What does that mean - “as contagious as chicken pox” - practically speaking?
Someone sneezes on you and you are infected?
It’s those sorts of statements by the CDC and experts that mean essentially nothing, if they can’t quantify such a statement with real world examples.
Chicken pox spreads as an aerosol and has an R-value of 10 - 12 and a herd immunity threshold of about 90%, so pretty contagious. However, as several posters pointed out no mention of the virulence of the mutated SARS-CoV2 virus.
I haven’t looked for more up-to-date reports but in most countries (like the UK and Sweden) where the Indian variant is taking over, the number of hospitalizations, ICU-patients and deaths have not increased. Could mean that the virulence of the mutated virus is lower, or the fact that most people in the risk groups have been vaccinated, and are protected at least to some extent.
The fact that younger and healthier people get infected is really no problem. On the contrary, the immunity in the “herd” builds up and soon this plague will be nothing but a memory, just like the Russian flu of 1894 (which probably also was a Corona virus).
So the change here comes from one key change in this variant, which is that it's vastly improved its ability to infect the types of cells in the respiratory tract. The result is two key factors in transmission: 1) people infected with Delta carry a massively higher viral load in their lungs, throat, mouth, and nose and 2) a smaller exposure is now more likely to trigger a new infection.
From a practical perspective, this means that being close to someone who's infected is now much more likely to result in you becoming infected and being further away in an enclosed space is now a much greater risk as well. Think of it as a difference in someone who hasn't showered for a day or two (previous COVID-19 variants) versus someone who never bathes at all and just finished a 5 mile bike ride in 95 degree heat. If you can smell him, you're at a pretty high risk of becoming infected.
The risk goes up even for people who are vaccinated or who had COVID-19 previously. This is a combination of higher exposure level (more virions getting dumped into you by the infected person) and antibody resistance due to structural changes in the proteins of the Delta variant. I believe the last numbers I saw indicated that reinfections with Delta have gone to about 1 in every 6,000 or so? Which would mean we would expect to see about 167 out of every 1 million previously infected with COVID-19 come down with Delta as well. However, usually that would come with less severe symptoms than the previous infection.
"Someone sneezes on you and you are infected?"
Sneezes are particularly bad because there's a ton of pressure against the normal droplets in your mouth. This causes tens of thousands of microscopic drops (aerosols) to be ejected at high speed. From a practical standpoint, if you're in a typically small sized Subway sandwich shop getting a $5 footlong and somebody in the restaurant has Delta and sneezes, everyone in there is getting a dose of the virus and is at risk of contracting it. Sneezing is the worst, but coughing, singing, and speaking loudly are also going to eject a lot of material. Breathing is less of an issue. For any of these, the concern indoors is an infected person (or persons) saturating the space with virus particles.
Based on the data so far about just how contagious Delta is, I would say that if you're unvaccinated and in the same room as someone who has the Delta variant for more than a few minutes, you have a pretty decent chance of becoming infected. I've seen extremely limited data about anything happening outdoors; not really enough to convince me that Delta is that much of a concern outside at this point. Certainly it was clear that all previous variants were no concern outdoors at all. If you're vaccinated and don't have regular contact with people for whom COVID-19 should be a major problem (folks over 65 or people with significant health issues), I honestly wouldn't worry about it. Same if you've had COVID-19 before. If you're unvaccinated and don't want to catch COVID-19, I would be very cautious about what situations I put myself in. Outdoors is your safest bet for any social interactions.
Varicella (chicken pox) is the prototype of "most infectious".
There is a famous case where a child on the 12th floor of a children's hospital, confined to his room, caught it from a child on the third floor, proven to be identical by molecular analysis.
Now, I read this on Twitter at 2 AM and I was up all night.
But I seriously doubt they have data that shows identity with varicella for transmissibility, and, as others have pointed out, "planted" is probably more correct than "leaked".