that can straightforwardly find
answers in messy study data without many judgment calls by scientists, who are
humans with their own strength, weaknesses, and eccentricities. A meta-analysis, after all, can’t
meta-analyze itself.
“I was really
surprised that the Cochrane group let this go through,” Jake Eaton, a public
policy and global health researcher who was the lead researcher on a Cochrane review of childhood
nutrition, told me. “The fact it’s looking at masks across different
settings and with different diseases makes it really tricky. Cochrane reviews
are very good if you really want to assemble the most rigorous evidence and
say, ‘Do we have conclusive signal that this works?’ This is something of a
perverse use of a Cochrane review.”
The big problem is the
sheer scope of the question: not "does masking reduce Covid transmission
during a pandemic?" but "does masking help against all respiratory
illness across contexts?" Because of the questions it chooses to consider
and the ways it chooses to aggregate its evidence, it has arrived at a
mask-skeptical conclusion with limited applicability to the hot-button question
we all care about: whether there’s evidence for masking during
high-transmission periods in the Covid-19 pandemic.
We want science to be
objective and impartial, for scientists to get the same answers regardless of
their starting worldviews. On some questions, like the efficacy of antibiotics against
bacterial infections, the evidence really is overwhelming. But on some, like
masks, it’s limited.
How you interpret limited evidence depends substantially on
where you start. A meta-analysis is a useful tool, but the results it gets will
be substantially informed by how the question is posed and which research is
included in answering it. There often aren’t easy answers even to seemingly
easy questions, and there sometimes aren’t any answers — even to questions that
we care about deeply and that have been studied in some depth.
—Kelsey Piper
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