What Exactly Is Herd Immunity — and Will We Ever Get There?

Experts once believed it was the key to eradicating the COVID-19 pandemic. Now, they're not so sure. Here's what you need to know about herd immunity.

When the novel coronavirus was new back in March 2020, there were countless questions circulating — some of which have now been answered: "How can I protect myself from contracting or spreading the virus?" (Answer: Wear a mask and social distance.) "Can I get seriously ill, or even have lasting, serious side effects from the virus?" (Answer: Yes to both.)

But one of the most critical (and anxiety-inducing) questions surrounding the pandemic has yet to be addressed: When will things go back to normal?

Early in the pandemic, many experts pointed to two factors that could potentially contribute to an end: First, a vaccine for the virus (which now exists 🙏, and second, something called herd immunity.

While the term might draw up images of farmers injecting hordes of cattle, herd immunity (aka population immunity) actually plays a pivotal role in the how humans eradicate (or coexistwith) viruses on this planet. "[Herd immunity] is when the majority of a population is immune to an infectious disease, either by natural infection or a vaccine providing indirect protection," explains Barun Mathema, Ph.D,assistant professor of epidemiology at Columbia University. (

If you haven't come down with a case of the measles as of late, you have herd immunity to thank. That's because roughly 93 percent of children are vaccinated for measles, according to the Centers for Disease Control (CDC). This means 93 percent of the population can't contract or transmit the virus, making it more difficult for it to jump from host to host. In other words, herd immunity is at play in keeping the virus under control.

Now that there's an effective vaccine against COVID-19, what does this all mean in regard to achieving that coveted herd immunity threshold and getting back to some semblance of normalcy? Unfortunately, as is the case with many aspects of this pandemic, the answer isn't so clear-cut.

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What's Standing In the Way of Herd Immunity from COVID-19?

Early on in the pandemic, a report from the Center for Infectious Disease Research and Policy at the University of Minnesota estimated that, in order for the U.S. to achieve herd immunity against COVID-19, roughly 60 to 70 percent of the population would need to be immune to the virus — whether from being vaccinated or from developing antibodies as a result of being infected with the virus. However, in a recent WBUR interview, Anthony Fauci, M.D., director of the U.S. National Institute of Allergy and Infectious Diseases, estimated that figure as more like 70 to 80-plus percent.

But now, those initial projections don't really hold up, says Mathema. "[With COVID-19], we are now unlikely to reach herd immunity," he says. This is due to three main factors: 1) new strains of COVID-19 are emerging with mutated characteristics and rates of infectiousness 2) a significant percentage of people in the U.S. (25 percent according to a Monmouth University poll) are refusing to get the vaccine, and 3) global vaccination and mitigation efforts have been particularly messy. (

First Up, the Variants

"Viruses are constantly mutating due to random errors during replication, but the rate of those mutations is affected by how quickly the virus is reproducing (and spreading)," says Andrea Love, Ph.D., an immunologist at Nexcelom Bioscience. "Eventually, mutations that offer the virus an advantage will persist and become more dominant, which is what we're seeing with B.1.1.7 and others." (FYI, the B.1.1.7. variant made up nearly 58 percent of all COVID-19 cases as of March 27th, according to the CDC. It's also upwards of 70 percent more contagious than the original strain.)

"If we can't get everyone protected by vaccination quickly enough, we give the virus more opportunities to mutate — and some of these mutations might potentially affect the efficacy of the vaccines," Love adds.

Next, Vaccinations

In the U.S., a little more than half of all adults have received at least one dose of a COVID-19 shot (either from Moderna, Pfizer, or the paused and now un-paused Johnson & Johnson) and roughly 32 percent are fully vaccinated as of May 5th. But as mentioned, recent polls have indicated that nearly 30 percent of Americans are hesitant to get vaccinated. And while vaccine manufacturers are starting to authorize shots for children under age 18 in the U.S., only one-third of parents said they'd try to get their child vaccinated right away, according to the Kaiser report. This is problematic for herd immunity because it could keep us from reaching that goal of 70 to 80-plus percent of immunity within the population, says Mathema. "A very aggressive and speedy push to vaccinate everyone is the best way achieve herd immunity for COVID-19," he reiterates.

Lastly, Global Factors

And the final thing delaying the possibility of herd immunity against COVID-19? Global access to the vaccine has been fragmented in many poorer countries, with limited widespread distribution campaigns and lack of funding. "While the U.S. is doing comparatively well with vaccine deployment, the rest of the world is not in as good a position," says Love. "In places like India that only have about two percent of the population vaccinated, we're seeing the devastating effects of rapid spread of new variants. We can't reach herd immunity for a global outbreak if we can't protect the rest of the world as well." According to data from The New York Times, roughly 83 percent of vaccinations for COVID-19 have been given to individuals in higher-income countries, with 0.3 percent divvied to low-income countries. (

What Will Actually Lead to an End of COVID-19 Restrictions

Unfortunately, it looks like herd immunity isn't just around the corner. But, the question that's plagued us since the pandemic's early weeks remains: When can I finally go back to my old life?

"If we continue to make progress on vaccination coverage, it is conceivable that parts of society may be able to open up again by late fall," says Mathema. However, just because more folks are vaccinated for COVID-19 doesn't mean we won't still have spikes in infection (and potential shutdowns). "If a significant percentage of the population is unvaccinated yet mixing for the holidays, we could certainly see an uptick again," says Mathema.

That makes the push for vaccinations even more critical. "Getting vaccines into the arms of everyone who wants them is key," notes Brandon Brown, Ph.D., an epidemiologist at the University of California Riverside. "Then we need to work to educate those who don't want the vaccines and help them understand why everyone getting vaccinated is the only way for all of us to be best protected."

The need to social distance and wear a mask probably won't subside anytime soon, as any mitigation can help reduce transmission and thus prevent mutations from occurring, says Love. "Anything that can slow the spread will enable us to protect people as we work to make the vaccine available around the world," she says. (

And while a semblance of "normal" life might be achievable by fall, Brown adds that it's helpful to rethink what "normal" might look like while there are still so many unknowns.

"I'm willing to say goodbye to a part of our reality prior to COVID-19," says Brown. "Universal masking is common in some countries, and in this past year, masks have helped prevent the spread of many respiratory infections not limited to SARS-CoV-2." (If you didn't come down with a non-COVID infection this year, such as the flu, you might have have mask-wearing and social distancing to thank!)

"The handshake may change to an elbow bump or fist bump permanently. Maybe employers will grant more sick days, or we will get frequent booster shots to prevent COVID-19," he says. The important thing is that "our new normal should allow people the opportunity to protect themselves from pandemics like COVID-19." And that means doing what we can, whether or not herd immunity is on the horizon.

The information in this story is accurate as of press time. As updates about coronavirus COVID-19 continue to evolve, it's possible that some information and recommendations in this story have changed since initial publication. We encourage you to check in regularly with resources such as the CDC, the WHO, and your local public health department for the most up-to-date data and recommendations.

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