Very few people in South Africa have been fully vaccinated, relatively speaking, roughly a quarter of the population.
So the very rapid rise in new cases there looks likely to include the very large number of the previously infected.
Daily new cases in the 7-day rolling average have absolutely soared, from about 290 in mid-November to almost 8900 by Dec 4.
There have been almost 2.8 million infections pre-Omicron, and fewer than 1.3% had been reinfected pre-Omicron, according to the new paper discussed in detail here:
