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 (nih.gov)
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 (nih.gov)
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 (nih.gov)
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.