For more than a year since COVID-19 upended our lives, the concept of herd immunity has floated in our future as the goal to be reached: Once enough of us attained immunity by recovering from COVID-19 or vaccination, we could put the pandemic in the rearview mirror and return to life as we knew it.

Experience with the likes of Spanish Flu and measles gave us reason to believe the virus will fade away once it can’t find enough unprotected people to infect.

But even with the vaccination effort ramping up and access expanding this month to everyone 16 and older, experts and authorities are backing away from the notion that herd immunity is right around the corner, or even achievable. Not least among them is California Gov. Gavin Newsom.

“I fear it’s a little illusory, this notion of herd immunity,” Newsom said Tuesday when asked if his administration had a projection for when California would reach that goal.

“You have people that simply are not going to take the vaccine,” Newsom explained. “You have a population, if you’re going to reach herd immunity, that must include all of our children as well, and yet we don’t have … authorization broadly for people below the age of 16. You also have to be mindful of the variants… combined with resistance.” All that makes that question “very, very difficult to answer.”

Health experts for the most part said the governor isn’t off the mark.

“It’s a very nuanced issue,” said Dr. John Swartzberg, professor emeritus of infectious disease and vaccinology with the UC Berkeley-UCSF Joint Medical Program. “I don’t think the governor is off the wall, but do think it deserves a better explanation.”

Herd immunity comes down to a matter of math — the point where enough people have immunity to a disease to snuff out its spread. But a number of variables can make solving the equation maddening. The more contagious the disease, the more people have to be immune to slow the spread.

Andrew Noymer, an epidemiologist and population health scientist at UC Irvine, noted that herd immunity isn’t the same as disease eradication, as occurred with smallpox through a global vaccination campaign. With herd immunity, there still will be infections, even small outbreaks, as there are with measles. They just won’t spread far.

Newsom said it’s been suggested that COVID-19 may require immunity in 70%, 85% even 90% of the population, and indicated he didn’t have much faith in any particular figure.

“It’s not a magical number — the governor is right in that — it’s more of a conceptual idea,” Swartzberg said. “We want to get to the point where the chance of us being around somebody who’s infected is exceedingly low. Once it gets to that point, the virus will have a very difficult time finding a person in whom it can propagate. When we get to a very high degree of community immunity it’s very unlikely this virus will be able to sustain itself in any fashion to strongly influence how we live.”

Experts at the University of Washington School of Medicine’s Institute for Health Metrics and Evaluation, which runs a widely respected computer model for projecting COVID-19 outbreaks, don’t expect herd immunity in the U.S. until next year.

Ali H. Mokdad, a professor of Health Metrics Sciences at the institute, estimates 85% of the population would need immunity to the virus to achieve herd immunity because it is so contagious. But only about 65% are likely to have immunity by the winter, he said, because children younger than 16 aren’t currently eligible for the vaccine and many Americans don’t want to take it. More contagious variants of the virus that may evade some of the vaccines’ protection add to the concerns.

“We will reach herd immunity, but not before this winter,” Mokdad said. He expects the virus to subside over the summer and fall but to return in winter again with the onset of cold, drier weather more favorable to its spread. It’s unlikely to drive a surge as bad as last winter’s, he said, but it’s likely that health officials will need to reimpose travel and mask requirements and restrictions on gathering.

Dr. Anthony Fauci, the nation’s top infectious disease expert, told a Senate hearing last month that unvaccinated children impede herd immunity, saying “we should be careful about wedding ourselves to this concept of herd immunity because we really do not know precisely, for this particular virus, what that is.”

The unknowns make projecting herd immunity difficult — how long will vaccines or immunity from past infections protect us and will they stop new viral mutations in the U.S. and around the world, which eventually find their way here?

“Herd immunity is a moving target simply because we don’t have enough vaccine to vaccinate the world,” Mokdad said.

Not all experts are buying the pessimism over herd immunity. Dr. Monica Gandhi, an epidemiologist at UC San Francisco, cites several reasons why it’s reasonable to expect herd immunity in the near future.

The vaccines currently authorized in the United State by Pfizer and Moderna “are more effective than we would have ever hoped, preventing 100% of severe disease in roll-out studies and 95% of symptomatic infections, making it likely we will achieve herd immunity,” Gandhi said.

She notes studies from Israel, which has led the world in vaccinating its population, have shown that children do not need to be vaccinated to reduce transmission and that vaccinating just those age 16 and older brings down infections among children substantially.

Israel and the United Kingdom, which also has been a world leader in vaccinating its people, have seen very low transmission rates of the virus with vaccination levels of 60% and 48% respectively, even with lockdown restrictions lifting and more people gathering, Gandhi said. The vaccines have demonstrated good protection against the known variants, she added, and Californians have shown themselves to be more receptive to the vaccines than those in other states.

Even so, other experts advise against placing bets on when herd immunity will come.

Epidemiologist Lauren Ancel Meyers, executive director of the University of Texas at Austin COVID-19 Modeling Consortium, said “it depends on the efficacy and duration of immunity acquired through infection or vaccination, whether we have pockets of low immunity in our communities, and whether there are emerging variants that can evade immunity.”

“Because COVID-19 is such a stealthy virus — it spreads quickly and silently — it won’t start to fade away until the vast majority of the people are immunized,” Meyers said. “And we won’t get to this point as long as there are pockets of people who have low levels of immunity.”

UC Irvine’s Noymer said it’s likely we won’t know when we’ve reached herd immunity until months after the fact, and even then, it may be temporary.

“A lot of people think this thing just goes away,” Noymer said. “Herd immunity wanes as well as waxes. I think we can get to herd immunity, but that doesn’t mean we’re going to stay there.”