Increase in new cases per day and increase in positivity rate in South Africa over the last 7-10 days are troubling to say the least.
But not as much as immune response decrease in people previously infected with COVID-19. Infection with, e.g., the initial wave of COVID-19 would provide some immunity from, e.g., Delta, but regardless of what strain of COVID-19 you were infected with in the past, you are 3× more likely to be infected by Omicron because of its ability to dodge the acquired immune response of those previously infected.
Could vaccine-conferred immune response be similarly lessened in efficacy by Omicron? No definitive answer yet.
Cheers,
Ian