Several health experts and scientists have used the term “herd immunity” to describe the overall immunity of a population against a virus like COVID-19.
But what exactly is herd immunity, and how is it achieved?
Many have questioned whether the phenomenon could be the key to ending the coronavirus pandemic, with some speculating if the United States could reach the sought-after stage by as early as April.
Turns out, the answer depends on several variables. Here is what we know about herd immunity so far.
What is herd immunity, and how is it achieved?
According to the World Health Organization, herd immunity is the indirect protection of an infectious disease that happens when the vast majority of the population is vaccinated, or has developed immunity through previous infection.
In other words, it is a “biological phenomenon” that occurs when a significant percentage of a population is inoculated, creating a shield for people who have not yet been vaccinated.
"One of the aims with working towards herd immunity is to keep vulnerable groups who cannot get vaccinated (e.g. due to health conditions like allergic reactions to the vaccine) safe and protected from the disease," the WHO said.
While fighting off a COVID-19 infection naturally can contribute to herd immunity, the WHO says it is better achieved through widespread vaccination.
“Vaccines train our immune systems to create proteins that fight disease, known as ‘antibodies’, just as would happen when we are exposed to a disease but – crucially – vaccines work without making us sick,” the WHO said.
Dr. Aileen Marty, an infectious disease expert at Florida International University, added that the protection an individual gets from a COVID-19 vaccine is arguably greater than the protection gleaned by developing antibodies naturally.
She explained that vaccines approved in the United States have an added “stiffener” type feature which hinges on the spike of the virus, so that it stays fixed in a particular shape.
“That's the best shape for making the best antibodies to keep the virus from causing disease,” she said. “And that's why the immunity that people get from the vaccine is better quality. You get better quality T cells and better quality B cells.”
How many people need to be vaccinated to accomplish herd immunity?
So, what percentage of the population needs to be vaccinated against COVID-19 in order for herd immunity to become a reality?
Experts say that number was somewhere between 70 to 75 percent -- before the arrival of COVID-19 mutations.
“Now that we have these escaped mutants, we realize that we really need to get closer to 90 percent of us having protection from this virus to protect all of us and to really allow us to have normal living,” Dr. Marty said.
When will herd immunity be reached?
A recent op-ed from the Wall Street Journal suggested that a state of herd immunity could be reached by as early as April. But several scientists -- including the nation's leading health expert -- are wary of this estimate.
Dr. Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Disease, says we could see effects of herd immunity by early-to-mid fall -- if the vaccine roll-out goes according to plan.
However, Fauci also says herd immunity is an "elusive number" and the primary focus should be on vaccination.
"We should just be concerned about getting as many people vaccinated as quickly as we possibly can because herd immunity is still somewhat of an elusive number," he said. "So rather than fixating on that, why don’t we just say, get as many people vaccinated as quickly as you possibly can."
The bottom line?
A substantial portion of the population needs to be vaccinated in order to achieve a state of herd immunity. But even then, experts caution that it's not a cure-all.
"Remember, herd immunity is not an on off switch," Dr. Marty said.
"It's more like a dimmer so that the further we get along, the better all of us are in terms of protection and in terms of getting back to normal."