A scientist breaks it down.
Herd immunity has been discussed in Congress, pushed as a strategy to control the virus by Trump administration aides, and debated by scientists.
"Herd immunity is not a containment strategy."
— Gypsyamber D'Souza, a professor at Johns Hopkins Bloomberg School of public health.
And where it fits (and doesn't fit) in the fight against Covid-19.
The definition of herd immunity:
Herd immunity is the idea that, once the majority of a population is exposed to infection (and has some form of immunity) it becomes harder for even non-immune people to be exposed.
When no one has immunity to a virus, it can tear through the population – as seen in the top panel.
When a few people develop immunity, the virus begins to run out of potential footholds (seen in the middle panel)
Eventually, enough people develop immunity, leaving the virus with few footholds.
Even those without immunity run lower risks of encountering it.
What's the best way to get there?
Vaccination, D'Souza says.
The measles vaccine essentially eliminated the highly contagious disease from North America in 2000.
(The virus resurged in 2019, beginning in places that did not have high enough rates of vaccination to achieve herd immunity.)
Douglas Sacha/Getty Images
How many people need to be vaccinated to achieve herd immunity?
For Covid-19, the World Health Organization estimates that between 60 and 80 percent of the population would need to be immune for herd immunity to be achieved.
Some scientists place their estimates as low as 50 percent, The New York Times reported.
D'Souza says that the threshold is "certainly above 50% for coronavirus and we don't know exactly where it is."
How has this played out during the pandemic so far?
Herd immunity is not a strategy for controlling the pandemic for a clear reason:
It comes at an extremely high cost.
JONATHAN NACKSTRAND / Getty Images
Sweden tried to achieve herd immunity naturally by letting the virus run uncontrolled. Sweden didn't impose social distancing or lockdowns this winter.
"They have seen higher mortality and morbidity than all of the countries that surrounded them," says D'Souza.
Sweden has averaged 57.7 coronavirus deaths per 100,000 people.
Johns Hopkins Coronavirus Resource Center
Norway, its neighbor, has averaged about 5 deaths per 100,000 people
Finland has averaged 6.22 deaths per 100,000 people.
That includes Sweden, where only an estimated 15 percent of the population had antibodies, according to an August paper in The Journal of the Royal Society of Medicine.
Is it possible to achieve herd immunity in a small community?
68 percent of people had coronavirus antibodies at one clinic in Corona, Queens. In pockets of Mumbai's poorest neighborhoods, between 51 and 58 percent of people had antibodies.
NurPhoto / Contributor/ Getty Images
That's not enough to truly guarantee immunity in the US for a few reasons.
Bernhard Lang/ Getty Images
And even self-isolated herd immunity bubbles can still be popped.
A final note on herd immunity...
KATERYNA KON/SCIENCE PHOTO LIBRARY/Getty Images
Scientists are still unsure how long immunity to the coronavirus — if encountered naturally — even lasts.
"If having had that infection does not prevent you from getting reinfected at all, well then the idea of herd immunity doesn't apply at all," says D'Souza.
"Of course it can still apply if there is an effective vaccine," she adds.
"You get protection that way."
Read more about how vaccines get approved here.