Are We Even Close to Reaching Widespread Immunity for COVID-19?
There’s one phrase that regularly comes up in the discussion about life in a post-COVID world: herd immunity. After all, this form of mass protection has been essential in slowing and squashing the spread of various contagious diseases in the past, including polio, measles, chicken pox, and mumps, says Susan Besser, M.D., a primary care physician at Mercy Medical Center in Baltimore.
A community gains herd immunity when a significant portion of the population (the “herd”) becomes immune to an infectious disease, according to the Centers for Disease Control and Prevention (CDC). A person can achieve that immunity through vaccination or building an immune response due to prior infection, making person-to-person spread of the disease unlikely.
When it comes to COVID-19, the road to herd immunity isn’t simple. Anthony Fauci, M.D., director of the National Institute of Allergy and Infectious Diseases, said in late February on the Pod Save America podcast that the U.S. will “hopefully” reach herd immunity “by the end of the summer and the early fall” in 2021.
A recent CNN analysis estimates that 70% of the U.S. population could be fully vaccinated against COVID-19 by the end of July, and that 85% may be vaccinated by mid-September—but we’re nowhere near that number just yet.
Even with highly infectious variants spreading, experts agree that timeline may be possible if vaccination rates continue to ramp up. “I do think in late summer we will be closer to herd immunity,” says infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security. Here, experts break down how far we have left to go.
How does herd immunity work, exactly?
First, it’s important to understand how we build immunity to various contagious illnesses in the first place. When you’re exposed to a disease-causing pathogen, like the novel coronavirus, your body launches an immune response to attack it.
Whether you develop symptoms or not, your body will produce unique antibodies—proteins that fight against what your system sees as invaders—that will help you ward off the harmful pathogen if you encounter it again. This protection can be established through natural infection or through an effective vaccine.
So, when a large amount of people become immune to an infectious agent, it makes it that much harder for people who aren’t immune to contract. The virus can’t break through the herd, and even people who can’t be vaccinated, like newborns, people with certain health conditions, or people with severe allergies to a vaccine ingredient, are given some protection because the disease doesn’t have a lot of opportunity to spread, the CDC says.
A good example: Measles was eliminated from the U.S. in 2000 due to herd immunity. While clusters of cases show up here and there, the risk for the majority of the population is still low thanks to high rates of vaccination, the CDC says.
Herd immunity varies by disease, though. “The percentage of people that need to be immunized depends on the particular disease. It could be anywhere from 30% to 90%,” Dr. Besser says.
Is herd immunity possible with COVID-19?
That’s the hope—but it has to be acquired safely and natural infection isn’t the answer due to the high risk, experts say. A vaccine is the “surest way” to develop herd immunity in a population, says Dr. Adalja. While some immunity will come from natural infection—those who fell ill with COVID-19 and recovered from it—experts don’t think “natural infection is sufficient to slow the virus,” he says, as it has killed more than 530,000 people in the U.S. alone at the time of publication.
Plus, COVID-19 is caused by a newly discovered coronavirus (yes, a year old is still new), and experts aren’t sure how long immunity even lasts once a person naturally builds antibodies. “If you get a measles infection, the protection lasts for your entire life, but that’s not the case with some other infections. For COVID-19, it’s currently an open question,” says William Schaffner, M.D., an infectious disease specialist and professor at the Vanderbilt University School of Medicine. Data so far does seem to suggest that antibodies from a natural infection don’t last forever, he adds.
Measles is the most infectious virus that experts know of, Dr. Schaffner says, and “more than 90% of the population must be immunized against the virus,” for herd immunity to have some impact. Most respiratory viruses aren’t as infectious as measles, though, and those generally need about 85% of a protected population to achieve herd immunity.
For COVID-19, estimates given by Dr. Fauci range from 70% to 85%, but that number could grow higher as variants continue to spread. The CDC emphasizes that we do not yet know how many people have to be vaccinated against COVID-19 before herd immunity is reached.
The available COVID-19 vaccines are helping us get closer to herd immunity.
“Vaccines induce immunity, and the more people who are vaccinated, the closer you come to that threshold of herd immunity,” Dr. Adalja says. To date, more than 100 million vaccine doses have been administered in the U.S., according to CDC data, reaching a little more than 22% of the population.
Nearly 30 million people in the U.S.—or 9% of the population—has had COVID-19, according to data collected by The New York Times. But Dr. Adalja says it’s important to get those people vaccinated, too, to help them build up lasting antibodies and T-cell immunity (T-cells kick into action after you’ve been infected with a pathogen). “Vaccine-induced immunity is also more robust,” he says.
Reaching herd immunity with a vaccine is a little more complicated than it seems, though. Not everyone has the same risk of getting and passing on the virus—some are more likely to be infected and spread it (think: essential workers), while others may not have as high of a risk of passing it on (think: people who have been working from home and mainly staying indoors), Dr. Adalja says. “You may get the benefits of herd immunity before you actually cross the threshold of herd immunity if the people who are responsible for the majority of cases get vaccinated,” he says.
What happens after we finally reach herd immunity for COVID-19?
Doctors think COVID-19 will eventually be a smaller threat. “I doubt it will go away,” says Richard Watkins, M.D., an infectious disease specialist and a professor of internal medicine at the Northeast Ohio Medical University. He predicts that “COVID-19 will probably become seasonal, like the flu, with increases in the winter.”
Dr. Adalja also thinks COVID-19 will become “a much more manageable respiratory illness,” and anticipates that cases will drop. “They’ll be much lower, because the virus will have a harder time finding people to infect,” he says.
Finally, keep this in mind, per Dr. Adalja: “We should go back to some semblance of normalcy even before we reach herd immunity.”
This article is accurate as of press time. However, as the COVID-19 pandemic rapidly evolves and the scientific community’s understanding of the novel coronavirus develops, some of the information may have changed since it was last updated. While we aim to keep all of our stories up to date, please visit online resources provided by the CDC, WHO, and your local public health department to stay informed on the latest news. Always talk to your doctor for professional medical advice.
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