I couldn't find a source for that. I found this, which is from December 2020 - after either of the two above
https://www.who.int/news-room/questi...id-19-serology
So it mentions it can be achieved either way, although recommends vaccination as that doesn't involve people getting ill.'Herd immunity', also known as 'population immunity', is the indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection. WHO supports achieving 'herd immunity' through vaccination, not by allowing a disease to spread through any segment of the population, as this would result in unnecessary cases and deaths.