What are the detailed implications for transmissibility—e.g., what’s happening now?
What’s different about Delta—and is far more disturbing—is that the actual levels of the virus in a vaccinated person’s nasopharynx is approximately 1,000 times higher than the alpha variant type, reported Dr. Anthony Fauci to MSNBC during a recent interview.
There was some early evidence that COVID disease severity depended on initial dose. Given a higher initial dose the immune response is more easily overwhelmed - therefore serious disease more likely.
Researchers are now pointing to this cruise ship outbreak, in which all passengers were provided surgical masks, as evidence that universal masking may result in a higher proportion of asymptomatic COVID-19 cases. Other outbreaks of mostly asymptomatic cases where widespread masking was implemented, in places like jails and meatpacking plants, provide epidemiological data that masks could reduce viral inoculum -- and as a result, decrease the severity of illness.
Writing in the New England Journal of Medicine, Monica Gandhi, MD, and George Rutherford, MD, of the University of California in San Francisco, hypothesized that widespread population masking may act as a sort of "variolation," exposing individuals to a smaller amount of viral particles and producing an immune response.
Gandhi told MedPage Today that the viral inoculum, or the initial dose of virus that a patient takes in, is one likely determinant of ultimate illness severity. That's separate from patients' subsequent viral load, the level of replicating virus as measured by copies per mL.
The "variolation" hypothesis holds that, at some level, the inoculum overwhelms the immune system, leading to serious illness. With less than that (and the threshold may vary from one person to the next), the individual successfully fights off the infection, with mild or no clinical illness.
"Diseases in which your immune system has a big role to play in how sick you get -- and your immune system contributes to pathogenesis -- do not seem to be able to handle a large viral inoculum," Gandhi said in an interview.
One (possible - I'm not very confident) take home from this is that vaccinated people should worry less about very small doses, but take care to avoid very high doses. The problem is that exact dose is very very difficult to gauge, what you think is small may turn out to be high.
What are the known very high doses? 1. Talking face-to-face unmasked with somone who is infected, where a significant quantity of aerosol droplets pass directly from mouth to mouth.
2. Staying in a badly ventilated area where infected people have also been for some time, so that aerosols can build up in the air and be inhaled in decent quantity.
in case 1. masks help particularly the other person, because large droplets get caught or diverted by the mask. they probably help you a bit, but becasue by the time they get to you the droplets will have had time to evaporate a bit and become smaller i'm less sure about this.
in case 2. masks do not help you personally. they do help the environment because a significant fracation of contaminated droplets are caught by the mask.
I'm not sure how much masks help in these cases, maybe delta is now so transmissable that a factor of 2 or so in reduction is irrelevant. But, because there is a lot of uncertainty, and anyone can see that masks might help, it seems really nasty to everyone else not to wear one in public in these high risk scenarios.