Herd immunity in the US could be around the corner - but maintaining it is likely to be a perpetual battle

• The US could reach herd immunity for COVID-19 in weeks or months, but the metric is a moving target.
• Vaccines bring the US closer to that goal. More-contagious variants make it harder to reach.
• Maintaining herd immunity could require annual booster shots.

Scientists want you to know that the threshold for herd immunity isn't a single, static number.

But it isn't an unattainable goal, either.

Many experts believe the US is approaching a point at which enough people will have either gotten COVID-19 or been vaccinated that the coronavirus can no longer spread easily - the definition of herd immunity. But that threshold should never be thought of as a fixed goalpost, scientists say. And reaching it wouldn't definitively end the pandemic either.

Rather, herd immunity in the US is a moving target that the country would have to work to maintain over time.

"Technically, whenever we see cases going down, that suggests that there is enough population immunity or herd immunity to drive cases down, given the other conditions that are going on," David Dowdy, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, told Insider.

Since January, new daily coronavirus cases in the US have fallen 75%, on average. In the past two weeks, cases have dipped 27%. If that pattern continues, herd immunity could be around the corner - at least for a while.

However, Dowdy said, "as people change their behavior, as the seasons change, as new variants emerge, etcetera, this number will change - so there's not a magic number that once we get there, we're always there."

Whether the US can hold onto herd immunity depends on our ability to develop and administer booster shots for new variants (assuming that the virus continues to mutate and that current vaccines aren't as effective against emerging strains).

"You have to look at: How frequently are we tracking the virus? How frequently are we producing new vaccines and getting people vaccinated?" Rahul Subramanian, a data scientist at the University of Chicago, told Insider. "So it's almost like an equilibrium."

Herd immunity will most likely require vaccinating 70% to 85% of the US population

In simple terms, herd immunity involves a calculation: how quickly the coronavirus spreads multiplied by the share of a population that's susceptible to infection. The resulting value, known the effective reproductive number, represents the average number of secondary cases that arise from a single infection. If that number is less than 1, a population has technically reached herd immunity.

Immunity developed in response to vaccinations and natural infections brings this reproductive number down. But more contagious variants and increased social interaction bring the number up. So it's hard to know exactly when the virus will stop spreading - especially if we're not catching every case through testing or are failing to track variants through genomic surveillance.

"It's almost, in a way, like economists trying to determine whether we're in a recession," Subramanian said. "Usually you find out that you're officially in a recession long after."

Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, has suggested that 70% to 85% of the US population would need to be vaccinated to reach herd immunity.

Subramanian agrees. That range, he said, accounts for lingering questions about how many people have been infected already and how long immunity after infection lasts (the best guess so far is at least eight months). Many people who previously got sick have also gotten vaccinated, which makes the vaccination rate a more reliable indicator.

"You can vaccinate 50% of the population, but if it's the wrong 50% - the 50% who are at the lowest risk of getting COVID to start with - then it doesn't mean that you magically then cross a threshold," Dowdy said. "The key is to get those numbers high enough so that even in the populations that are at highest risk of getting infection, you're having enough vaccination to make a difference."

The final hurdle: vaccine hesitancy

If the US continues vaccinating people at the current rate - about 2.3 million doses a day - then at least 70% of the population will have received at least one dose within the next five weeks. But a quarter of US adults in a recent CNN poll said they wouldn't seek out a shot.

Another way to push the US closer toward herd immunity, Subramanian said, is authorizing vaccines for children.

Moderna and Pfizer are enrolling children as young as 6 months in clinical trials. The Food and Drug Administration is expected to authorize the Pfizer-BioNTech vaccine for 12- to 15-year-olds as early as next week, The New York Times reported. Some scientists expect shots to become available to all kids this summer.

Already, scientists are encouraged by the fact that nearly half of US adults have been fully vaccinated.

"The probability of seeing another wave like what we did in December is pretty low now that we have over 40% of the population with at least one vaccine dose," Dowdy said.

"We do still need to maintain some level of vigilance," he added, "because if the virus has taught us one thing, it's that it's difficult to predict the future."

The coronavirus could follow the path of either measles or seasonal flu

In an ideal world, Subramanian said, the coronavirus would come to resemble measles: a virus that lingers at low levels in the population but requires only two vaccine doses in childhood.

Before the US approved a measles vaccine in 1968, about 3 million to 4 million people in the country got measles every year. By 2000, measles had been practically eliminated, with the exception of sporadic outbreaks among the unvaccinated.

"It's not like we've eradicated measles," Subramanian said. "It's still there in other countries, but in the US we have herd immunity against it by vaccination."

But a likelier scenario, he said, is that the coronavirus would be a yearly threat like seasonal influenza: a constantly mutating virus that makes herd immunity difficult to maintain.

"The question of whether we have herd immunity to flu, it's always changing, because we might have good immunity when you're vaccinating one season, then as soon as the virus changes, we need to vaccinate again," Subramanian said.

Dowdy said he also expected sporadic coronavirus booster shots to be necessary - though perhaps not annually.

"I think we will see this as an endemic infection," he said, "and I think we will, at some point, need to update and provide booster shots."