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COVID cases are suddenly falling in 4 hard-hit Northeastern states. Does that mean herd immunity is on the way?

Eager to know when parts of America are finally approaching herd immunity against COVID-19? Then pay close attention to what’s happening in New York, New Jersey, Massachusetts and Connecticut.

Triggered by reopenings and fueled by the rise of more contagious variants, COVID numbers suddenly shot up across all four states last month. But then, just as suddenly, cases began to plummet right at the start of April — and they’re still plummeting today, even as restrictions are being lifted.

Could this swift reversal — which occurred despite rising mobility and mixing — herald the local onset of something like herd immunity (that is, the point when the virus starts to run out of unprotected hosts to infect)?

People walk by a sign for both a Covid-19 testing clinic and a Covid vaccination location outside of a Brooklyn hospital on March, 29 2021 in the Brooklyn borough of New York City. (Spencer Platt/Getty Images)People walk by a sign for both a Covid-19 testing clinic and a Covid vaccination location outside of a Brooklyn hospital on March, 29 2021 in the Brooklyn borough of New York City. (Spencer Platt/Getty Images)

A person walks by a sign for a COVID-19 testing site and a COVID vaccination location outside a Brooklyn hospital in March. (Spencer Platt/Getty Images)

“I do think this pattern is significant, and the leading factor is the combination of natural immunity from infection and vaccine-induced immunity,” says Yahoo News Medical Contributor Dr. Kavita Patel, a Brookings Institution health scholar and a primary care internist. “Between the two, you’re starting to cover the majority of the population in these states. We’re progressing toward herd immunity kind of by hook or crook.”

We may never actually know when a particular state (or the U.S. as a whole) crosses the herd immunity threshold, which experts describe as the point when 75 to 90 percent of a particular population is protected by antibodies. For one thing, true herd immunity — the goal of getting so many Americans vaccinated that COVID-19 can never spread again — is probably unattainable; vaccine demand is declining, hesitancy persists in certain pockets of the country, variants keep emerging and most of the rest of the world remains unshielded by prior infection or immunization.

But there’s another, more immediate way to look at what vaccination and natural immunity have the power to do, together: end the emergency of the U.S. pandemic, reduce COVID-19 to a manageable risk and let normal life resume even before 75 percent of a particular population has been fully dosed.

This is not to say that the pandemic isn’t still raging across the country. “I do not think we are even close to real herd immunity,” Howard Forman, a professor of public health at Yale University, tells Yahoo News.

To his point, cases continue to rise in states such as North Carolina, Oregon and Nevada — and on Thursday, Washington Gov. Jay Inslee said his state, which was also hit hard last spring, is “seeing the beginning of a fourth surge” that is “starting, unfortunately, at a higher level than where the other waves started from.”

Still, “the more people who are immune,” Forman explains, “the less steep the inclines.”

Patel agrees. “We’re not likely to get to ‘zero COVID,’” she says. “But it’s like a dimmer switch. When you have about 30 percent immunity, you start to see cases go down. At 50 percent, you start to see precipitous drops. And certain pockets of the country seem to be getting there first.”

Which is where New York, New Jersey, Massachusetts and Connecticut come in. 

People prepare to be administered the Johnson & Johnson coronavirus (COVID-19) vaccine at the Northwell Health pop-up coronavirus (COVID-19) vaccination site at the Albanian Islamic Cultural Center in Staten Island on April 08, 2021 in New York City. (Michael M. Santiago/Getty Images)People prepare to be administered the Johnson & Johnson coronavirus (COVID-19) vaccine at the Northwell Health pop-up coronavirus (COVID-19) vaccination site at the Albanian Islamic Cultural Center in Staten Island on April 08, 2021 in New York City. (Michael M. Santiago/Getty Images)

The Northwell Health pop-up COVID-19 vaccination site at the Albanian Islamic Cultural Center on April 8 in New York City. (Michael M. Santiago/Getty Images)

The stats are suggestive. The daily average of new COVID-19 cases increased by 79 percent in New York during the last week of March. In New Jersey, it increased by 59 percent between Feb. 21 and April 1. In Connecticut, it increased by 99 percent over the last three weeks of March. And in Massachusetts, it increased by 64 percent over the same period.

Yet since the start of April, average daily cases have declined by 46 percent in New York, 29 percent in New Jersey, 30 percent in Massachusetts and 40 percent in Connecticut — even as rules on bars, restaurants, movie theaters and other businesses have been getting looser and looser.

Testing across all four states, meanwhile, has hovered around the same level for months, suggesting that the actual number of infections is falling, as opposed to just the number of infections being detected. 

The question is, why now?

In theory, if New York, New Jersey, Massachusetts and Connecticut were progressing toward herd immunity, two factors would be contributing: immunity from prior infection and immunity from vaccination.

The reason it’s tempting to see the recent downturn in cases across these four states as an early sign of emerging population-level protection is that all four of them boast some of America’s highest levels of both.

They were hit hardest, for starters, by the pandemic’s first wave last spring, suffering the highest death and hospitalization rates in the country. At the same time, testing was scarce, so case counts — though also higher than anywhere else in the U.S. early on — could capture only a tiny fraction of the number of residents who got infected, survived and emerged with some degree of natural immunity. Then all four states experienced sizable winter surges, just like the rest of the U.S., expanding that existing base of protection.

Medical workers tend to a patient at a Brooklyn hospital that has seen a rise in coronavirus-related cases on December 15, 2020 in New York City. (Spencer Platt/Getty Images)Medical workers tend to a patient at a Brooklyn hospital that has seen a rise in coronavirus-related cases on December 15, 2020 in New York City. (Spencer Platt/Getty Images)

Medical workers tend to a patient at a Brooklyn hospital in December. (Spencer Platt/Getty Images)

At covid19-projections.com, Youyang Gu, an independent data scientist trained at the Massachusetts Institute of Technology, spent much of the first year of the pandemic using a data-driven approach with a layer of artificial intelligence to forecast the virus’s trajectory. Gu stopped updating his true infection estimates on Feb. 21, but at the time, he estimated that 38.9 percent of New Jerseyans, 34.5 percent of New Yorkers, 26.4 percent of Connecticuters and 26.3 percent of Bay Staters had already gotten COVID.

Some other states — namely the ones with low levels of mask wearing and lax restrictions — have seen higher rates of infection; South Dakota (47.5 percent as of Feb. 21) comes to mind.

But few other states have also administered at least one vaccine dose to as many of their residents as New York (44 percent), New Jersey (47.4 percent), Massachusetts (50 percent) and Connecticut (50.3 percent), on top of such a large existing foundation of natural immunity.

It is the combination of these two kinds of immunity that could be at least starting to starve the coronavirus of vulnerable people to infect and depressing case counts across four of America’s previously hardest-hit states, even as they’ve continued to phase out official mitigation measures and combat more contagious variants.

Meanwhile, the march toward immunity may be getting an additional boost, experts theorize, from the particular politics and demographics of such states. All states, for instance, gave older Americans early access to vaccination; younger adults qualified only recently. Yet while vaccination rates among seniors have been relatively consistent regardless of their partisan leanings, that’s not the case among non-seniors.

According to a New York Times analysis, the rate of full vaccination for older adults in Republican-leaning counties is just 5 percent lower than the national average. But the rate for younger adults in those same counties is 18 percent below average. In other words, bluer states such as New York, New Jersey, Massachusetts and Connecticut are likely vaccinating younger Americans at a higher rate than redder states — and last year younger Americans accounted for more than 70 percent of the virus’s spread, according to a study by Imperial College London’s Department of Mathematics.

A woman squints as she receives her second dose of the Moderna Covid-19 vaccine at the mobile Covid-19 vaccination clinic,run by Hartford Healthcare at Saint Charles Borromeo Catholic Church's McGivney community center in Bridgeport, Connecticut on April 20, 2021. (Joseph Prezioso/AFP via Getty Images)A woman squints as she receives her second dose of the Moderna Covid-19 vaccine at the mobile Covid-19 vaccination clinic,run by Hartford Healthcare at Saint Charles Borromeo Catholic Church's McGivney community center in Bridgeport, Connecticut on April 20, 2021. (Joseph Prezioso/AFP via Getty Images)

A woman receives her second dose of the Moderna COVID-19 vaccine at a community center in Bridgeport, Conn., on Tuesday. (Joseph Prezioso/AFP via Getty Images)

“Elderly people are being vaccinated at a relatively high rate no matter where you are, but getting younger people vaccinated (versus older) is underrated in getting this under control,” Forman says. “Vaccinating 18-to-40-year-olds has a much bigger impact on cases than vaccinating the same number of 65-to-90-year-olds.”

Likewise, Patel believes that the specific demographics of more urban communities may also be working in favor of immunity.

“Poor Black and brown communities got hit harder in all these regions, which means they have a higher level of natural immunity,” Patel says. “So even though they’re not getting as much vaccine, that helps. Then we’ve got the whiter, wealthier communities [in big cities] getting vaccinated at a really high rate. You do the math, you add them up and you get a lot more people with immunity. It may actually be a better demographic formula for getting to herd immunity than the mix in some smaller rural areas.”

To be sure, other forces might be at work here as well. Warming weather and seasonality likely play a part. So does the fact that New York, New Jersey, Massachusetts and Connecticut have kept their public mask mandates in place; that simple precaution, combined with commonsense behavior like not gathering indoors, unmasked and unvaccinated, probably acts to further slow the virus’s spread among the shrinking number of residents who still don’t have any immunity.

But unlike previous declines, this one doesn’t seem to be happening because people are changing their behavior and taking more precautions. They’re taking fewer, yet cases continue to fall. Something else seems to be exerting downward pressure on the virus.

In this photo made Tuesday, Dec. 5, 2017, Kavita Patel poses for a photo in Creve Coeur, MO. (Jeff Roberson/AP Photo)In this photo made Tuesday, Dec. 5, 2017, Kavita Patel poses for a photo in Creve Coeur, MO. (Jeff Roberson/AP Photo)

Dr. Kavita Patel in Creve Coeur, Mo., in 2017. (Jeff Roberson/AP)

It would be premature to claim that herd immunity has arrived in New York, New Jersey, Massachusetts or Connecticut. And declining infections certainly don’t mean these states are anywhere near “zero COVID” or that they ever will be. Vaccination is still critical, and medical experts continue to advise that everyone able to get vaccinated should do so as soon as possible.

But April’s sudden, simultaneous downturn in cases across four states with some of America’s highest combined levels of immunity is a sign of progress — and a glimpse at what might lie ahead in communities that keep their foot on the gas.

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