The initial data says that even though South Africa has a much lower vaccination rate, omicron is mild.
From today:
> lack of high death and hospitalization rates, despite the fact that Omicron has spread at breakneck speed across the country and accounts for most of the infections over the past three weeks, is the most glaring difference.
https://www.cbsnews.com/news/covid-omicron-variant-south-afr...
If you don't correct for it, it's easy to see "10x as many cases but few deaths" just because cases went up so quickly that deaths didn't start yet. Of course, this can be corrected for, and I'd expect a good study to do so - but I would NOT trust a typical news article to make the distinction, nor would I trust it to skip the temptation to misinterpret the study for a more interesting headline.
“South Africa has a quite high seroprevalence of prior infection, particularly after delta, and in some parts of South Africa up to 80 percent of people were exposed to previous infection,” she said. “We don’t think it’s a question of virulence, but more a question of exposure to vaccination and prior infection, so we would be cautious to try and interpret that this is a less virulent strain. We’ll have to see what happens in other parts of the world before we make a call on this.”
>By May 2021, the combined infection- and vaccination-induced seroprevalence estimate increased to 83.3% (95% CI, 82.9%-83.7%)
https://jamanetwork.com/journals/jama/fullarticle/2784013
The delta variant surge didn't hit the States until after this study concluded, so one would expect the numbers to be quite a bit higher today.
The UK provides some estimates of seroprevalence in blood donors (thus a skewed sample): https://www.gov.uk/government/publications/covid-19-vaccine-... (Figure 3). These estimates aren't quite what you are looking for, but one test is sensitive to previous infection and estimates 20% previously infected, whereas another test is sensitive to (infection OR vaccine) and estimates ~100%. Basically, in that population, pretty much everybody has some kind of antibodies.
To answer your question, I would say that SA doesn't look particularly unique.
> Professor Shabir Madhi, a vaccinologist at Wits University in Johannesburg who ran trials on AstraZeneca's COVID vaccine, believes it is the substantial percentage of the population in places like Gauteng province — which includes the urban hubs of Pretoria and Johannesburg and has seen a dramatic uptick in new infections — with underlying T-cell immunity that is preventing the disease from becoming more severe.
It should bring deep shame on people to see their own source material used directly in contradiction to their claim.
There are not a lot of people out there still who are not partially immune at this point, due to vaccine, recovery or both. It varies by region of course, but it's certainly true in most parts of the US.