“Don’t fight yesterday’s war,” Mayor Bill de Blasio told WNYC’s Brian Lehrer about the prospect of new lockdowns as Omicron surges — and this time the mayor has it exactly right. Let’s hope Gov. Kathy Hochul, President Joe Biden and other leaders show the same common sense.

Yes, the mayor’s “No, no, no” was mainly about possible of public-school shutdowns, but his larger point holds across the board: “This is not March of 2020.”

And it’s not just that New York is “one of the most highly vaccinated places” in the country, as de Blasio noted: A lot else has changed these last two years, including what we know, what treatments work — and even the virus itself.

Most important, while Omicron looks to be more contagious than past strains, it’s also proving less virulent, report authorities in South Africa, where the new variant first broke out.

South African Medical Association chief Dr. Angelique Coetzee says: “Even if you get breakthrough infections, it’s mild cases.” Take precautions, but “don’t hype it up.”

A month into her country’s Omicron wave, she says her patients are suffering much milder symptoms. None have died except one who had HIV and other comorbidities.

“Preliminary data does suggest that while there is increasing rate of hospitalization . . . it looks like it is purely because of the [case] numbers rather than as a result of any severity of the variant itself, this Omicron,” says the nation’s health minister, Joe Phaahla. Indeed, he reports that just 1.7 percent of cases are leading to hospitalization, vs. 19 percent when Delta hit.

And it looks like infections have already peaked in the capital region, Bloomberg reports.

Yet South Africa is only 38 percent fully vaccinated, vs. 71 percent for New York City.

Bottom line: Our area, and much of America, will likely see surging case numbers in coming weeks — but cases stopped being the best indicator of the threat even before Omicron landed here.

On top of all this, it’s now clear that COVID in some form will be with us forever — just as influenza variants are endemic a full century after the Spanish Flu devastated the globe. But the logic of evolution drives viruses to be less deadly as well as more contagious: A bug that kills its host tends to die out, while variants that only sicken you (at most) keep spreading.

Equally important, the last two years have proven that the coronavirus is overwhelmingly a threat only to older people, with three quarters of US deaths coming in the 65+ age bracket. Kids under 12 are largely unaffected (except for cancer patients and other young ones with serious co-morbidities), nor do they seem to spread it.

All this argues for taking major precautions only for older Americans, not broad lockdowns or mask mandates for everyone — and certainly not closing schools. (New York City, which is now seeing a wave of COVID-driven school closures, ought to rethink the triggers for such shutdowns, just as the CDC is moving toward doing.)

As Stanford prof Dr. Jay Bhattacharya and his colleagues have argued since releasing their Great Barrington Declaration last October, a focus on protecting the old is the most rational strategy, hands-down. Lockdowns and other major restrictions impose huge costs of their own — economic, psychological and even to physical health.

Bottom line, he writes at The Daily Mail, rather than indulge in senseless new mask mandates (as Hochul has done), “We can end the pandemic if only we can muster the courage and political will to do so.” In areas “that have eschewed lockdowns, the pandemic is effectively over, even as the virus continues to circulate.”

It is early days but as long as we don’t give in to hysteria, “the vast majority will find that living with the virus is not so hard after all.”