The Omicron variant, which is spreading far faster than previous versions of the coronavirus, is not likely to help countries achieve so-called herd immunity against Covid-19, in which enough people become immune to the virus that it can no longer spread, leading disease experts say.

From the earliest days of the pandemic, public health officials have expressed hope that it was possible to achieve herd immunity against Covid-19, as long as a high enough percentage of the population was vaccinated or infected with the virus.

Those hopes dimmed as...