April 14, 2021 – After more than a year of living with COVID-19, the United States is beginning to see a light at the end of the tunnel. More than 3 million shots are administered every day. With over 74 million people fully vaccinated — more than 22% of the population — the nation is steadily building its immunity to the virus.

Herd immunity is consistently mentioned as the ultimate goal of vaccinations, but what exactly does the term mean, and when will the United States reach it?

Here are answers to five of the biggest questions about herd immunity:

What Is Herd Immunity?

When enough people have immunity to an infectious disease — whether through previous infection or vaccination — it becomes more difficult for the disease to continue spreading. This provides indirect protection throughout a group (or a "herd"), even for those without individual immunity to the disease.

Experts can estimate the percentage of a population that needs to be immune to prevent a disease's spread, what's called the herd immunity threshold. The more contagious the disease, the higher the threshold.

One person with measles, for example, can infect between 12 and 18 other unvaccinated people, and roughly 95% of a population must have immunity to measles to prevent transmission. A disease will eventually sputter out when an infected individual transmits the disease to, on average, fewer than one person.

But even when a population as a whole has herd immunity, local outbreaks are still possible, said Samuel Scarpino, PhD, an assistant professor in the Network Science Institute at Northeastern University in Boston. "What the herd immunity threshold is telling us is, 'How likely is it that a small number of cases will grow into a large number of cases?' " he explains.

What Is the Threshold for COVID-19?

Experts have estimated that 70% to 90% of the population needs to be vaccinated to curb the spread of COVID-19, but there is still much to learn about this disease.

So far, the three COVID-19 vaccines authorized in the United States have remained effective against more contagious and deadly variants of the virus that have emerged, though slightly less effective than they are against the original virus.

However, if a new strain emerges that can dodge the body's immune response to the vaccines, that will dampen the nation's efforts at achieving herd immunity.

And while the vaccine rollout across the nation has been promising, the country's overall vaccination numbers don't give the full story of the state of the pandemic, says Abraar Karan, MD, an internal medicine physician at Harvard Medical School and Brigham and Women's Hospital in Boston. "We're not dealing with one huge outbreak as much as we are dealing with many local outbreaks," he said, and the immunity needed to prevent virus spread varies from place to place.

Crowded cities, where people constantly interact, will have a higher herd immunity threshold than more sparsely populated areas, Karan notes. While certain communities may already have achieved herd immunity, others are still vulnerable to outbreaks.

"It's also not going to be the case that unvaccinated individuals are scattered around randomly," Scarpino said. "They're going to be clustered in the same households, same neighborhoods, and the same towns."

Identifying and delivering vaccines to these higher-risk areas is key to controlling the spread of the virus, Karan adds.

Does Herd Immunity Mean a Return to Normal?

Although vaccinations are a key step to containing the virus, "immunity is only one part of the equation," Karan says. "It's not an on and off switch."

The virus will continue to circulate even once the herd immunity threshold is reached. It's like taking your foot off the gas of a moving car: The car will eventually come to a stop, but first it will travel some distance. Those with COVID-19 will still infect others, and more infections will occur downstream.

Take, for example, a scenario in which herd immunity is achieved, and on average, one sick individual infects 0.8 other people. If 100,000 people have the virus, they would go on to infect 80,000 more. Those 80,000 will transmit the disease to 64,000 more people, and so on.

If a virus is still spreading throughout a population, "unless 100% of people are immunized, there's still a risk that more people will get infected and more people end up in the hospital and die from the virus," says Lauren Ancel Meyers, PhD, director of the University of Texas at Austin COVID-19 Modeling Consortium.

"The moment we have herd immunity is not the moment where we can totally let down our guard," she said.

In the moving-car scenario, social distancing protocols that slow the virus' spread, such as masking and avoiding large gatherings, act as a brake and influence how quickly a pandemic shrinks.

Can We Lose Herd Immunity?

Maintaining herd immunity is a "constant battle that we have with infectious diseases," Karan said, as viruses evolve to evade immune responses. That's why a new flu vaccine is administered every year, and why we need a booster shot for diphtheria every 10 years to restore immunity to the disease as it wanes over time.

This is also why there were measles outbreaks in 2019; lower vaccination rates in some communities left residents more vulnerable to the disease.

Experts still don't know how long individual immunity to coronavirus lasts. Data suggest immunity may last at least 6 to 8 months after infection or vaccination. But it is likely that additional vaccinations will be necessary to prime the immune system to fend off the virus, Karan says.

Human behavior also influences herd immunity. The protective threshold for a mask-less, pre-COVID-19 society is higher than a masked, socially distanced world.

"We can definitely be in and out of herd immunity as our behavior changes," Scarpino adds.

Is Herd Immunity Necessary to Stop the Pandemic?

Even with the nation's successful vaccine rollout, there are still a number of hurdles to reaching herd immunity, Meyers said. The vaccines' lower efficacy against more infectious and virulent strains of COVID-19 means a greater percentage of the population needs immunity to achieve broad protection.

Additionally, vaccines aren't yet authorized for children younger than 16. Recent polling data suggest 25% to 30% of U.S. adults are hesitant to receive a vaccine, which makes the minimum herd immunity threshold of 70% difficult to cross without children and adolescents.

But that doesn't condemn the United States to a continued battle against the virus with no end in sight, said Ali Khan, MD, dean of the College of Public Health at the University of Nebraska Medical Center in Omaha.

"We're trying to achieve traditional herd immunity with vaccines while forgetting the fact that public health alone can get us to control this disease," he said, citing results in countries like China, New Zealand, and Australia.

While increasing the number of vaccinations is important, herd immunity has become somewhat of a "false god," he says, when we should be paying more attention to deaths, hospitalizations, and cases in a community. "If those numbers are going down, does it matter if the number of people vaccinated in the community is 5%, 55%, or 99%?"

Both growing vaccination numbers as well as a concerted public health effort to control spread is key, Karan adds, as there is no single magic solution to stopping COVID-19.

And the fight is far from over. "The end of COVID will be a long, drawn-out process," he said. "How quickly that happens will be dependent on what we do."

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