When will California reach herd immunity? Maybe June, maybe never

Herd immunity went from a phrase most of us had never heard of to the thing we're all waiting for in just about a year's time.

"It's kind of shorthand for back to normal," said Dr. Bob Wachter, chair of UCSF's Department of Medicine. Herd immunity will happen, he explains, when COVID-19 "has so little opportunity to find a happy home here that it begins to die off and isn't an important part of our lives anymore."

Sounds amazing, doesn't it?

Herd immunity can essentially be accomplished by two methods: "The hard way is to get sick, the easy way is to get the vaccine," said Dr. George Rutherford, an epidemiologist at UCSF.

In reality, we're taking a blended approach, aiming to have between 70% and 85% of the population immune to COVID-19 either via vaccine or from a past infection.

Eager to finally be rid of this virus, we asked four experts when we might cross that finish line. It turned out herd immunity was less of a finish line and more of a moving target. The answer is complicated and laden with asterisks, caveats and hypotheticals -- a few of which we'll lay out here. (Hear more from the doctors directly in the Q&A video at the top of this story.)

When will California hit herd immunity?

So first things first, what if California were an island? (We know it's not.) When would the state reach herd immunity?

Doing some rough math (emphasis on rough), we'd need at least 27.7 million Californians immune to the virus to reach the 70% threshold. So far, California has vaccinated about 9 million people and has had about 3.5 million confirmed cases. (That real number of cases could be anywhere from two to three times higher than that figure due to a lack of testing early in the pandemic, asymptomatic cases, and other factors, experts estimate.)

Working with those rough numbers, I asked the doctors to give me a rough estimate of when California might be able to reach herd immunity and -- the exciting part -- return to normalcy.

Rutherford estimated we'll get there somewhere between April and June of this year. Wachter suggested sometime in the fall. Dr. Alok Patel, an ABC7 special correspondent, thought late summer to early fall was a good guess.

Dr. Monica Gandhi, a UCSF professor of medicine, estimated July of 2021. "I think we could get there faster if we hurried," she added.

Four doctors and four different answers? That's because we're dealing with quite a few unknowns and even more complicating factors.

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Roadblock no. 1 to herd immunity: COVID-19 variants

First of all, let's talk about the variants.

"Viruses mutate constantly, so most mutations do nothing," said Wachter. "What we're seeing now with the U.K., the Brazilian and the South African variant is interesting, because it shows that variants can change in all sorts of ways. They can make (viruses) more infectious, they can make them more serious -- meaning equally infectious, but if you happen to get it you're likely to get sicker -- and/or they can make them resistant to immunity."

Any or all of those things are concerning because they make herd immunity harder to attain. Take the U.K. variant, which is said to be about 50% more infectious than last year's predominant strain. That means you'd need to vaccinate more people to really curb its spread. Variants that are resistant to immunity would provide an even bigger challenge and force a real re-tinkering of the vaccine.

As long as the virus keeps circulating, it will continue to have a chance to mutate. That's one of the major reasons there's been such a big emphasis on getting the vaccine into people's arms as quickly as possible. Not only does it keep people from getting sick, it keeps the virus from creating more -- potentially more contagious or serious -- mutations.

Roadblock no. 2: California isn't an island

Gov. Gavin Newsom recently announced an ambitious goal of vaccinating 4 million Californians per week. That hasn't happened yet -- not even close -- but that would hypothetically speed us toward herd immunity in no time.

But California isn't an island. Nor is the United States, for that matter. And this is where things get really complicated.

If you think the vaccine rollout is going slowly in the United States, it's going worse in almost every other country. If people around the world are still getting sick, they will continue to circulate the virus on airplanes or cruise ships, much like they did in the beginning of this pandemic.

Plus, as long as the virus is in circulation, there are more opportunities for it to mutate.

"I don't actually think we're going to eradicate this virus anytime soon," said Gandhi. Total eradication has happened before with smallpox, she points out, but the virus was substantially different from this one.

"The answer is several years, to many years, to never," said Wachter. "We never reached herd immunity with the flu. What we have (with the flu) is a virus that mutates every year and we've got to rejigger our vaccines every year. Luckily, it's not that infectious and not that deadly. But 30,000 or 40,000 people a year die of the flu in the United States."

As long as the virus continues to mutate somewhere in the world, Wachter thinks this coronavirus will be with us indefinitely, much like the flu is.

"It's seasonal, it comes in the winter, there's a new version with variants, and so we've got to rejigger vaccines every year or two," he said. "That probably is more likely than the scenario of two years from now, we never ever see this thing again."

That's why the doctors aren't exactly framing herd immunity as some grand finish line.

The ending (Spoiler: There's no ending)

"The most common misconception is that it's an on/off switch. That, you know, on Oct. 13 (for example), we will hit herd immunity. Somebody is going to ring a bell, like the stock exchange, and we'll all go back to partying. It's probably not going to be like that," said Wachter.

It'll look more like the phased reopening we've seen a few times here in California, where things slowly start to loosen up sector-by-sector. Except this time, we won't be waiting for the other shoe to drop.

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"For a while, it's gonna be weird," said Wachter. "I mean, the first time I'm gonna walk into a crowded bar without a mask, I'm not sure I can do it. It's going to be too uncomfortable. But then there will come a time where we can kind of say, 'Now people are acting pretty normal. And it's OK. Because we have reached that point.'"

See more insights on herd immunity from Dr. Monica Gandhi, Dr. Alok Patel, Dr. George Rutherford and Dr. Bob Wachter in our full Q&A at the top of this story.