There’s one phrase that regularly comes up in the discussion about life in a post-coronavirus 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.
But when it comes to COVID-19, the road to herd immunity isn’t simple. Here, experts break down what we know so far, and how far we left to go.
How does herd immunity work, exactly?
First, it’s important to understand how we build immunity to various 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 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 and people with certain chronic illnesses, 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, and 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 the novel coronavirus?
That’s the hope—but it has to be acquired safely and natural infection just 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 infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security. While some immunity will come from natural infection—those who contracted COVID-19 early on and effectively recovered from it—experts don’t yet know if “natural infection is sufficient to slow the virus.”
Plus, COVID-19 is caused by a newly discovered coronavirus, 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.
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, “it will need to be well above 80%,” Dr. Schaffner says.
Can antibody testing bring us closer to understanding herd immunity?
Because creating a viable vaccine will take well over a year, experts are turning to antibody testing to try and figure out who may have already had COVID-19 and recovered, which may help guide public health decisions surrounding stay-at-home orders.
A new, preliminary pilot study from Massachusetts General Hospital found that nearly a third of 200 Boston residents who gave a drop of blood to researchers on the street tested positive for antibodies linked to COVID-19. The study found that none of the 64 people who had antibodies had tested positive for COVID-19 before, per The Boston Globe.
But a third of the population isn’t nearly enough, says Dr. Schaffner. He says it’s unlikely that this much of the general population would have antibodies to the virus. (This particular study was done in an area where there was a fairly large outbreak.)
“Most of my epidemiology colleagues think that the portion of the U.S. population that’s immune is 10% or even less,” he says. “It may be higher in other parts of the country with larger outbreaks, like New York City.”
The bottom line: To achieve herd immunity, we need a vaccine.
“A vaccine would make us breathe a lot easier and would reduce the risk of COVID-19 infection very substantially,” Dr. Schaffner says. “At that point, we could all go back to normal activities without a care.”
Until then, “the only thing we can do to keep the number of cases down is social distancing,” Dr. Adalja says, and those restrictions are likely to come and go as second and third waves of COVID-19 outbreaks hit after communities reach their initial peak in cases.
In fact, a new study from Harvard’s T.H. Chan School of Public Health projects that some level of social distancing could last until 2022, requiring certain measures to be in place between 25% and 75% of that time, depending on the seasons, “unless critical care capacity is increased substantially or a treatment or vaccine becomes available.”
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