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There is a graph being shared around on the internet that implies that the reason that South Korea, Japan, Singapore and Hong Kong have better control of Covid-19 is that the people wear masks as a protective measure.

Is this actual cause / effect or correlation?

enter image description here

Note that is similar but not the same as this question Does a surgical mask help prevent catching COVID-19?; the question is not does a mask prevent Covid 19, the question is are the differences in the graph because of wearing facemasks.

Additionally, it was reported in Science that a top China CDC official said that the West is making a big mistake by not adopting masks for the general population:

Science has tried to interview George Gao, director-general of the Chinese Center for Disease Control and Prevention (CDC), for 2 months. Last week he responded. [...]

Q: What mistakes are other countries making?

A: The big mistake in the U.S. and Europe, in my opinion, is that people aren’t wearing masks.

Also the Czech PM claimed in a tweet carried by the (US) media that:

Mr. President @realDonaldTrump, try tackling virus the Czech way. Wearing a simple cloth mask, decreases the spread of the virus by 80 %! Czech Republic has made it OBLIGATORY for its citizens to wear a mask in the public.

So that's a rather more concrete claim. Meanwhile, the WHO has convened a panel to re-analyze the problem, in view of latest research.

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    I see the US is listed under “no masks”. Typically that would be pretty accurate; however recently people have been buying and using face masks in much greater quantities, to the point where the surgeon general had to tweet about it. – Laurel Apr 1 at 13:11
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    I don't think it is meaningful to show a graph with absolute number of cases and not say, per million. One thing: did South Koreans all start wearing masks at day 10-15 after the 1000th case? – Weather Vane Apr 1 at 16:01
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    Remember those countries listed under No Masks also have other differences that mean you could not isolate this one factor. Correlation, I'm afraid. – Rory Alsop Apr 1 at 16:07
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    "Unresolved current event". I can even make your question fit the Skeptics format better, by quoting a China CDC official, who says that the West is making a big mistake by not using masks widely in the general population. Alas, there would be no way prove him right or wrong at the moment with anything other than correlations, which are plagued by [other] confounders. – Fizz Apr 1 at 17:29
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    Most people in Singapore did not use masks until late March/early April 2020, when the government (1) first officially announced it would stop discouraging the use of masks; (2) began its "circuit-breaker" (soft lockdown); and (3) started mandating the use of masks at certain places. This was also when daily cases started rising sharply. So, in the case of Singapore, we would, if anything, find a strong (but of course spurious) positive correlation between use of masks and cases. – JerryS1988 Apr 12 at 4:30
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This question likely cannot be answered for several months, because this is an "ongoing current event" and involves the larger question of how masks work in collaboration with nationwide and international pandemic control tactics. However, SARS-CoV-2 is a type of coronavirus which has been long known to scientists, and there was already a 2003 outbreak of SARS-CoV-1 that inspired several studies about masks. I think it is in the interest of Skeptics.SE and the public interest to discuss these studies here.

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    For completeness, the last paper (and insofar the only one on Covid-19), says that the 1st bus trip took 2 hours (and 10 minutes) and that few others wore masks on that bus. The 2nd trip was on a minibus and it took 50 minutes. No info is given whether anyone but the guy infected wore a mask on the minibus. Also there were 39 passengers on the bus and 14 on the minibus. While the sample sizes differ, the "population density" was probably comparable given the difference in sizes between the two vehicles. – Fizz Apr 3 at 0:01
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    Thanks, this is very informative and points to the direction of what I wanted an answer to, but is not quite the answer yet. I suspect the required material to answer this won't be available much before 2021. I also suspect the answer will refer to it being a percentage difference and not the reason. – user1605665 Apr 5 at 23:36
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Interestingly, there's already a paper from Yale jointly authored by some of their economics and medicine faculty that tries to answer this, basically by a regression model that tries to include the other possible public-health factors/confounders... And long story short, the differences persist even when such known confounders are included, suggesting that the putative effect of masks is not easy to "explain away" by some other factor.

The pattern in the figure is quite stark: countries with pre-existing norms that sick people should wear masks – including South Korea, Japan, Hong Kong and Taiwan – have been among the most effective at containing the spread of the epidemic. The average daily growth rate of confirmed positives is 18% in countries with no preexisting mask norms and 10% in countries with such norms.

This evidence is far from definitive: norms do not perfectly predict actual mask availability and use, these countries may have instituted other policies which contained the spread of the epidemic (such as widespread testing in South Korea), and infection rates are imperfectly measured and may appear higher in countries with more testing among other factors.

To aid in interpreting the graph, we conduct several regression analyses, shown in Table 1.

enter image description here

The goal of this analysis is to examine whether mask norms have a relationship to case growth, controlling for other factors. Column (1) shows the measured relationship between the growth rate of cases and an indicator variable for countries with pre-existing mask norms. In Column (2), we control for the timing of school closings, workplace closings, the cancellation of public events and the closing of public transport as “Baseline Policy Controls”. Specifically, specifications with “Baseline Policy Controls” control for policies in place at time 0 (100 cases or 10 deaths). Column 3 undertakes a more dynamic view of policy variables. In this specification, “Average Policy over 8 Days” adds controls for these same policy variables, but averaged over the first 8 days after time 0. When adding these controls we find that the estimated effect of masks is unchanged or grows slightly larger. [footnote: These policy variables come from the Oxford COVID-19 Government Response Tracker] In column (4), we control for an “Asia” fixed effect since growth rates may differ due to cultural differences common to Asian countries; the estimated effect on cases is slightly larger with this control.

One measurement concern is that cases are measured with error and rates of testing and measurement vary across countries. Thus, deaths from COVID-19 may be better measured than cases. We repeat the analyses above using deaths as the outcome variable in the right panel of Table 1. We find that the growth rate of deaths is 21% in countries with no mask norms and 11% in countries with such norms. Even with the small number of observations, the impact of masks in all reported analyses is statistically significant at the 5% level, and usually at the 1% level.

While we control for major policy variables, there are many factors that cannot be controlled for in an ecological study of this type. [footnote: For example, in addition to norms of mask wearing, handshakes are rare in Japan which may slow the spread of the virus. However, one comment that is often made is that a variety of factors may distinguish the Asian countries from the non-Asian countries. Including an indicator for “Asian” actually increases our estimates of the case reduction from mask norms.] These results are far from the final word, but they do complement and provide a measure of external validity the epidemiological studies of masks cited above. While our analysis principally concerns the impact of norms that sick people wear masks, it has direct implications for universal mask adoption. If the causal interpretation of the above results is correct, the impact of mask norms (which increase the likelihood of mask wearing relative to no norm countries) should understate the impact of universal mask adoption for both visibly sick and healthy individuals (who are potentially asymptomatically infected).

Our economic analysis suggests that that even if masks are far less effective than the evidence above suggests, the potential benefits are substantial. If masks reduce the transmission rate of the virus by only 10%, epidemiological models suggest that hundreds of thousands of deaths could be prevented globally, creating trillions of dollars in economic value. According to one commonly used epidemiological model, a 10% reduction in transmission probabilities would generate $3,000-6,000 in value per household from reduced mortality risk in the US alone. This estimate is conservative with respect to the benefits, as it does not include the economic benefits from a quicker resumption of normal activity. And our estimates above suggest that the effect of masks could be 5-6 times as large. Of course, all such estimates are only as reliable as the underlying epidemiological models. But even if these models overstated risk by a factor of ten, the benefits of cloth masks, would conservatively be $300 per household.

-- "The case for universal cloth mask adoption and policies to increase the supply of medical masks for health workers", Jason Abaluck, Judith Chevalier, Nicholas A. Christakis, Howard Forman, Edward H. Kaplan, Albert Ko and Sten H. Vermun


I should mention that there have been (systematic) reviews of the kinds of papers that Avery mentioned. Generally, the problem with mask use by the general public has been pinned to inconsistent and improper use. Results in HCW (healthcare worker) context have generally been (much) superior. McIntyre and Chughtai's 2015 BMJ review discusses three categories of studies: RCTs in HCWs, RCTs in the general public, and epidemiological studies. The first and last of these generally provide more encouraging results for maks use than the middle category, which also has been the least studied. I'm only giving you their summary table for that category further below.

What I also want to say before we get to those prior studies, is that a recent (April 2) Lancet paper from China on Covid-19 says that mask shortages would occur if China mandated their universal use:

Three scenarios with respect to wearing facemasks were considered: (1) a universal facemask wearing policy implementation in all regions of mainland China; (2) a universal facemask wearing policy implementation only in the epicentre (Hubei province, China); and (3) no implementation of a universal facemask wearing policy. [...]

During the prediction period, the largest daily facemask shortages were predicted to be 589.5, 49.3, and 37.5 million in each of the three scenarios, respectively. [...]

Implementing a universal facemask wearing policy in the whole of China could lead to severe facemask shortage. Without effective public communication, a universal facemask wearing policy could result in societal panic and subsequently, increase the nationwide and worldwide demand for facemasks. These increased demands could cause a facemask shortage for healthcare workers and reduce the effectiveness of outbreak control in the affected regions, eventually leading to a pandemic.

I guess we're lucky China controlled the outbreak in Hubei, or else we'd be half a billion masks short elsewhere. Also an interesting fact from this paper:

to control the COVID-19 outbreak, the Chinese government imported more than 2.0 billion facemasks between 24 Jan 2020 and 29 Feb 2020

I have to wonder from where...

As far as attributing China's success in Hubei (insofar) just/mainly to masks, that's probably a tall order. There's one recent paper in Science (March 25) about the contribution of the severe travel restrictions in controlling the outbreak:

Major behavioral, clinical, and state interventions have been undertaken to mitigate the epidemic and prevent the persistence of the virus in human populations in China and worldwide. It remains unclear how these unprecedented interventions, including travel restrictions, affected COVID-19 spread in China. We use real-time mobility data from Wuhan and detailed case data including travel history to elucidate the role of case importation on transmission in cities across China and ascertain the impact of control measures. Early on, the spatial distribution of COVID-19 cases in China was explained well by human mobility data. Following the implementation of control measures, this correlation dropped and growth rates became negative in most locations, although shifts in the demographics of reported cases were still indicative of local chains of transmission outside Wuhan. This study shows that the drastic control measures implemented in China substantially mitigated the spread of COVID-19.

So yeah, there are potentially confounding factors to worry about when ascertaining the overall impact of a public health measure.


A newer (April 3) CDC press release says:

We now know from recent studies that a significant portion of individuals with coronavirus lack symptoms (“asymptomatic”) and that even those who eventually develop symptoms (“pre-symptomatic”) can transmit the virus to others before showing symptoms. This means that the virus can spread between people interacting in close proximity—for example, speaking, coughing, or sneezing—even if those people are not exhibiting symptoms. In light of this new evidence, CDC recommends wearing cloth face coverings in public settings where other social distancing measures are difficult to maintain (e.g., grocery stores and pharmacies) especially in areas of significant community-based transmission. [...]

The cloth face coverings recommended are not surgical masks or N-95 respirators. Those are critical supplies that must continue to be reserved for healthcare workers and other medical first responders, as recommended by current CDC guidance.

(Only the word "especially" was emphasized in the original.)

This CDC announcement was also highlighted by president Trump in a press conference, but Trump also said that he personally doesn't plan to wear a mask. Additional comment in the Hill that this is a change in CDC's stance on masks:

The guidance from the Centers for Disease Control and Prevention (CDC) is a turnaround for the agency, which told the public at the beginning of the outbreak that healthy people should not buy or wear masks because it would not protect them from contracting the disease.

Somewhat similarly, NYT reports (on the same day) that the WHO has said:

At a W.H.O. briefing on Friday, Dr. Michael J. Ryan, executive director of the health emergency program, said that while the agency still recommended masks only for front line health workers and those who are sick or caring for the sick, “we can certainly see circumstances in which the use of masks, both homemade or cloth masks, at community level may help in an overall comprehensive response to this disease.”

(Emphasis mine.) So yeah, while we have no definitive data on their effectiveness in the general public, the public health authorities seem to (increasingly) entertain the idea of masks in (general) community use, if it doesn't interfere with HCW supplies of masks.

In contrast, in an equally recent UK briefing:

Meanwhile, the Government confirmed that it does not recommend that healthy people wear face masks in light of the US adding it to their advice.

Professor Jonathan Van-Tam, deputy chief medical officer for England, said the practice seemed “wired into” some South East Asian cultures but there was no evidence it helped stop the spread.

He said: “There is no evidence that general wearing of face masks by the public who are well affects the spread of the disease in our society. What matters is social distancing.”

So yeah, some public health officials are more skeptical than others about this.


As for the review of prior studies on mask effectiveness in the general public:

enter image description here

Due to (substantial) variations in study design, no meta-analysis was performed. In the review authors' summary

Of the nine trials of facemasks identified in community settings, in all but one, facemasks were used for respiratory protection of well people. They found that facemasks and facemasks plus hand hygiene may prevent infection in community settings, subject to early use and compliance. [...]

Facemasks are used in the community in Asian countries, not only to protect people from acquiring respiratory infections but also to minimise spread of infection from the wearer. Such use often increases during outbreaks and pandemics. Cloth masks were reportedly used by the general public during the 1918 influenza pandemic. During the SARS outbreaks, masks were widely used in diverse community settings.

We identified nine RCTs of facemasks in various household and community settings, and in all but one they were used for respiratory protection. In one household trial the use of facemasks was tested as source control to prevent the spread of infections from the wearer. These RCTs had diverse settings, designs, and interventions— many of which were mixed, such as hand washing and facemasks (fig 2).

An RCT in Hong Kong randomised index cases (198 laboratory confirmed influenza cases) and their households into medical masks, hand hygiene, or a control arm. Rates of laboratory confirmed influenza and influenza-like illness were not significantly different in the medical mask arm versus the control arm (influenza: odds ratio 1.16, 0.31 to 4.34; influenza-like illness: 0.88, 0.34 to 2.27). In a second trial by the same group, medical masks plus hand hygiene and hand hygiene alone groups were compared with a control group (total 407 index cases). There was no significant difference across the three arms, although medical masks plus hand hygiene were protective when the intervention was implemented early (within 36 hours of onset of symptoms in the index case, adjusted odds ratio 0.33, 0.13 to 0.87).

An Australian study randomised 145 index cases and their household members to one of three arms—medical masks, P2 respirators (equivalent to N95), or control. In contrast to the second trial above, where both index cases and household members used a mask, only household contacts used a medical mask in this study. No significant difference in the risk of influenza-like illness was seen between the three arms in the per protocol analysis, but risk was significantly lower with the adherent use of P2 or medical masks (hazard ratio 0.26, 0.09 to 0.77).

Two RCTs in university residence halls in the United States over two influenza seasons randomised well students into medical masks plus hand hygiene, medical masks alone, or control. Influenza-like illness and laboratory confirmed influenza were not significantly reduced after either intervention, although during the first four to six weeks, influenza-like illness was significantly lower in the medical masks plus hand hygiene arm in both trials (P<0.05). This suggests that hand hygiene might have been the major contributor to protection.

An RCT in the US randomised 617 households to education, hand sanitiser alone, or hand sanitiser plus medical masks. Although the rates of upper respiratory tract infections, influenza-like illness, and laboratory confirmed influenza were low in the hand sanitiser and hand sanitiser plus medical masks groups, the difference was not significant after adjusting for other factors. However, the hand sanitiser plus medical masks group had significantly lower secondary attack rates for influenza, influenza-like illness, and upper respiratory tract infections (odds ratio 0.82, 0.70 to 0.97) compared with the education group. Results for the hand sanitiser only group were not significant (1.01, 0.85 to 1.21).

An RCT in Thailand randomised 465 index patients and their families to hand hygiene, hand hygiene plus medical masks, and a control arm. No significant difference between secondary influenza rate was seen. In a cluster randomised controlled trial in Germany, 84 index cases and 218 household contacts were randomised into a mask arm, masks plus hand hygiene arm, and a control arm. There was no significant difference in rates of laboratory confirmed influenza and influenza-like illness in all arms by intention to treat analysis. However, the risk of influenza was significantly lower if the data from two intervention arms were pooled and the intervention was applied within 36 hours of the onset of symptoms (odds ratio 0.16, 0.03 to 0.92).

A household trial in France examined the role of medical masks as source control—index patients were randomised into medical mask (52 household and 148 contacts) and control groups (53 household and 158 contacts). There was no difference between the groups (0.95, 0.44 to 2.05), and the trial was finished early owing to low recruitment and subsequent H1N1-pdm09 infection.

[... in the conclusion section:]

Compliance is a determinant of protection, and it decreases with increasing duration of continuous mask use. Policies and guidelines on mask use worldwide are inconsistent, perhaps reflecting the relatively small number of RCTs available to inform them.

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    It's good to see statements to the effect of "better than nothing" vs. "we don't know they are useful" or "they are useless" or "no evidence so we recommend against them". There's an asymmetry of risk that it seems people often fail to consider. If there is little or no downside and huge potential upside to wearing even a home-made mouth and nose covering (particularly in comparison with other non-pharmaceutical interventions), it makes no sense to tell people not to wear a mouth and nose covering. A tiny difference to the infection rate and viral load can have a huge effect over time. – Lag Apr 7 at 9:32
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    @Lag: "if there is little or no downside" - that is where this advice falls down – Oddthinking Apr 11 at 19:07
  • @Oddthinking no doubt by now you will have read Face masks for the public during the covid-19 crisis BMJ 2020;369:m1435 bmj.com/content/369/bmj.m1435. – Lag Apr 12 at 7:21
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    @Lag: By now, I am aware this is a major controversial point being argued back and forth by many health professionals, so a single paper in which the authors express their opinions on the evidence is interesting, but not enough to post medical advice without warnings on Skeptics.SE. – Oddthinking Apr 12 at 8:21
  • @Lag: Eh? That was just posted on Apr 9; I'll give it a read sure, but saying people should have have red it already... Not everyone is glued to the medical journal pages. (As you can probably see I've noted/updated this answer with WTO, CDC etc. changes, as I became aware of them. A paper getting published in BMJ is not that super-remarkable in comparison.) – Fizz Apr 12 at 8:28
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Widespread mask wearing is something those countries have in common but it is not the only difference between their efforts and those of other countries.

I think the point of the chart is not to suggest that mask use is a silver bullet but something (a) that the countries with the 'better' curves have in common with each other and not in common with the other countries and (b) it happens to be a relatively inexpensive non-pharmaceutical intervention that members of the public can individually do for themselves rather than rely on the authorities or NGOs (mass testing programmes and such interventions are not things members of the public can do themselves). Perhaps some people are intentionally or unintentionally over-egging the point.

The difficulty in trying to answer your question is that those countries have done / are doing several different things, as indicated in that chart: instituting early on in their timelines school closures, mass testing, strict quarantine rules, contact tracing, door-to-door monitoring, smartphone alerts about the movements of people who have tested positive, identifying infection hotspots and supplying hotspots with extra resources, temperature checks and free hand sanitiser in shops and cafés. How do we separate out the impacts of each individual intervention?

Here is a comparison table - I don't know the origin, but it was tweeted by Anthony Costello, a paediatrician, professor at University College London and ex-director at WHO, and it looks about right. However, it doesn't mention masks.

Comparison table of the death rates per million in countries with 'protective shield' of non-pharmaceutical interventions vs. countries without that shield

What individual contribution was made by any one aspect of the so-called 'protective shield' in that table - (a) community case detection, (b) contact tracing, (c) isolation, (d) mass testing programme - we don't know yet (it may be impossible to know). However, taken at face value, there does seem to be a large difference in the death rates per million and there isn't an immediately obvious explanation other than the difference in interventions.

As I said, the table doesn't mention masks but masks are in widespread use in some - not all - of the countries in the 'protective shield' column and none of the countries in the 'without protective shield' column except parts of Spain and the USA very recently (the USA's CDC issued its Recommendation Regarding the Use of Cloth Face Coverings on 3 April 2020). But because of the incubation period of the virus we won't be able to detect any impact of an intervention on infection rates (if there is a detectable impact) until say two weeks from the intervention - we'd expect the proportion of those infected 'today' who will show symptoms, to show them in 1-14 days.

The Wikipedia articles for the 2020 coronavirus pandemic in each country respectively have more details about the country's responses. For example: South Korea, Japan, Singapore, United Kingdom, United States of America. (If there is hunger here for a summary, I might have a go at that.)

If you look at what studies there have been of mask use, some of which are summarised in other answers on this page, you'll see the evidence is that masks make either no significant difference or some significant positive difference particularly in conjunction with hand hygiene which we should all be doing already. Now, it stands to reason (simple maths) that a small impact on infection rate can have a huge beneficial impact over time. Compare an infection rate of 3 (that is, 3 people infected by every one person every day) vs. 2.99 or even 2.999. It might be a difference that is undetectable in an N = 200 to 1500 size study such as linked to in other answers but may well be detectable with N = tens of thousands to millions. Compare the numbers of infections after ten days or 15 days. Also consider a hospitalisation rate of say 10% and a death rate of say 1%.

Two countries to keep an eye on in the coming weeks are Czechia and Slovakia, which at 30 March were the only countries in Europe to make mask-wearing mandatory following campaigning by members of the public. Again, because of the incubation period, if there is an impact from mask-wearing we won't be able to observe it before say the middle of April - and if their other efforts changed around this same time we might not be able to disentangle the impacts.

We know "the virus can spread between people interacting in close proximity—for example, speaking, coughing, or sneezing—even if those people are not exhibiting symptoms" (USA's CDC). "Studies to date suggest that the virus that causes COVID-19 is mainly transmitted through contact with respiratory droplets ... The risk of catching COVID-19 from someone with no symptoms at all is very low. However, many people with COVID-19 experience only mild symptoms" (WHO). And it's not counter-intuitive that covering one's nose and mouth might reduce the stuff getting in and out of one's nose and mouth.

Readers may be interested in "Parachute use to prevent death and major trauma when jumping from aircraft: randomized controlled trial" BMJ 2018;363:k5094.

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