PPE corruption: Masuku fights back, challenges report's findings
"This translates to about 20% of the population."
South Africa was conducting a national study aimed at providing accurate figures about the prevalence of coronavirus antibodies - a sign of infection - among its 58 million people.
Mkhize said the drop in numbers of daily detected infections "raises the question of the level of immunity that may already be existing in society".
Leading vaccinologist Shabir Madhi, a professor at the University of the Witwatersrand, suggested infection rates could be as high as 40% in densely-populated parts of the country.
"What we have seen in South Africa - at least based on early data... is that in densely-populated areas of the country... between 35 to 40 percent of people have been infected with this virus," Madhi, who is also in charge of the South African leg of the Oxford Covid-19 vaccine trial, told an online meeting on Sunday.
Case numbers in South Africa fell from 10 000 to 15 000 a day in July to just under 2 000 in recent days.
On Monday the number of daily new infections plunged to 956, the second time in under a week that the numbers dropped below 1 000.