"It's a hypothesis. Some level of pre-existing cross-protective immunity… might explain why the epidemic didn't unfold [the way it did in other parts of the world]," said Prof Madhi, explaining that data from scientists in the US appeared to support the hypothesis.

Colds and flu are, of course, commonplace around the world.

But the South African scientists wondered whether, because those viruses spread more effectively in overcrowded neighbourhoods, poorer communities might have been more widely exposed and therefore, enjoy a larger degree of immunity towards Covid-19.

The same, of course, could apply to other parts of the world, like India, with similar challenges.

"The protection might be much more intense in highly populated areas, in African settings. It might explain why the majority [on the continent] have asymptomatic or mild infections," said Prof Madhi.

"I can't think of anything else that would explain the numbers of completely asymptomatic people we're seeing. The numbers are completely unbelievable," he said, savouring the irony of the possibility that Africa's entrenched poverty might "for once" work in the continent's favour.

Sceptics though might point to countries like Brazil, with its crowded favelas, and its high infection rate.