COVID-19 | Governments and officials are voicing hopes that Covid-19 vaccines could bring "herd immunity", with some calculating that immunising just two-thirds of a population could halt the pandemic disease and help protect whole communities or nations.

But the concept comes with caveats and big demands of what vaccines might be capable of preventing. Some experts say such expectations are misplaced.

For a start, figuring out what's needed to achieve herd immunity with Covid-19 vaccines involves a range of factors, several of which are unknown...