Amid Gov. Gavin Newsom’s scenario of a post-sheltering world, there are two chilling words: herd immunity.

Tomorrow’s tableau — waiters with masks, distant desks, split-shift schools — will be the new normal, he told reporters in his Tuesday press briefing, “at least until we have herd immunity.”

The phrase came up again as he explained what’s ahead, as “we begin to transition into suppression, ultimately, on our way to herd immunity” and a vaccine. He repeated it later, describing progress “towards herd immunity.”

What’s herd immunity? It’s when so many people have been infected and develop protective antibodies that a virus runs out of hosts. That’s likely what happened in the 1918-1919 flu pandemic, which killed 20 million to 40 million people and then vanished.

What does “herd immunity” look like in the age of COVID-19? Without a vaccine, about 28 million infected Californians.

Based on current estimates, about 5 percent of infected people — or roughly 1.4 million Californians — would get severely ill. Of these, 840,000 could die, although there’s hope of holding that number down.

This bleak strategy may be the only way through a pandemic that is causing profound economic, social and education paralysis. A vaccine, which also could provide herd immunity, is 12 to 18 months away, with likely additional months needed to scale up manufacturing and distribution.

It’s also very scary. The governor’s promised “light at the end of the tunnel” could instead be the glaring halogens over an ICU bed.

As if in synchrony, on Tuesday scientists at Harvard’s prestigious Harvard T.H. Chan School of Public Health also conceded the inevitability of continued infections.

In a set of mathematical models published in the journal Science, they proposed a strategy of intermittent restrictions that would help us approach herd immunity as slowly as possible, so hospitals aren’t overwhelmed.

Rather than hiding from the virus, a goal is to spread out the number of infections at any one time, so fewer people die, they concluded.

“Several rounds of social distancing will be required to get us to ‘herd immunity’ in the absence of vaccination,” said Harvard epidemiologist and study co-author Dr. Marc Lipsitch.

This is the concept: If a large number of people — the “herd” — are immune, then a vulnerable person in the middle of the herd is unlikely to be exposed. Life goes on.

Based on early estimates of this virus’s infectiousness, we likely will need at least 70% of the population to be immune to have herd protection, according to Johns Hopkins School of Public Health epidemiologists Gypsyamber D’Souza and David Dowdy.

Why 70%? The more contagious the virus, the more people need to be vaccinated for herd immunity to work; contagious viruses are more likely to find that unprotected person in the herd. Immunity to measles is reached at 90-95%; for polio, it’s 80-85%.

Right now, we’re nowhere near that. It’s unclear exactly how far away we are, because COVID-19 antibody tests just now are coming online. Back in late February and early March, only two out of nearly 3,000 people with respiratory symptoms were later found to have had the virus, according to a Stanford study.

There’s a lot else we still don’t know. For instance, how many Californians would die with this strategy? We can only make estimates. So far, the virus has killed 3% of Californians known to be infected. But the true death rate is almost certainly much lower, because many of those infected haven’t been tested. The vast majority of infected people – 80%, it is estimated – have only mild symptoms.

How much protection do antibodies confer on people who have recovered? The jury is still out on that.

When will new medicines blunt the impact of illness? We don’t know that, either.

Could we reach that magic 70% target by infecting just young people, segregating the sick and elders? Perhaps. Hospitalization rates increase with age, with a rate per 100,000 of 0.3 in persons under age 4, 0.1 in ages 5 to 17 years and 2.5 in ages 18 to 49 years.

The goal is to reach “herd immunity” without overshooting it, incurring unnecessary heartbreak, the Harvard experts said.

Dr. Sonia Angell, director of the California Department of Public Health, seems braced for this hard choice — and rather than denying it, is preparing.

“As we change interventions, as we make it possible for people to move around more freely, we do know that infections could increase,” she said Tuesday. “And indeed, there may be more demands on our hospitals. So it is essential that we ensure our ability to care for the sick.”

Diplomatically, she warned: “As we start to loosen up, there is the possibility for more movement of COVID-19. And we need to make sure that our hospitals are prepared. Some of the key questions we’ll be asking ourselves include, do we have adequate bed capacities, staff and supplies such as ventilators and masks?”

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Coronavirus: Santa Clara County passes 20,000 cases amid statewide decline It would be easy to create herd immunity very quickly. Lifting all restrictions now would accelerate the process. It would also swamp our hospitals and fill our cemeteries.

A go-slow approach means that more of us will survive to join the herd. Studies shows that death rates are highest — surpassing 10% — in Italy and Spain, where health systems were overwhelmed by a huge explosion of cases. In contrast, the death rate is 2.4% in Germany, 2.1% in South Korea and 0.5% in Iceland.

The goal is to infect the most while killing the least. That demands an exquisitely calibrated state strategy: counting each illness, then lifting or imposing very targeted restrictions to create smaller and better-managed outbreaks.

To reach herd immunity, “there’s no light switch here” of all-or-nothing restrictions, said Newsom. “It’s more like a dimmer … toggling back and forth.”