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 the coronavirus mutated into new variants in quick succession over the past year, enabling it to reinfect people who were vaccinated or had previously contracted Covid-19.

Some health officials have revived the possibility of herd immunity since Omicron emerged late last year.

The fact that the variant spreads so quickly and causes milder illness might soon expose enough people, in a less harmful way, to the SARS-COV-2 virus and provide that protection, they argue.

Disease experts note, however, that Omicron’s transmissibility is aided by the fact that this variant is even better than its predecessors at infecting people who were vaccinated or had a prior infection. That adds to evidence that the coronavirus will continue to find ways to break through our immune defenses, they said.

“Reaching a theoretical threshold beyond which transmission will cease is probably unrealistic given the experience we have had in the pandemic,” Dr. Olivier le Polain, an epidemiologist with the World Health Organization (WHO), told Reuters.

That is not to say that prior immunity offers no benefit. Instead of herd immunity, many experts interviewed by Reuters said there was growing evidence that vaccines and prior infection would help boost population immunity against Covid-19, which makes the disease less serious for those who are infected, or become reinfected.

“As long as population immunity holds with this variant and future variants, we'll be fortunate and the disease will be manageable,” said Dr. David Heymann, a professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine.

Not like Measles

Current Covid-19 vaccines were primarily designed to prevent severe disease and death rather than infection. But clinical trial results in late 2020 showing that two of the vaccines had more than 90% efficacy against the disease initially sparked hope that the virus could be largely contained by widespread vaccination, similar to the way measles has been curbed by inoculation.

With SARS-CoV-2, two factors have since undermined that picture, said Marc Lipsitch, an epidemiologist at Harvard T.H. Chan School of Public Health.

"The first is that immunity, especially to infection, which is the important kind of immunity, wanes quite quickly, at least from the vaccines that we have right now," he said.

The second is that the virus can quickly mutate in a way that enables it to elude protection from vaccination or prior infection - even when immunity has not waned.

"It changes the game when vaccinated people can still shed virus and infect other people," said Dr. David Wohl, an infectious disease specialist at the University of North Carolina at Chapel Hill School of Medicine.

He cautioned against assuming that infection with Omicron would increase protection, especially against the next variant that might arise. "Just because you had Omicron, maybe that protects you from getting Omicron again, maybe," Wohl said.

Vaccines in development that provide immunity against future variants or even multiple types of coronaviruses could change that, said Pasi Penttinen, the top influenza expert at the European Centre for Disease Prevention and Control, but it will take time.

Still, the hope for herd immunity as a ticket back to normal life is hard to shake.

"These things were in the media: 'We’ll reach herd immunity when 60% of the population are vaccinated.' It didn't happen. Then for 80%. Again, it didn't happen,” Francois Balloux, professor of computational systems biology at University College London, told Reuters.

“As horrible as it sounds, I think we have to prepare ourselves to the fact that the vast majority, essentially everyone, will get exposed to SARS-CoV-2," he said.

Global health experts expect that the coronavirus will ultimately become endemic, circulating persistently in the population and causing sporadic surges. The emergence of Omicron, however, has raised questions about exactly when that might happen.

“We will get there," said the WHO's le Polain, "but we are not there at the moment.”