Take a look at India, where Delta had originated. The population is much younger (and thinner) than the U.S. So my educated guess is that when it burned through the population, it had infected everyone and a lot of their elderly (their percentage of the population is much lower than ours) died in the surge. But it was only really bad in India for around three months so they got through it relatively quickly.
Our population is much older and more spread out so it's going to take a while for the fuel to burn out. Just my .02
“ Our population is much older and more spread out” (than India’s).
That is for sure. People are jammed together a lot more in India, and the sense of personal space is a lot smaller.
I rode a crowded bus there once, and had everyone within earshot leaning in to listen about how wide open Alaska is - a population density of one million acres per family, and no highway connection to the State Capital of Juneau. Eyes were wide with amazement.
India in general is among the most favorable places for disease spread on Earth. The daily commute on the Mumbai trains ensures blazingly fast and widespread transmission of respiratory viruses.
It seems that it takes natural immunity to strongly suppress transmission of Delta, even if the vaccines are effective in reducing the severity of disease and death rates. Perhaps nasal vaccines would better impede initial colonization (infection) and transmission, but nobody is going to have one approved until next year.
By then, Delta and Omnicron will probably have had their run.