By CARLA K. JOHNSON

AP – The omicron wave that assaulted the United States this winter also bolstered its defenses, leaving enough protection against the coronavirus that future spikes will likely require much less — if any — dramatic disruption to society.

Millions of individual Americans’ immune systems now recognize the virus and are primed to fight it off if they encounter omicron, or even another variant.

About half of eligible Americans have received booster shots, there have been nearly 80 million confirmed infections overall and many more infections have never been reported. One influential model uses those factors and others to estimate that 73% of Americans are, for now, immune to omicron, the dominant variant, and that could rise to 80% by mid-March.

This will prevent or shorten new illnesses in protected people and reduce the amount of virus circulating overall, likely tamping down new waves. Hospitals will get a break from overwhelmed ICUs, experts agree.

“We have changed,” said Ali Mokdad, a professor of health metrics sciences at the University of Washington in Seattle. “We have been exposed to this virus and we know how to deal with it.”

The coronavirus — the current variant or future ones that are sure to pop up — remains a dangerous germ. It is still infecting more than 130,000 Americans and killing more than 2,000 every day. Tens of millions of people remain vulnerable.

And there will be future outbreaks. The notion of a “herd immunity” that could stop the virus has slipped away under the harsh reality of new variants, waning immunity, and the rejection of vaccines by some Americans.

But the coronavirus is no longer new. Two years ago it arrived in a nation where nobody’s immune system had seen it before. The entire population — 330 million people — were immunologically naive, that is, susceptible to infection.

“I am optimistic even if we have a surge in summer, cases will go up, but hospitalizations and deaths will not,” said Mokdad, who works on the Institute for Health Metrics and Evaluation model, which calculated the 73% figure for The Associated Press.

With varying degrees of relief and caution, many Americans are starting to return to their pre-pandemic lifestyles.

Sarah Rixen, 41, of Bismarck, North Dakota, started singing again with a civic chorus after taking a year off. Now, with omicron winding down, she said she feels more confident than at any time since the crisis began.

“But I am still a little leery that there could be another variant around the corner,” said Rixen, noting that her family and most of her relatives are fully vaccinated. “I am still going to wear a mask.”

As mask mandates ease, workers return to offices and flights fill up, experts are trying to understand whether this return to normal can last, or if another setback is looming.

To address that, researchers are trying to answer questions about the virus, the vaccine, and how our bodies respond: How fast is booster protection waning against omicron? How long does protection from infection last? How many mild infections were never reported? How many people got infected but had no symptoms?

To find clues, they use health data from other countries such as Britain, Denmark, South Africa and Qatar to project what could be in store.

Scientists at Johns Hopkins University Bloomberg School of Public Health estimates that about three out of four people in the United States will have been infected by omicron by the end of the surge.

“We know it’s a huge proportion of the population,” said Shaun Truelove, an epidemiologist and disease modeler at Johns Hopkins. “This varies a lot by location, and in some areas we expect the number infected to be closer to one in two.”

That means different regions or groups of people have different level of protection — and risk. In Virginia, disease modelers are thinking about their population in terms of groups with different levels of immunity.

They estimate about 45% of Virginians have the highest level of immunity through boosted vaccination or through vaccination plus a recent infection with omicron. Another 47% have immunity that has waned somewhat; and 7% are the most vulnerable because they were never vaccinated and never infected.

In all, the vast majority of Virginians have at least some immunity, said Bryan Lewis, a computational epidemiologist who leads University of Virginia’s COVID-19 modeling team.

“That’s going to be a nice shield of armor for our population as a whole,” Lewis said. “If we do get to very low case rates, we certainly can ease back on some of these restrictions.”

Still, while the population is better protected, many individuals are not. Even by the most optimistic estimates for population immunity, 80 million or so Americans are still vulnerable. That’s about the same as the total number of confirmed infections in the U.S. during the pandemic.

“The 26% who could still get omicron right now have to be very careful,” Mokdad said.

Andrew Pekosz, a virus researcher at the Johns Hopkins Bloomberg School of Public Health, is concerned that people — particularly unvaccinated omicron survivors — may have a false sense of security. “In an ideal world, unvaccinated individuals infected with omicron would be lining up for a vaccine shot,” he said.

Also, estimating protection is far from an exact science. It’s a moving target, as immunity wanes and new variants circulate. Protection varies widely from person to person. And it’s impossible to know for sure how many people are protected at all. The IHME model estimates a wide range — from 63% to 81% of Americans.

“We’ve reached a much better position for the coming months, but with waning immunity we shouldn’t take it for granted,” Mokdad said.

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AP writer Dave Kolpack in Fargo, North Dakota, contributed.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education. The AP is solely responsible for all content.

By VLADIMIR ISACHENKOV, YURAS KARMANAU and LORNE COOK

KYIV, Ukraine (AP) — Ukrainians defied pressure from Moscow with a national show of flag-waving unity Wednesday, while the West warned that it saw no sign of a promised pullback of Russian troops from Ukraine’s borders despite Kremlin declarations of a withdrawal.

While a feared Russian invasion of Ukraine on Wednesday did not materialize, the United States and its allies maintain that the threat remains strong, with Europe’s security and economic stability in the balance.

Russian President Vladimir Putin has signaled he wants a peaceful path out of the crisis, and U.S. President Joe Biden promised that the U.S. would continue to give diplomacy “every chance,” but he struck a skeptical tone about Moscow’s intentions. Biden also insisted that Washington and its allies would not “sacrifice basic principles” respecting Ukraine sovereignty.

Russia has massed about 150,000 troops east, north and south of Ukraine. Moscow denies it has any plans to invade and announced a pullback of some forces and weapons, though it gave few details. Even though Putin has not committed to a full withdrawal, the Russian statements also seemed to lower the political temperature following weeks of escalating East-West tensions.

Russian Defense Ministry video showed a trainload of armored vehicles moving across a bridge away from Crimea, the Black Sea peninsula that Russia annexed from Ukraine in 2014. It also announced that more tank units of the Western Military District were being loaded on trains to move back to their permanent bases after training exercises.

“We haven’t seen a pullback,” U.S. Secretary of State Antony Blinken told ABC News. “He (Putin) can pull the trigger. He can pull it today. He can pull it tomorrow. He can pull it next week. The forces are there if he wants to renew aggression against Ukraine.”

NATO Secretary-General Jens Stoltenberg said the alliance also saw no sign of decreasing troop levels around Ukraine.

“At the moment, we have not seen any withdrawal of Russian forces,” he said, before chairing a meeting of NATO defense ministers in Brussels. “If they really start to withdraw forces, that’s something we will welcome but that remains to be seen.”

Ukrainian President Volodymyr Zelenskyy similarly dismissed the Russian withdrawal claims, noting the lack of evidence of a pullback. He has repeatedly sought to project calm as well as strength during the crisis, declaring Wednesday a “Day of National Unity.”

“We are united by a desire to happily live in peace,” Zelenskyy said in an address to the nation. “We can defend our home only if we stay united.”

Across the country, Ukrainians of all ages waved flags in the streets and from apartment windows.

Hundreds unfolded a 200-meter (650-foot) flag at Kyiv’s Olimpiyskiy Stadium, while another was draped in the center of a shopping mall in the capital.

In the government-controlled part of Ukraine’s eastern region of Luhansk, where Russian-backed separatists have been fighting Ukrainian troops since 2014, residents stretched another huge flag across a street.

Russia wants the West to keep Ukraine and other former Soviet nations out of NATO, halt weapons deployments near Russian borders and roll back forces from Eastern Europe. The U.S. and its allies have roundly rejected those demands, but they offered to engage in talks with Russia on ways to bolster security in Europe.

Speaking after meeting with German Chancellor Olaf Scholz, Putin said Tuesday that the West agreed to discuss a ban on missile deployment to Europe, restrictions on military drills and other confidence-building measures — issues that Moscow put on the table years ago. He added that Russia was willing to discuss those issues, but only in conjunction “with the main issues that are of primary importance for us.”

While Scholz reiterated that NATO’s eastward expansion “is not on the agenda — everyone knows that very well,” Putin retorted that Moscow will not be assuaged by such assurances.

“They are telling us it won’t happen tomorrow,” Putin said. “Well, when will it happen? The day after tomorrow? We want to solve this issue now as part of negotiation process through peaceful means.”

On Wednesday, Russian fighter jets flew training missions over neighboring Belarus and paratroopers held shooting drills at firing ranges there as part of massive war games that the West feared could be used as cover for an invasion of Ukraine. Belarusian Foreign Minister Vladimir Makei reaffirmed that all Russian troops and weapons will leave the country when the maneuvers end Sunday.

Russia has mocked Western warnings about an imminent invasion as “paranoia” and “madness.” Kremlin spokesman Dmitry Peskov pointed sarcastically at warnings of Wednesday’s invasion, saying that Russian officials had a good sleep.

Asked by German daily Welt if Russia was going to attack Wednesday, Russia’s ambassador to the European Union Vladimir Chizhov quipped: “Wars in Europe rarely start on a Wednesday.”

“There won’t be an escalation next week either, on in the week after, or in the coming month,” he said.

But British Defense Secretary Ben Wallace told Sky News that there has been no evidence of a Russian withdrawal. “In fact, we’ve seen continued buildup of things like field hospitals and strategic weapons systems,” he said.

On Tuesday, a series of cyberattacks knocked out the websites of the Ukrainian army, the defense ministry and major banks, and Serhii Demediuk, the No. 2 official at Ukraine’s National Security and Defense Council, blamed Russia.

“The action continued a hybrid aggression of the Russian Federation,” Demediuk told The Associated Press, adding that his group was was working “to stem the attacks and track down their source.”

The Kremlin denied any involvement.

Demediuk said the attacks were intended to sow panic, but Ukrainian authorities moved quickly to restore the websites.

In Moscow, Russian lawmakers sent an appeal to Putin urging him to recognize rebel-held areas in eastern Ukraine as independent states — where Russia has supported rebels in a conflict that has killed over 14,000 in nearly eight years. Putin signaled that he wasn’t inclined to back the motion, which would effectively shatter a 2015 peace deal that was a diplomatic coup for Moscow.

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Isachenkov reported Moscow and Cook from Brussels. Dasha Litvinova in Moscow, Angela Charlton in Paris, Jill Lawless in London, Ellen Knickmeyer and Aamer Madhani in Washington and Frank Jordans in Berlin contributed.

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More AP coverage of the Ukraine crisis: https://apnews.com/hub/russia-ukraine