Your quote is the scientific understanding of the nature of viruses and their mutations.
The Delta variant of SARS-CoV-2 is, as far as anyone who has paid attention to India can tell, a virus that has far exceeded the infectivity and mortality of the common cold. It was the main variant of COVID-19 in India. India has recorded almost 400,000 deaths officially attributed to COVID-19. The real number of people dying of COVID-19 in India is probably five to ten times that number. It was not that many weeks ago that hospitals in India were overwhelmed, running out of not just hospital beds but oxygen supplies. Crematoriums were overwhelmed, burning people in car parks. Bodies floating down the Ganges.
If you are suggesting that the reason that the number of new cases and new deaths has abated somewhat in India is due to a new mutation, you are probably wrong. It is more likely due to natural herd immunity in multiple areas in India.
