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

In this thread, @puzzlegal, @ThelmaLou, @Jay_Z, and I were discussing the robustness in protection of masks against the new Delta variant. I started this thread to avoid hijacking that one.

The question is, does the higher transmissibility of the Delta variant decrease the effectiveness of masks? Why The Delta Variant Is So Contagious: A New Study Sheds Light : Goats and Soda : NPR

A few things to consider:

  1. The viral load in the respiratory tract (1000X higher in Delta variant), will effect the concentration of virus. The lower the concentration of virus (vaccination, universal masks, good ventilation, social distancing, etc.), the lower the probability to the point where even moderate quality masks actually work to protect you.

  2. The ability of a virus to evade the filtration ability of a mask. The variables for this would largely be physical as in the “stickiness” of the virus to mask material, the “viability” of the virus in smaller droplets like aerosols, etc. I am assuming these parameters are not changing with the new variants, but I could be wrong.

Please make an effort to keep the content in the thread factual without hijacking it regarding whether or not masks work at all. Masks can work as both source control and protect the wearer, depending on the quality of the mask material and how well gaps are sealed.

Any thoughts? My first thread, so I hope this works.

The virus probably hasn’t gotten any smaller, so in theory, the N-95 should still screen out most of the particles. The problem with the newer variants is that the viral load is greater; the implication is that it is now more important for everyone to be wearing masks, not just the person who wants to avoid infection. If you’re the only masked person in a room of people without masks and shedding virus, the protective value of the mask is likely greatly reduced, which is why I really, really do not understand what is going on at CDC when it comes to their mask guidance. They’ve been behind the curve on this pandemic the entire time, and I think time will reveal that this is yet another unforced error. I hope I eat my words.

I apologize for a long rambley post. I’m afraid I don’t have time now to make it tighter. Hopefully, you can see why I am wondering if Delta might not be effectively stopped by most facemasks.

So… I was trolling the internet looking for evidence that masks help prevent the spread of covid. And I found a bunch of stuff.

Some was mostly focused on how the “6 foot” rule really isn’t helpful. and instead, we should be opening windows and wearing masks.
A guideline to limit indoor airborne transmission of COVID-19 | PNAS
It explores the implications of covid being largely spread via aerosols. It’s quite long and has a lot of math, but among its conclusions are

the interpretation of which is immediately clear. To minimize risk of infection, one should avoid spending extended periods in highly populated areas. One is safer in rooms with large volume and high ventilation rates. One is at greater risk in rooms where people are exerting themselves in such a way as to increase their respiration rate and pathogen output, for example, by exercising, singing, or shouting. Since the rate of inhalation of contagion depends on the volume flux of both the exhalation of the infected individual and the inhalation of the susceptible person, the risk of infection increases as Q2bQb2. Likewise, masks worn by both infected and susceptible persons will reduce the risk of transmission by a factor p2mpm2, a dramatic effect given that pm≤0.1pm≤0.1 for moderately high-quality masks (74, 75).

That is, that masks (especially good ones) decrease the spread of covid, especially in indoors settings with modest ventilation.

I also read this, which concluded that surgical masks can probably reduce the spread of corona virus and perhaps flu, but not the common cold
Respiratory virus shedding in exhaled breath and efficacy of face masks | Nature Medicine

We identified seasonal human coronaviruses, influenza viruses and rhinoviruses in exhaled breath and coughs of children and adults with acute respiratory illness. Surgical face masks significantly reduced detection of influenza virus RNA in respiratory droplets and coronavirus RNA in aerosols, with a trend toward reduced detection of coronavirus RNA in respiratory droplets. Our results indicate that surgical face masks could prevent transmission of human coronaviruses and influenza viruses from symptomatic individuals.

Then I read
Effectiveness of Face Masks in Preventing Airborne Transmission of SARS-CoV-2 (
which said

When a mannequin exposed to the virus was equipped with various masks (cotton mask, surgical mask, or N95 mask), the uptake of the virus droplets/aerosols was reduced. A cotton mask led to an approximately 20% to 40% reduction in virus uptake compared to no mask (Fig. 2B). The N95 mask had the highest protective efficacy (approximately 80% to 90% reduction) of the various masks examined; however, infectious virus penetration was measurable even when the N95 mask was completely fitted to the face with adhesive tape (Fig. 2B). In contrast, when a mask was attached to the mannequin that released virus, cotton and surgical masks blocked more than 50% of the virus transmission, whereas the N95 mask showed considerable protective efficacy (Fig. 2C). There was a synergistic effect when both the virus receiver and virus spreader wore masks (cotton masks or surgical masks) to prevent the transmission of infective droplets/aerosols (Fig. 2D and ​andEE).

But here’s where I start wondering if masks might be less effective against Delta:

Slight reduction in SARS-CoV-2 exposure viral load due to masking results in a significant reduction in transmission with widespread implementation - PubMed (

It basically says that it takes (in general) a critical threshold of viral particles to infect a person, (see also)

Viral load and contact heterogeneity predict SARS-CoV-2 transmission and super-spreading events (

And so a relatively small decrease in how many particles are passed back and forth can create a large decrease in disease transmission, by dropping the expected number of particles below that threshold. This is related to why masks appear to be more effective for preventing the spread of covid that for preventing the spread of the flu, fwiw.

So… I wonder if the much more infectious Delta variant which needs fewer particles to create an infection might be more resistant to being prevented by masks.

According to an article I just read today (can source if desired), 6 of every 10,000 new infections in a Los Angeles County surge are vaccinated individuals. The remaining 9,994 are unvaccinated.

I’m just wondering, is all of this directed at the 6, or the 9,994?

My intention was to discuss the effectiveness of masks in general at reducing the spread of covid, not at giving advice on who should wear one. I was actually looking at the increase of covid in counties that haven’t yet had huge numbers of cases. In many cases, those are countries that adopted high levels of mask-wearing early on. And I speculated that perhaps that preventive measure was not working as well.

Also, yes, i would appreciate a source for those numbers. I’ve been looking for data about the relative incidence of covid and its results among the vaccinated and the unvaccinated. (But i think that’s really a topic for a different thread.)

Do you know which variant they referenced? A study out of Israel (not a perfect study) suggests that the Pfizer vaccine is only 67% effective against infection by Delta (while still highly effective against hospitalization and death).

The article I posted is suggesting something similar. There’s a viral concentration threshold under which even moderately effective masks greatly reduce the probability of infection. But if the concentration is beyond that threshold, you need a better mask. If it’s really high (covid ward), even good masks aren’t good enough. You need additional PPE.

I guess the next question is, how does vaccination change all this? Vaccination may reduce viral load in breakthrough infections which should make masks more effective as source control even for a Delta-infected person. Initial report of decreased SARS-CoV-2 viral load after inoculation with the BNT162b2 vaccine | Nature Medicine.

On the other side of the equation, a vaccinated person would be able to handle a larger inoculum. A decent quality mask may be good enough to protect a vaccinated person against infection by Delta.

An unvaccinated person in a region with low vaccination rates and anti-maskers will have to double mask or use an N95 every time they go indoors in public. Unvaccinated and no mask in a region with high infection rates? It seems like it’s going to be a matter of time.

Delta variant was specifically referenced:

Citing rise of delta variant, Los Angeles reports 165 percent increase in Covid cases (

Now the article says “0.06 percent.” This translates to 0.0006%. They also cite 839 new infections. 0.0006% of 839 is half a person and impossible mathwise. So something’s off there.

Regardless, if you’re counting up to 839, the highly touted delta variant is infecting the unvaccinated at an extremely higher rate than the vaccinated.

I think that article mixed up data in the news release. The 0.06% covid positive is 2822 out of all 4.6 million fully vaccinated people in LA county since vaccines were first rolled out.

Among 4.6 million fully vaccinated people in L.A. County, Public Health identified 2,822 people who tested positive for a COVID-19 infection contracted more than two weeks after they were fully vaccinated. That means that about 0.06% of all fully vaccinated people tested positive for COVID-19. A total of 195 people, 0.004% of those fully vaccinated, were hospitalized for infections contracted while fully vaccinated. And 21 people died of their infections, 0.0004%. These numbers are very similar to the numbers seen last week.

Even so, with numbers that good, even a slight decrease in protection will still mean excellent protection.

0.06 percent doesn’t get “translated” into anything to get percent. It’s already in percent (%). If you mean it translates to 0.0006 of the total, that would be accurate.

I mistated, didn’t need the %. 0.0006.

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