SARS-CoV-2 Variants and Mask Effectiveness (Factual)

This simulation was performed before the Delta mutant came out, but it probably still holds true if outbreak is defined as serious disease. All scenarios involve loosening social distancing restrictions. Note that if everyone wore masks at the highest levels for that area, only 25-50% of the people would have to be vaccinated with a moderately (80%) effective vaccine to avert another outbreak. Even if mask use drops 50% from prior levels, a moderately effective vaccine requires only 32-57% coverage to avert a new outbreak. However, if no one wears masks, vaccine coverage would have to be up to 78% for a moderately effective vaccine to suppress another major outbreak. Also note that natural immunity was taken into consideration and assumed to be protective.

Without a vaccine (scenario 1), the spread of COVID-19 could be suppressed in these states by maintaining strict social distancing measures and face mask use levels. But relaxing social distancing restrictions to the pre-pandemic level without changing the current face mask use would lead to a new COVID-19 outbreak, resulting in 0.8–4 million infections and 15,000–240,000 deaths across these four states over the next 12 months. Under this circumstance, introducing a vaccine (scenario 2) would partially offset this negative impact even if the vaccine effectiveness and coverage are relatively low. However, if face mask use is reduced by 50% (scenario 3), a vaccine that is only 50% effective (weak vaccine) would require coverage of 55–94% to suppress the epidemic in these states. A vaccine that is 80% effective (moderate vaccine) would only require 32–57% coverage to suppress the epidemic. In contrast, if face mask usage stops completely (scenario 4), a weak vaccine would not suppress the epidemic, and further major outbreaks would occur. A moderate vaccine with coverage of 48–78% or a strong vaccine (100% effective) with coverage of 33–58% would be required to suppress the epidemic. Delaying vaccination rollout for 1–2 months would not substantially alter the epidemic trend if the current non-pharmaceutical interventions are maintained.

If you want more reading material, these are links to articles regarding mask effectiveness that the head of my local county health department provided today when challenged by one of the judges about mask recommendations. The delta variant, which is on the increase locally, had prompted the interaction between the health department and the judge. I don’t know if any of the articles touch specifically on the issue of equal effectiveness for all COVID variants. I would wonder if newer variants have existed long enough for anyone to do studies of variant-specific mask effectiveness.

A rapid systematic review of the efficacy of face masks and respirators against coronaviruses and other respiratory transmissible viruses for the community, healthcare workers and sick patients (

An evidence review of face masks against COVID-19 | PNAS

How effective is a mask in preventing COVID‐19 infection? (

Effectiveness of Mask Wearing to Control Community Spread of SARS-CoV-2 | Infectious Diseases | JAMA | JAMA Network

Making sense of the research on COVID-19 and masks (

Face masks effectively limit the probability of SARS-CoV-2 transmission | Science (

Thanks so much!

I also posted your last link in my OP. The paper addresses the probability of being infected as a function of the amount of virus in certain environments. This should inform the type of PPE required. Beyond a certain concentration threshold, even N95’s are not enough, such as intubating a covid patient in a covid ward. However, the chances of encountering a high concentration of virus in the grocery store is much lower. A decent quality, multilayer cloth mask can block enough of the small amount of virus floating around to prevent infection. I’ll add that being vaccinated reduces the chance of being infected even further.

I agree that the delta variant hasn’t been around long enough to collect data on mask efficacy. I’ve searched for papers, even preprints, but I can’t find any that even address the alpha variant.

A comparison of surgical masks as source control for coronaviruses, influenza viruses, and rhinoviruses. The masks didn’t work as well with rhinoviruses but that may be due to the different structure of the viruses. Coronaviruses and influenza viruses have a lipid envelope surrounding them. A serious caveat with these results is that the Delta virus is thought to shed 1000X more than the wild type.

Keep in mind too that masks can greatly reduce viral load, even if they allow enough virus particles to infect. This is just my take, but it seems very unlikely that we’re ever going to get rid of SARS-2-COVID-19. Therefore, in my estimation the real goal is not necessarily creating a world of zero risk; rather, we probably need to be focused on how to make the consequences of the virus less severe. Collective masking is one way to do that; collective vaccination is an even better way to achieve that goal.

Thanks! I will read those.

One point that should be kept in mind is that, for every mitigation tactic, we should look at a cost-benefit analysis. Closing all non-essential businesses, for instance, is certainly effective at preventing infection, but it comes at a tremendous cost: You need to decide whether the benefit is worth that tremendous cost. Masking, by comparison, has an extremely low cost, and so it takes only a very minor benefit to make masking worthwhile.

True. I look at the risks as series of probabilities of receiving a high enough dose that will overwhelm my immune system. I know the relationships aren’t linear but it helps. Masks lower the probability of inhaling any virus at all in grocery store settings because the particles will be too dilute.

Delta definitely changes the equation, but so does vaccination. Data out of UK demonstrate clearly that although cases increased deaths and hospitalizations are barely on the rise. Compare their earlier surges at this point of the surge. Deaths have risen but at least 4-fold less than they did during earlier surges. Hospitalizations are 1/3 to 1/2 half they were in earlier surges.

Israel is doing even better than the UK in terms of cases because they had more people fully vaccinated when the Delta variant was first detected. The virus is having a hard time finding kindling to burn through. However, cases are finally rising and some of these are breakthrough infections. Again, deaths and hospitalizations are minimal.

When I teach labs in the Fall, I’m going to wear a good mask for the introduction lecture, then put on a shield (instead of the usual goggles) when lab starts and I circulate among the students. I am requiring my students to wear a mask or they can drop the class. I’m already wearing a mask over to my parents. I just finished teaching fully remote, so I’m not as worried now. When my Fall semester starts, I’m thinking of wearing the shield with the mask when I get close to my mom.

Masks primarily work to mitigate the introduction of viral particles into the air. The fact that they likely also minimize inhaling particles is nice but secondary.

Masks being the general ones people are wearing not the N95/PPE type

In Israel, an overwhelming majority of cases are breakthrough cases.

The same is true in Provincetown, Massachusetts.

It’s actually not having a hard time finding people to infect; it’s infecting both vaccinated and unvaccinated. The vaccines do provide a much stronger layer of protection, which makes the worst outcomes much less likely. But I think we’re back at a point now where masking can no longer be considered optional, and I think offices should reconsider bringing employees back to work.

From the article about Provincetown

What makes the outbreak unusual is that two-thirds or more of the cases have been among fully vaccinated people, in an area boasting one of the country’s highest vaccination rates.

Since pretty much every one in Provincetown is vaccinated, that means that the risk of catching it was MUCH higher for the few unvaccinated people. It’s hard to know what the %vax rate is in Provincetown, since the official number is about 125%. That mostly represents a disconnect between the census number, which is permanent residents, and the seasonal residency – many people who were vaccinated did so during the summer season. But it highlights the general fact that data is never clean and tidy.

But the 12-15 year olds are reported as 95% vaccinated, and the 16-19 year olds as 87% vaccinated, so I think we can assume the underlying rates of vaccination are very high.

If 1/3 of the infected were unvaccinated, and only 1/10 of the total population (a guess) was vaccinated that means that the odds of a vaccinated person catching it would have been about 2/9 the odds of an unvaccinated person catching it. Or a 78% effectiveness rate. Not nearly as good as the 95% we were looking at with the original virus, but still pretty damn good protection.

Oops – the official rate for “at least partially vaccinated” is 124% (published as >95%, but the state includes the underlying data). The official rate for fully vaccinated is only 115%. My numbers for kids above were fully vaccinated rates. I don’t think 90% vaxxed is an unreasonable guess, given the data.

That’s not completely true if you do things to improve your mask. This is important if you live in an area full of anti-maskers. For surgical masks, it’s only necessary to close the gaps. You can do it by tying the loops and tucking or, even better, wearing a nicely fitted cloth mask over it. This converts a mask used for source control into a mask for PPE. Some surgical masks already fit fairly snug and likely act as PPE. See the link in my OP for data on probabilities of different masks as PPE. Remember, we’re talking about probabilities in low concentrations of virus like a well ventilated grocery store. High viral concentrations like ER or covid ward, even N95’s are not enough.

Tying your surgical mask. I know this works with the perfume smell test.

Doubling up masks to improve fit and add more filtration to protect the wearer

Fix the mask with a brace or rubber bands - this looks weird but it makes a point.


There are excellent cloth masks out there with filter inserts so that you don’t have to wear the surgical mask. You can wear to masks but combine fabrics so that you have filtration through both mechanical and electrostatic mechanisms. The better the mask is for source control, the better it will be for PPE as well (as long as you seal the gaps). Again, I test my masks with the perfume test.

A video on the different types of filtration: shear mechanical for the large droplets, increasing the maze and sticking chances for small particles moving via brownian motion, and electrostatic for the medium-sized particles.

In both of these papers below, the materials were subjected to something sprayed directly on them at a certain flow rate. It was testing the material, not gaps.

A useful overview of laboratory testing and real world data (updated last November)

Yes, because an overwhelming majority of adults are vaccinated. I think it’s over 85%. If you read larger articles on this, you’ll see that many researchers are questioning effectiveness results. The sample size of unvaccinated adults is too small relative to vaccinated. For me, the large vaccination rate in the population demonstrates the utility of vaccination. It’s why cases are so low even though they had Delta cases way back in April. Compare that to the doubling rate in a place like Louisiana in only the past three weeks. Compare Israel’s deaths and hospitalizations to Louisiana. Their cases have been rising for over a month. Deaths are a blip and hospitalizations are rising but nowhere near to what they had in the past. Now look at Louisiana. Deaths are already rising and hospitalizations are skyrocketing. It is true that Israel has a mask mandate, but that would only prove that masks and vaccines are working together to keep hospitalizations, deaths, and even cases low.

It is having a hard time finding people to infect compared to the unvaccinated. Again, compare Mass to Louisiana. Compare California to Louisiana. I’m choosing Louisiana because its surge started more recently and its vaccination rate is crap so it’s a conservative comparison. Again, the differences could be due to both vaccination and masks. That’s why I’m wearing a mask in public.

Yes. The data out of San Antonio is that 96% of the people currently in the hospital are unvaccinated. The Delta variant accounted for 83% of cases a week ago so it’s probably 100% of cases now. So our surge of hospitalizations are unvaccinated people. The good news is that I started to see people wearing masks again a couple of weeks ago.

Is there any actual evidence that wearing two masks is better than one? The only argument I’ve seen for it is “Well, it’s just common sense”, but common sense would argue the opposite: If doubling up were better, they’d have been made that way in the first place. To see why it might not be better, consider the hypothetical of a mask made out of cellophane: Nothing at all is going to get through that mask, but that means that 100% of the airflow is going to find its way around the edges of the mask, and so it won’t actually stop anything.

Yes. Look at some my cites. The value depends on the mask. If you have a good mask like a surgical mask but it has gaps, simply adding a cloth mask on top will seal the gaps. Sometimes masks fit well but they have limited filtration. Doubling up adds layers. Mixing fabrics adds variety of filtration.

Obviously a well fitted n95 doesn’t need doubling for most purposes.

In terms of actual evidence, the jury still seems to be out. Double masking almost certainly provides additional filtration; however, it’s one more item that has to be decontaminated once you take it off. If you decontaminate the inner mask but fail to do so with the outer mask, you could inadvertently contaminate yourself unwittingly, which was one of the original concerns or cautions about masking in the first place. Probably wasn’t a big concern with the earlier variants of COVID but considering the viral load of the Delta variant, I think that concern is legitimate and back in play.

Another concern is that the additional layering cannot pull the seal of the mask that’s protecting you. If you’re getting additional filtration but impacting the integrity of the seal that prevents air from directly flowing underneath your mask, then you’re actually doing more harm than good.

All of that is to say, it probably does help, but only if you do it right. There’s an article from the Cambridge University Press

Note the mask isn’t filtering individual virions; it’s filtering droplets, especially those advice about a micron in diameter. I suppose a virus could change respiratory behavior to alter the droplet distribution, but I haven’t read anything one way or another.

To the OP, I don’t see mask effectiveness changing at all from one variant to another. The effectiveness is its ability to filter droplets. If Delta droplets happened to have much more virus in them, that would increase the risk of infection, but the mask isn’t any less effective at filtering.

Correct - I meant that the aerosols that contain the virus haven’t gotten any smaller but worded it inaccurately. So in that sense, the masks are no less protective. But with the delta variant, there is potentially more virus being shed whenever those droplets or aerosols are released. The virulence of the Delta variant makes both vaccines and masks less effective relative to the previous variants. Still, we’re absolutely better off with both vaccines and masks because these are layers of protection that can make the virus less severe. The data, both statistical and anecdotal, are beginning to show a clear pattern, which is that breakthrough infections are much more likely but that the vaccines are doing a pretty good job of beating the virus back once infected – and that is the value of the vaccine. Masking could also be the difference between not getting the virus at all or getting much less of it. I just worry we may not be so lucky with the next variant of concern.